Photosynthesis:  Calvin Cycle

WEBVTT
Kind: captions
Language: en

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I think we're now ready to learn
a little bit about the

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dark reactions.

00:00:04.570 --> 00:00:07.300
But just to remember where we
are in this whole scheme of

00:00:07.300 --> 00:00:12.750
photosynthesis, photons came
in and excited electrons in

00:00:12.750 --> 00:00:14.520
chlorophyll in the
light reactions.

00:00:14.520 --> 00:00:17.500
and as those photons went to
lower and lower energy

00:00:17.500 --> 00:00:20.250
states-- we saw it over here
in the last video-- as they

00:00:20.250 --> 00:00:22.860
went to lower and lower energy
states, and all of this was

00:00:22.860 --> 00:00:26.130
going on in the thylakoid
membrane right over here.

00:00:26.130 --> 00:00:27.920
You can imagine-- Let me do
it in a different color.

00:00:27.920 --> 00:00:29.980
You can imagine it occurring
right here.

00:00:29.980 --> 00:00:32.619
As they went into lower and
lower energy states, two

00:00:32.619 --> 00:00:33.360
things happened.

00:00:33.360 --> 00:00:37.470
One, the release of energy was
able to pump the hydrogens

00:00:37.470 --> 00:00:38.640
across this membrane.

00:00:38.640 --> 00:00:40.800
And then when you had a high
concentration of hydrogens

00:00:40.800 --> 00:00:44.380
here, those went back through
the ATP synthase and drove

00:00:44.380 --> 00:00:47.350
that motor to produce ATP.

00:00:47.350 --> 00:00:50.440
And then the final electron
acceptor, or hydrogen

00:00:50.440 --> 00:00:51.840
acceptor, depending on how
you want to view it.

00:00:51.840 --> 00:00:55.540
The whole hydrogen atom
was NAD plus.

00:00:55.540 --> 00:00:59.575
So the two byproducts, or the
two byproducts that we're

00:00:59.575 --> 00:01:03.550
going to continue using in
photosynthesis from our light

00:01:03.550 --> 00:01:06.540
cycle, from our light
reactions I guess.

00:01:06.540 --> 00:01:09.780
I shouldn't call it the light
cycle-- were-- I wrote it up

00:01:09.780 --> 00:01:13.290
here-- ATP and NADPH.

00:01:13.290 --> 00:01:16.040
And then the byproduct was that
we needed the electron to

00:01:16.040 --> 00:01:17.950
replace that first
excited electron.

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So we take it away from water.

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And so we also produce oxygen,
which is a very valuable

00:01:22.740 --> 00:01:24.420
byproduct of this reaction.

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But now that we have this ATP
and this NADPH, we're ready to

00:01:28.960 --> 00:01:31.690
proceed into the
dark reactions.

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And I want to highlight again,
even though it's called the

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dark reactions it doesn't mean
that it happens at night.

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It actually happens at the same
time as light reactions.

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It occurs while the
sun is out.

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The reason why they call it
the dark reactions is that

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they're light independent.

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They don't require photons.

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They only require ATP, NADPH,
and carbon dioxide.

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So let's understand what's
going on here

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a little bit better.

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So let me go down to
where I have some

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clean space down here.

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So we had our light reactions.

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And they produced-- I just
reviewed this-- produced some

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ATP and produced
some and NADPH.

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And now we're going to take some
carbon dioxide from the

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atmosphere.

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And all of this will go into
the-- I'll call it the light

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independent reactions.

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Because dark reactions
is misleading.

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So the light independent
reactions, the actual

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mechanism is called
the Calvin Cycle.

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And that's what this video
is really about.

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It goes into the Calvin Cycle
and out pops-- whether you

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want to call it PGAL-- we talked
about it in the first

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video-- or G3P.

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This is glyceraldehyde
3-phosphate.

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This is phosphoglyceraldehyde
They are the exact same

00:03:09.890 --> 00:03:12.420
molecule, just different
names.

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And you can imagine it as
a 3-carbon chain with a

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phosphate group.

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And then this can then be used
to build other carbohydrates.

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You put two of these together
you can get a glucose.

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You might remember in the first
stage of glycolysis, or

00:03:28.950 --> 00:03:31.730
the first time we cut a glucose
molecule we ended up

00:03:31.730 --> 00:03:34.490
with two phosphoglyceraldehyde
molecules.

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Glucose has six carbons.

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This has three.

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Let's study the Calvin
Cycle in just a

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little bit more detail.

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So let's say exiting the light
reactions, let's say we have--

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well let's start off with
six carbon dioxides.

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So this is independent of
the light reactions.

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And I'll show you why I'm
using these numbers.

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I don't have to use these
exact numbers.

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So let's say I start
off with six CO2s.

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And I could write a CO2 because
we really care about

00:04:05.760 --> 00:04:06.760
what's happening
to the carbon.

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We can just write it as a single
carbon that has two

00:04:09.320 --> 00:04:11.000
oxygens on it, which
I could draw.

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But I'm not going to draw
them right now.

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Because I want to really
show you what

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happens to the carbons.

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Maybe I should draw this
in this yellow.

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Just to show you only
the carbons.

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I'm not showing you the
oxygens on here.

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And what happens is the CO2,
the six CO2s, essentially

00:04:30.170 --> 00:04:33.240
react with-- and I'll talk a
little bit about this reaction

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in a second-- they react with
six molecules-- and this is

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going to look a little bit
strange to you-- of this

00:04:41.640 --> 00:04:45.100
molecule, you could
call it RuBP.

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That's short for ribulose
biphosphate.

00:04:48.630 --> 00:04:52.000
Sometimes called ribulose-1
5-bisphosphate.

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And the reason why it's called
that is because it's a

00:04:54.430 --> 00:04:57.630
5-carbon molecule.

00:04:57.630 --> 00:04:59.710
So, three, four five.

00:04:59.710 --> 00:05:02.840
And it has a phosphate on
the 1 and 5 carbon.

00:05:02.840 --> 00:05:05.830
So it's ribulose bisphosphate.

00:05:05.830 --> 00:05:12.400
Or sometimes, ribulosee-1-- let
me write this-- that's the

00:05:12.400 --> 00:05:14.906
first carbon.

00:05:14.906 --> 00:05:16.460
5-bisphosphate.

00:05:16.460 --> 00:05:17.850
We have two phosphates.

00:05:17.850 --> 00:05:20.710
So that's ribulose-1
5-bisphosphate.

00:05:20.710 --> 00:05:23.760
Fancy name, but it's just
a 5-carbon chain with 2

00:05:23.760 --> 00:05:24.950
phosphates on it.

00:05:24.950 --> 00:05:27.570
These two react together.

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And this is a simplification.

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These two react together.

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There's a lot more going on
here, but I want you to get

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the big picture.

00:05:35.960 --> 00:05:45.430
to form, 12 molecules of PGAL,
of phosphoglyceraldehyde or

00:05:45.430 --> 00:05:53.340
glyceraldihyde 3-phosphate of
PGAL, which you can view as

00:05:53.340 --> 00:06:01.000
a-- it has three carbons and
then it has a phosphate group.

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And just to make sure we're
accounting for our carbons

00:06:03.300 --> 00:06:06.560
properly, let's think
about what happens.

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We have 12 of these guys.

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You can think of it that
we have-- 12 times

00:06:10.520 --> 00:06:12.510
3-- we have 36 carbons.

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Now did we start with
36 carbons?

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Well we have 6 times 5 carbons.

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That's 30.

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Plus another 6 here.

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So, yes.

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We have 36 carbons.

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They react with each other
to form this PGAL.

00:06:25.260 --> 00:06:28.690
The bonds or the electrons in
this molecule are in a higher

00:06:28.690 --> 00:06:32.020
energy state than the electrons
in this molecule.

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So we have to add energy
in order for

00:06:33.900 --> 00:06:35.420
this reaction to happen.

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This won't happen
spontaneously.

00:06:37.480 --> 00:06:40.620
And the energy for this
reaction, if we use the

00:06:40.620 --> 00:06:44.160
numbers 6 and 6 here, the energy
from this reaction is

00:06:44.160 --> 00:06:51.890
going to come from 12 ATPs-- you
could imagine 2 ATPs for

00:06:51.890 --> 00:06:54.040
every carbon and every ribulose

00:06:54.040 --> 00:07:02.990
bisphosphate; and 12 NADPHs.

00:07:02.990 --> 00:07:04.900
I don't want to get you confused
with-- it's very

00:07:04.900 --> 00:07:07.220
similar to NADH, but I don't
want to get you confused with

00:07:07.220 --> 00:07:08.560
what goes on in respiration.

00:07:08.560 --> 00:07:17.100
And then these leave as 12 ADPs
plus 12 phosphate groups.

00:07:17.100 --> 00:07:25.460
And then you're going to have
plus 12 NADP pluses.

00:07:25.460 --> 00:07:27.970
And the reason why this is a
source of energy is because

00:07:27.970 --> 00:07:30.420
the electrons in NADPH, or you
could say the hydrogen with

00:07:30.420 --> 00:07:33.320
the electron in NADPH, is at
a higher energy state.

00:07:33.320 --> 00:07:35.290
So as it goes to lower
energy state, it

00:07:35.290 --> 00:07:36.680
helps drive a reaction.

00:07:36.680 --> 00:07:40.360
And of course ATPs, when they
lose their phosphate groups,

00:07:40.360 --> 00:07:42.372
those electrons are in a very
high energy state, they enter

00:07:42.372 --> 00:07:45.550
a lower energy state, help drive
a reaction, help put

00:07:45.550 --> 00:07:47.010
energy into a reaction.

00:07:47.010 --> 00:07:50.630
So then we have these
12 PGALs.

00:07:50.630 --> 00:07:54.150
Now the reason why it's called
a Calvin Cycle-- as you can

00:07:54.150 --> 00:07:55.680
imagine-- we studied
the Kreb Cycle.

00:07:55.680 --> 00:07:58.140
Cycles start reusing things.

00:07:58.140 --> 00:08:00.960
The reason why it's called the
Calvin Cycle is because we do

00:08:00.960 --> 00:08:04.570
reuse, actually, most
of these PGALs.

00:08:04.570 --> 00:08:12.660
So of the 12 PGALs, we're going
to use 10 of them to--

00:08:12.660 --> 00:08:15.400
let me actually do
it this way.

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So we're going to
have 10 PGALs.

00:08:17.780 --> 00:08:23.320
10 phosphoglyceraldehydes 10
PGALs we're going to use to

00:08:23.320 --> 00:08:26.150
recreate the ribulose
bisphosphate.

00:08:26.150 --> 00:08:27.440
And the counting works.

00:08:27.440 --> 00:08:29.530
Because we have ten 3-carbon
molecules.

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That's 30 carbons.

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Then we have six 5-carbon
molecules.

00:08:33.200 --> 00:08:34.309
30 carbons.

00:08:34.309 --> 00:08:36.580
But this, once again, is
going to take energy.

00:08:36.580 --> 00:08:41.520
This is going to take the
energy from six ATPs.

00:08:41.520 --> 00:08:45.590
So you're going to have six ATPs
essentially losing their

00:08:45.590 --> 00:08:46.260
phosphate group.

00:08:46.260 --> 00:08:47.740
The electrons enter
lower energy

00:08:47.740 --> 00:08:49.482
states, drive reactions.

00:08:49.482 --> 00:08:54.660
And you're going to have six
ADPs plus six phosphate groups

00:08:54.660 --> 00:08:56.000
that get released.

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And so you see it as a cycle.

00:08:57.590 --> 00:09:00.050
But the question is, well
gee I used all of these.

00:09:00.050 --> 00:09:01.250
What do I get out of it?

00:09:01.250 --> 00:09:03.410
Well I only used 10
out of the 12.

00:09:03.410 --> 00:09:05.895
So I have 2 PGALs left.

00:09:09.280 --> 00:09:12.700
And these can then be used--
and the reason why I used 6

00:09:12.700 --> 00:09:14.920
and 6 is so that
I get 12 here.

00:09:14.920 --> 00:09:16.150
And I get 2 here.

00:09:16.150 --> 00:09:18.790
And the reason why I have 2 here
is because 2 PGALs can be

00:09:18.790 --> 00:09:22.900
used to make a glucose.

00:09:22.900 --> 00:09:25.860
Which is a 6-carbon molecule.

00:09:25.860 --> 00:09:31.010
It's formula, we've seen
it before, is C6H12O6.

00:09:31.010 --> 00:09:34.140
But it's important to remember
that it doesn't have to just

00:09:34.140 --> 00:09:34.660
be glucose.

00:09:34.660 --> 00:09:36.770
It can then go off and generate
longer chained

00:09:36.770 --> 00:09:39.400
carbohydrates and starches,
anything that

00:09:39.400 --> 00:09:41.140
has a carbon backbone.

00:09:41.140 --> 00:09:42.180
So this is it.

00:09:42.180 --> 00:09:43.350
This is the dark reaction.

00:09:43.350 --> 00:09:47.310
We were able to take the
byproducts of the light

00:09:47.310 --> 00:09:51.130
reactions, the ATP and the
NADHs-- there's some more ATP

00:09:51.130 --> 00:09:54.670
there-- and use it
to fix carbon.

00:09:54.670 --> 00:09:56.700
This is called carbon
fixation.

00:09:56.700 --> 00:10:00.060
When you take carbon in a
gaseous form and you put it

00:10:00.060 --> 00:10:03.890
into a solid structure, that
is called carbon fixation.

00:10:03.890 --> 00:10:08.170
So through this Calvin Cycle we
were able to fix carbon and

00:10:08.170 --> 00:10:11.200
the energy comes from these
molecules generated from the

00:10:11.200 --> 00:10:12.280
light reaction.

00:10:12.280 --> 00:10:14.720
And of course, it's called a
cycle because we generate

00:10:14.720 --> 00:10:18.160
these PGALs, some of them can
be used to actually produce

00:10:18.160 --> 00:10:21.580
glucose or other carbohydrates
while most of them continue on

00:10:21.580 --> 00:10:26.250
to be recycled into ribulose
bisphosphate, which once again

00:10:26.250 --> 00:10:27.920
reacts with carbon dioxide.

00:10:27.920 --> 00:10:30.970
And then you get this cycle
happening over and over again.

00:10:30.970 --> 00:10:33.060
Now we said it doesn't
happen in a vacuum.

00:10:33.060 --> 00:10:35.040
Actually if you want to know the
actual location where this

00:10:35.040 --> 00:10:39.510
is occurring, this is all
occurring in the stroma.

00:10:39.510 --> 00:10:42.770
And the fluid inside the
chloroplast but outside of

00:10:42.770 --> 00:10:43.950
your thylakoid.

00:10:43.950 --> 00:10:47.130
So in your stroma, this is where
your light independent

00:10:47.130 --> 00:10:49.850
reactions are actually
occurring.

00:10:49.850 --> 00:10:54.640
And it's not just happening with
the ADP and the NADPH.

00:10:54.640 --> 00:10:59.600
There's actually a fairly decent
sized enzyme or protein

00:10:59.600 --> 00:11:00.560
that's facilitating it.

00:11:00.560 --> 00:11:02.790
That's allowing the carbon
dioxide to bond at certain

00:11:02.790 --> 00:11:05.900
points and the ribulose
bisphosphate and the ATP to

00:11:05.900 --> 00:11:08.440
react at certain points, to
essentially drive these two

00:11:08.440 --> 00:11:10.380
guys to react together.

00:11:10.380 --> 00:11:15.560
And that enzyme, sometimes it's
called RuBisCo, I'll tell

00:11:15.560 --> 00:11:16.870
you why it's called RuBisCo.

00:11:16.870 --> 00:11:19.000
So this is RuBisCo.

00:11:19.000 --> 00:11:23.690
So rub-- let me get the
capitalization right--

00:11:23.690 --> 00:11:29.950
ribulose bisphosphate rub--
bis-- co-- carboxylase.

00:11:29.950 --> 00:11:31.040
And this is what
it looks like.

00:11:31.040 --> 00:11:34.490
So it's a pretty big protein
enzyme molecule.

00:11:34.490 --> 00:11:37.710
You can imagine that you have
your ribulose bisphosphate

00:11:37.710 --> 00:11:39.070
bonding at one point.

00:11:39.070 --> 00:11:41.690
You have your carbon dioxide
bonding at another point.

00:11:41.690 --> 00:11:43.120
I don't know what
points they are.

00:11:43.120 --> 00:11:45.550
ATP bonds at another point.

00:11:45.550 --> 00:11:46.640
It reacts.

00:11:46.640 --> 00:11:50.400
That makes this thing twist and
turn in certain ways to

00:11:50.400 --> 00:11:55.590
make the ribulose bisphosphate
react with the carbon dioxide.

00:11:55.590 --> 00:11:57.460
NADPH might be reacting
at other parts.

00:11:57.460 --> 00:12:01.040
And that's what facilitates
this entire Calvin Cycle.

00:12:01.040 --> 00:12:06.600
And you might-- I told you over
here-- that this R U B P,

00:12:06.600 --> 00:12:11.410
this is ribulose-1
5-bisphophate.

00:12:11.410 --> 00:12:16.800
This RuBisCo, this is short for
ribulose-1 5-bisphophate

00:12:16.800 --> 00:12:18.290
carboxylase.

00:12:18.290 --> 00:12:19.880
I won't write it all out;
you could look it up.

00:12:19.880 --> 00:12:23.260
But it's just telling you, it's
an enzyme that's used to

00:12:23.260 --> 00:12:28.040
react carbon and ribulose-1
5-bisphophate.

00:12:28.040 --> 00:12:28.900
But now we're done.

00:12:28.900 --> 00:12:30.750
We're done with photosynthesis.

00:12:30.750 --> 00:12:36.070
We were able to start off with
photons and water to produce

00:12:36.070 --> 00:12:40.100
ATP and NADPH because we had
those excited electrons, we

00:12:40.100 --> 00:12:45.610
had the whole chemiosmosis to
drive the-- that allowed the

00:12:45.610 --> 00:12:47.840
ATP synthase to produce ATP.

00:12:47.840 --> 00:12:50.660
NADPH was the final
electron acceptor.

00:12:50.660 --> 00:12:54.020
These are then used as the fuel
in the Calvin Cycle, in

00:12:54.020 --> 00:12:54.990
the dark reaction.

00:12:54.990 --> 00:12:56.820
Which is badly named, it should
be called the light

00:12:56.820 --> 00:12:57.650
independent reaction.

00:12:57.650 --> 00:12:59.270
Because it actually does
happen in the light.

00:12:59.270 --> 00:13:02.070
You take your fuel from the
light reactions with some

00:13:02.070 --> 00:13:05.840
carbon dioxide and you can fix
it using your-- I like to call

00:13:05.840 --> 00:13:08.090
it-- the RuBisCo enzyme
in the Calvin Cycle.

00:13:08.090 --> 00:13:11.210
And you end up with your
phosphoglyceraldehyde which

00:13:11.210 --> 00:13:14.000
could also be called your
glyceraldehyde 3-phosphate,

00:13:14.000 --> 00:13:17.860
which can then be used to
generate glucose, which we all

00:13:17.860 --> 00:13:21.400
use to eat and fuel
our bodies.

00:13:21.400 --> 00:13:23.780
Or we learn in cellular
respiration, that can then be

00:13:23.780 --> 00:13:27.550
converted into ATP
when we need it.


Photosynthesis:  Light Reactions and Photophosphorylation

WEBVTT
Kind: captions
Language: en

00:00:00.680 --> 00:00:02.009
I want to review a little
bit of what we

00:00:02.009 --> 00:00:02.700
did in the last video.

00:00:02.700 --> 00:00:05.520
And maybe draw a larger, more
spread-out diagram.

00:00:05.520 --> 00:00:08.560
Because I think in the last
video I started to cram things

00:00:08.560 --> 00:00:10.060
on the right-hand side here.

00:00:10.060 --> 00:00:12.180
And this is a very important
concept, so I want to do it

00:00:12.180 --> 00:00:14.650
nice and spread-out in a way
that we can breathe.

00:00:14.650 --> 00:00:17.860
And maybe in the process I can
fill in some more blanks.

00:00:17.860 --> 00:00:22.400
So let's go back and draw the
membrane of a thylakoid that's

00:00:22.400 --> 00:00:24.220
sitting inside of a chloroplast.
I'm going to draw

00:00:24.220 --> 00:00:26.225
this same membrane here.

00:00:26.225 --> 00:00:30.440
So let me draw it nice
and spread out.

00:00:30.440 --> 00:00:35.090
So let me draw a nice big
membrane like that.

00:00:35.090 --> 00:00:37.520
That's the inside
of the membrane.

00:00:37.520 --> 00:00:39.780
So you can imagine that this
loops around and that would

00:00:39.780 --> 00:00:41.510
form the thylakoid.

00:00:41.510 --> 00:00:44.630
On this side of the membrane
we have the lumen.

00:00:44.630 --> 00:00:47.120
And on the outside of the
membrane we have the stroma,

00:00:47.120 --> 00:00:50.840
where all the fluid that fills
up the choloroplast. So this

00:00:50.840 --> 00:00:54.880
is the stroma right there.

00:00:54.880 --> 00:00:57.830
And this is just a kind of
standard membrane that we see

00:00:57.830 --> 00:00:59.010
in a lot of organelles.

00:00:59.010 --> 00:01:02.510
But this is actually a membrane
within an organelle.

00:01:02.510 --> 00:01:05.870
And then maybe there will be
a phospho-bilipid layer.

00:01:05.870 --> 00:01:08.840
And I just say that, or I'm
pointing that out because I

00:01:08.840 --> 00:01:10.950
want to think a little bit
about, in this video, how

00:01:10.950 --> 00:01:13.240
protons can actually get
across this thing.

00:01:13.240 --> 00:01:16.770
How do we use the energy from
these electrons going to lower

00:01:16.770 --> 00:01:21.780
energy states to actually pump
protons across this membrane.

00:01:21.780 --> 00:01:24.370
So you know when you have these
bilipid layers, your

00:01:24.370 --> 00:01:26.510
outside is hydrophilic.

00:01:26.510 --> 00:01:30.560
And of course, it's hydrophilic
because it

00:01:30.560 --> 00:01:32.080
operates well in a polar
environment.

00:01:32.080 --> 00:01:34.350
And then the insides are
non-polar or they're

00:01:34.350 --> 00:01:35.880
hydrophobic and you
have these tales.

00:01:35.880 --> 00:01:38.510
So I could draw the whole
membrane like that, but I

00:01:38.510 --> 00:01:38.996
won't do that.

00:01:38.996 --> 00:01:40.570
It will take me forever.

00:01:40.570 --> 00:01:43.060
But let me draw some of the
components that I did in the

00:01:43.060 --> 00:01:43.680
last video.

00:01:43.680 --> 00:01:45.400
So we have these complexes that

00:01:45.400 --> 00:01:47.590
span across this membrane.

00:01:47.590 --> 00:01:50.795
And the place we started
off with was the

00:01:50.795 --> 00:01:52.380
photosystem II complex.

00:01:56.170 --> 00:01:59.660
And then later on we have the
photosystem I complex.

00:02:04.380 --> 00:02:09.699
And let me draw the ATP
synthase right here.

00:02:09.699 --> 00:02:12.330
So ATP synthase also
spans across it.

00:02:12.330 --> 00:02:15.060
Then it has little
motor part of it.

00:02:15.060 --> 00:02:17.750
And the hydrogens go through and
it spins the motor and it

00:02:17.750 --> 00:02:21.280
crams the phosphate groups
into the ADP to make ATP.

00:02:21.280 --> 00:02:23.160
I'll talk about that
in a second.

00:02:23.160 --> 00:02:25.890
But the first thing I want to
point out is, as I said in the

00:02:25.890 --> 00:02:29.370
last video, the first place
where the electrons get

00:02:29.370 --> 00:02:33.710
excited is in the chlorophyll
and photosystem II.

00:02:33.710 --> 00:02:36.600
And then it gets less and less
and less excited, it gets

00:02:36.600 --> 00:02:40.150
headed off from one complex
to another complex.

00:02:40.150 --> 00:02:42.120
And eventually ends up
in photosystem I.

00:02:42.120 --> 00:02:43.780
It gets excited again.

00:02:43.780 --> 00:02:46.100
Then it gets handed off, handed
off, the whole time

00:02:46.100 --> 00:02:49.890
that energy is being used to
transfer hydrogen protons from

00:02:49.890 --> 00:02:56.800
the stroma into the lumen .

00:02:56.800 --> 00:03:00.160
But the first question that I
would ask is, why is this

00:03:00.160 --> 00:03:03.710
called photosystem II, while
this is called photosystem I

00:03:03.710 --> 00:03:05.590
when we're starting over here?

00:03:05.590 --> 00:03:10.780
And the reason is, this was
discovered first. Even though

00:03:10.780 --> 00:03:14.520
in the light reaction it
actually comes into use, or it

00:03:14.520 --> 00:03:16.750
comes into play, second.

00:03:16.750 --> 00:03:18.780
This was discovered first.
That's why they call it

00:03:18.780 --> 00:03:19.970
photosystem I.

00:03:19.970 --> 00:03:22.890
But the reality is photosystem
II is where

00:03:22.890 --> 00:03:24.800
everything starts from.

00:03:24.800 --> 00:03:29.910
Now in some textbooks you'll
also see this written as P680.

00:03:29.910 --> 00:03:34.420
And you'll see photosystem
I written as P700.

00:03:34.420 --> 00:03:38.010
And these numbers come from the
wavelength of light that

00:03:38.010 --> 00:03:41.100
is best absorbed by the
chlorophyll in these

00:03:41.100 --> 00:03:47.590
photosystems. So 680 corresponds
to 680 nanometers.

00:03:47.590 --> 00:03:50.490
That's the wavelength of light
that this absorbs the best.

00:03:50.490 --> 00:03:53.390
700 corresponds to
700 nanometers.

00:03:53.390 --> 00:03:56.710
That's the wavelength of
light it absorbs best.

00:03:56.710 --> 00:04:00.730
Now what I want to do here is
draw a little diagram below

00:04:00.730 --> 00:04:02.980
here to kind of talk
more about the

00:04:02.980 --> 00:04:04.160
electron energy states.

00:04:04.160 --> 00:04:06.750
I just kind of handwaved it a
little bit in the last video.

00:04:06.750 --> 00:04:08.520
So I'm going to draw a
little diagram here.

00:04:11.440 --> 00:04:13.540
And over here I'm going to write
the different things

00:04:13.540 --> 00:04:15.020
that the electron can
be a part of.

00:04:15.020 --> 00:04:18.930
So right now the electrons
can be part of H2O.

00:04:18.930 --> 00:04:23.300
It could be part of
chlorophyll A.

00:04:27.500 --> 00:04:31.190
It could be a part of-- I'll
talk more about this in a

00:04:31.190 --> 00:04:37.540
little bit-- pheophytin And
then you have all of the

00:04:37.540 --> 00:04:40.980
molecules or the complexes
it can become a part of.

00:04:40.980 --> 00:04:43.060
I'll actually write
them down here.

00:04:43.060 --> 00:04:44.770
So let me write.

00:04:44.770 --> 00:04:47.100
I don't want to take
up too much space.

00:04:47.100 --> 00:04:56.250
Plastoquinone and then there's
a cytochrome B6F complex.

00:04:56.250 --> 00:04:57.500
I'll just write B6F.

00:05:01.270 --> 00:05:02.520
Then you have plastocyanin .

00:05:05.370 --> 00:05:06.610
I'll just write as PC.

00:05:06.610 --> 00:05:09.270
You don't have to
memorize these.

00:05:09.270 --> 00:05:11.030
You'll forget them in
a week if you do.

00:05:11.030 --> 00:05:13.710
But unless you're studying
photosynthesis, then it might

00:05:13.710 --> 00:05:15.650
make sense to memorize them.

00:05:15.650 --> 00:05:19.570
And this is in photosystem II.

00:05:19.570 --> 00:05:21.810
Then you have chlorophyll
in photosystem I.

00:05:27.710 --> 00:05:31.380
And then you have some other,
you know you have ferredoxin

00:05:31.380 --> 00:05:34.790
I'll just write FD for
ferredoxin Some other

00:05:34.790 --> 00:05:37.640
molecules, you keep going and
then you have your eventual

00:05:37.640 --> 00:05:41.650
electron acceptor NADP plus.

00:05:41.650 --> 00:05:44.020
Which, once it accepts the
electron, becomes NADPH.

00:05:48.660 --> 00:05:52.580
Now, electrons are very-- so
this is, if we go up that's a

00:05:52.580 --> 00:05:55.060
high energy state, down it's
a low energy state.

00:05:55.060 --> 00:05:56.940
So electrons are already
very comfortable.

00:06:00.210 --> 00:06:01.700
in water.

00:06:01.700 --> 00:06:03.780
And in chlorophyll A they're
even more comfortable.

00:06:03.780 --> 00:06:05.080
At least this is
how I view it.

00:06:07.660 --> 00:06:11.120
But left to its own devices,
this electron will never leave

00:06:11.120 --> 00:06:12.360
chlorophyll A.

00:06:12.360 --> 00:06:14.150
But we know what happens.

00:06:14.150 --> 00:06:17.820
A photon comes in from 93
million miles away.

00:06:17.820 --> 00:06:20.250
You could imagine photons as
little light packets or you

00:06:20.250 --> 00:06:22.130
could view it as a light
wave. Either way.

00:06:22.130 --> 00:06:25.700
And it excites-- not necessarily
directly the

00:06:25.700 --> 00:06:26.810
chlorophyll A.

00:06:26.810 --> 00:06:30.830
It might excite other antenna
chlorophyll or

00:06:30.830 --> 00:06:33.140
other pigment molecules.

00:06:33.140 --> 00:06:35.860
And then through resonance
energy, you can imagine them

00:06:35.860 --> 00:06:37.600
vibrating, and it eventually
will excite the

00:06:37.600 --> 00:06:39.240
photophorylation A directly.

00:06:39.240 --> 00:06:42.210
Or excite the electrons in
chlorophyll A directly.

00:06:42.210 --> 00:06:45.530
And this dude right
here gets excited.

00:06:45.530 --> 00:06:49.050
Let me do that in a
brighter color.

00:06:49.050 --> 00:06:51.670
So it goes to a high
energy state.

00:06:51.670 --> 00:06:54.020
So the electron here is in
a high energy state.

00:06:54.020 --> 00:06:55.200
Ignore that lumen right there.

00:06:55.200 --> 00:06:58.040
It has nothing to do
with this electron.

00:06:58.040 --> 00:07:00.030
And then it goes-- and actually
when it goes to the

00:07:00.030 --> 00:07:02.840
high energy state, maybe I
should draw it like this, it

00:07:02.840 --> 00:07:04.880
actually shows up
in pheophytin.

00:07:04.880 --> 00:07:07.720
That is the primary
electron acceptor.

00:07:07.720 --> 00:07:10.620
And it's actually a chlorophyll
A molecule.

00:07:10.620 --> 00:07:11.910
Actually, let me show you
what a chlorophyll A

00:07:11.910 --> 00:07:13.680
molecule looks like.

00:07:13.680 --> 00:07:17.070
This is what a chlorophyll
A molecule looks like.

00:07:17.070 --> 00:07:19.590
In general, it has a
hydrocarbon tail.

00:07:19.590 --> 00:07:20.840
You see that right here.

00:07:25.900 --> 00:07:28.510
And it has a porphyrin ring.

00:07:28.510 --> 00:07:30.710
Or porphyrin head, I guess
you could call it.

00:07:33.610 --> 00:07:35.380
This little group right here
is called a porphyrin.

00:07:39.240 --> 00:07:41.520
And right in the center of
it, you have a magnesium.

00:07:41.520 --> 00:07:44.860
That green right there, that's
a magnesium ion.

00:07:44.860 --> 00:07:48.620
And when the photon comes in or
when the resonance energy

00:07:48.620 --> 00:07:51.520
comes in from some of the
antenna molecules, electrons

00:07:51.520 --> 00:07:53.840
in the double bond sitting
here in the

00:07:53.840 --> 00:07:55.870
porphyrin head get excited.

00:07:55.870 --> 00:07:58.810
Those are the electrons that
we're talking about.

00:07:58.810 --> 00:07:59.590
And they get excited.

00:07:59.590 --> 00:08:04.800
And the first electron acceptor
is this pheophytin

00:08:04.800 --> 00:08:06.030
that I just talked about.

00:08:06.030 --> 00:08:06.750
Pheophytin.

00:08:06.750 --> 00:08:09.830
It actually looks just like a
chlorophyll, but it has no

00:08:09.830 --> 00:08:11.470
magnesium ion in the middle.

00:08:11.470 --> 00:08:14.110
And maybe I'm getting a little
bit into the weeds a little

00:08:14.110 --> 00:08:14.850
bit too much.

00:08:14.850 --> 00:08:16.960
But the pheophytin, you actually
see in this diagram

00:08:16.960 --> 00:08:17.260
right here.

00:08:17.260 --> 00:08:20.160
It's part of this photosystem
complex.

00:08:20.160 --> 00:08:21.940
So the electron, you can
imagine, jumping from the

00:08:21.940 --> 00:08:26.090
chlorophyll to the pheophytin
that does not have that

00:08:26.090 --> 00:08:27.390
magnesium in the center.

00:08:27.390 --> 00:08:29.220
And when it sits in the
pheophytin it's at a very,

00:08:29.220 --> 00:08:31.430
very, very, very, high
energy state.

00:08:34.350 --> 00:08:36.330
And then it keeps being
transferred from the

00:08:36.330 --> 00:08:36.880
pheophytin.

00:08:36.880 --> 00:08:41.860
It goes to the plastoquinone So
maybe we go to a slightly

00:08:41.860 --> 00:08:43.390
lower energy state here.

00:08:43.390 --> 00:08:46.650
We keep using the electron
in green.

00:08:46.650 --> 00:08:50.230
Then it keeps going to a
slightly lower energy state in

00:08:50.230 --> 00:08:52.750
the cytochrome B6F complex.

00:08:52.750 --> 00:08:55.210
And then you have the
plastocyanin complex, lower

00:08:55.210 --> 00:08:56.610
energy state.

00:08:56.610 --> 00:09:02.430
And then eventually it goes into
photosystem I at an even

00:09:02.430 --> 00:09:03.210
lower energy state.

00:09:03.210 --> 00:09:05.560
Maybe slightly higher than the
energy state that it was

00:09:05.560 --> 00:09:08.470
originally in the chlorophyll A
molecule in photosystem II.

00:09:08.470 --> 00:09:11.350
Another photon or another set
of photons comes and hits

00:09:11.350 --> 00:09:12.290
photosystem I.

00:09:12.290 --> 00:09:15.780
Maybe its antenna molecules,
through resonance energy, that

00:09:15.780 --> 00:09:17.530
excites the electron.

00:09:17.530 --> 00:09:20.380
It might directly hit the
chlorophyll in photosystem in

00:09:20.380 --> 00:09:23.010
its reaction center.

00:09:23.010 --> 00:09:24.510
And then this gets
excited again.

00:09:27.260 --> 00:09:32.350
And so once again we have an
electron with a high potential

00:09:32.350 --> 00:09:36.720
that can keep going to, from one
molecule to another as it

00:09:36.720 --> 00:09:38.070
gets more and more
comfortable.

00:09:38.070 --> 00:09:41.640
And this releases energy that
can drive the proton pump.

00:09:41.640 --> 00:09:45.110
And it eventually ends
up in the NADPH.

00:09:45.110 --> 00:09:47.120
At a fairly high level
of energy still.

00:09:47.120 --> 00:09:50.580
This electron can still be
transferred to other things

00:09:50.580 --> 00:09:51.510
and release energy.

00:09:51.510 --> 00:09:53.610
And we'll see that when we
talk about the light

00:09:53.610 --> 00:09:55.180
independent reactions.

00:09:55.180 --> 00:09:58.010
Now the whole point of me
showing you this is, I wanted

00:09:58.010 --> 00:10:01.810
to kind of depict graphically
that the electron is starting

00:10:01.810 --> 00:10:03.000
off at a pretty low
energy state.

00:10:03.000 --> 00:10:06.430
And the only way this happens
is by energy from light.

00:10:06.430 --> 00:10:08.140
This would not happen
on its own.

00:10:08.140 --> 00:10:12.880
Going from a low energy state
to a higher energy state.

00:10:12.880 --> 00:10:15.320
And I touched on in the last
video, you have this electron

00:10:15.320 --> 00:10:17.410
going here and it gets
transferred from one molecule

00:10:17.410 --> 00:10:18.150
to another.

00:10:18.150 --> 00:10:21.090
Gets excited again, then keeps
going all the way, eventually

00:10:21.090 --> 00:10:24.650
being accepted by the NAD
plus to become NADPH.

00:10:24.650 --> 00:10:26.820
And you're like, where
did that H come from?

00:10:26.820 --> 00:10:28.590
You could say, well that
H is a proton.

00:10:28.590 --> 00:10:31.160
It gets that electron and then
they merge together and you

00:10:31.160 --> 00:10:32.340
have NADPH.

00:10:32.340 --> 00:10:33.050
But either way.

00:10:33.050 --> 00:10:35.740
But the question is, what
replaces this electron?

00:10:35.740 --> 00:10:38.060
And that's where that amazing
thing that I talked about in

00:10:38.060 --> 00:10:39.420
the last video happens.

00:10:39.420 --> 00:10:41.210
Water gets oxidized.

00:10:41.210 --> 00:10:43.060
Oxidizing is losing electrons.

00:10:43.060 --> 00:10:44.670
OIL RIG.

00:10:44.670 --> 00:10:52.110
So water gets oxidized by
the water oxidation

00:10:52.110 --> 00:10:53.890
on photosystem II.

00:10:53.890 --> 00:10:58.210
And that electron ends up and
replaces the electron in the

00:10:58.210 --> 00:10:58.830
chlorophyll.

00:10:58.830 --> 00:11:00.910
So once again, that's an amazing
idea, that you're

00:11:00.910 --> 00:11:02.730
oxidizing oxygen.

00:11:02.730 --> 00:11:06.410
So the net effect of what
happens is, is you're using

00:11:06.410 --> 00:11:10.610
energy, using this photon
energy right here, to

00:11:10.610 --> 00:11:15.110
essentially strip electrons
off of water.

00:11:15.110 --> 00:11:17.510
And as you know, when it's on
water it's spending most of

00:11:17.510 --> 00:11:18.700
its time on the oxygen.

00:11:18.700 --> 00:11:22.770
So it essentially strips
electrons off of oxygen and

00:11:22.770 --> 00:11:24.710
put them in a higher energy
state and have

00:11:24.710 --> 00:11:27.360
them end up on NADPH.

00:11:27.360 --> 00:11:30.220
And in the process, it
had gone to an even

00:11:30.220 --> 00:11:31.290
higher energy state.

00:11:31.290 --> 00:11:35.200
And then as it goes down to
NADPH, you are pumping protons

00:11:35.200 --> 00:11:36.430
across the membrane.

00:11:36.430 --> 00:11:39.330
We learned in the last video,
through chemiosmosis,

00:11:39.330 --> 00:11:43.470
eventually goes through the ATP
synthase channel, turns

00:11:43.470 --> 00:11:47.680
around this part of this protein
complex or enzyme

00:11:47.680 --> 00:11:51.460
complex and actually
generates ATP.

00:11:51.460 --> 00:11:55.070
ATP from ADP and phosphate
groups.

00:11:55.070 --> 00:11:58.740
And in the electron transport
chain video, when I talk about

00:11:58.740 --> 00:12:02.680
cellular respiration, I give a
visual concept of how this

00:12:02.680 --> 00:12:03.630
actually might happen.

00:12:03.630 --> 00:12:06.470
How you could, as these go
through, you actually can jam

00:12:06.470 --> 00:12:09.450
together the ATP and the ADP.

00:12:09.450 --> 00:12:12.680
So that's another question in
my head is, we talk about

00:12:12.680 --> 00:12:14.600
these electrons going from
one molecule to another.

00:12:14.600 --> 00:12:17.780
But how does that actually
pump hydrogen through?

00:12:17.780 --> 00:12:20.210
And I'm just going to do a very
gross oversimplification.

00:12:20.210 --> 00:12:22.070
I'm sure it's much more
complicated in

00:12:22.070 --> 00:12:24.470
actual plant cells.

00:12:24.470 --> 00:12:31.140
But you could imagine that we
have our pheophytin right here

00:12:31.140 --> 00:12:33.760
that has that electron
on it right there.

00:12:33.760 --> 00:12:35.400
Maybe it has its electron
right there.

00:12:35.400 --> 00:12:37.430
This is a gross
oversimplification.

00:12:37.430 --> 00:12:40.130
And then you have your
plastoquinone right here.

00:12:42.940 --> 00:12:44.520
That's the next acceptor.

00:12:44.520 --> 00:12:49.020
Now maybe on this protein
complex right there, the point

00:12:49.020 --> 00:12:52.350
that wants to accept the
electron is right there.

00:12:52.350 --> 00:12:57.970
And let's say that there's
another point on it that can

00:12:57.970 --> 00:13:00.170
accept a hydrogen.

00:13:00.170 --> 00:13:03.170
Maybe it accepts a hydrogen
proton there.

00:13:03.170 --> 00:13:05.910
So you can imagine when it's on
this side of the membrane,

00:13:05.910 --> 00:13:10.130
a hydrogen can become attached
right there.

00:13:10.130 --> 00:13:13.000
And this guy will want to
be attracted to that.

00:13:13.000 --> 00:13:16.720
So he'll rotate around.

00:13:16.720 --> 00:13:19.860
So you can imagine this-- if
this is kind of a wheel-- this

00:13:19.860 --> 00:13:20.530
attraction.

00:13:20.530 --> 00:13:23.660
Because the electron wants to
go into a lower energy state

00:13:23.660 --> 00:13:24.370
right here.

00:13:24.370 --> 00:13:25.970
It'll rotate around.

00:13:25.970 --> 00:13:29.950
That'll allow, essentially, this
hydrogen as it rotates.

00:13:29.950 --> 00:13:32.820
As this molecule, as this
protein rotates around this

00:13:32.820 --> 00:13:35.050
hydrogen will be able to
cross the barrier.

00:13:35.050 --> 00:13:38.290
And then once this guy and
that guy meet, then the

00:13:38.290 --> 00:13:39.750
hydrogen will be on
the other side.

00:13:39.750 --> 00:13:41.940
And so it can freely
go away again.

00:13:41.940 --> 00:13:47.420
So that's, at least in my
head, how I imagine the

00:13:47.420 --> 00:13:49.680
electrons going from a high
energy state to a lower energy

00:13:49.680 --> 00:13:52.250
state, how that can actually
drive a reaction.

00:13:52.250 --> 00:13:53.950
Remember, the electrons
want to do this.

00:13:53.950 --> 00:13:55.760
So they'll attract the different
parts of the

00:13:55.760 --> 00:13:56.920
molecules together.

00:13:56.920 --> 00:14:00.260
And as those molecules turn and
rotate and move, that can

00:14:00.260 --> 00:14:04.140
help facilitate hydrogen going
from the stroma, the outside

00:14:04.140 --> 00:14:07.440
of the thylakoid, to the inside
of the thylakoid.

00:14:07.440 --> 00:14:10.270
That will drive the chemiosmosis
later on.

00:14:10.270 --> 00:14:13.310
Now there's one other point
I want to touch on here.

00:14:13.310 --> 00:14:15.590
Everything I've described so
far, we started with an

00:14:15.590 --> 00:14:17.020
electron in water.

00:14:17.020 --> 00:14:21.880
And obviously when water loses
its hydrogens, it loses both

00:14:21.880 --> 00:14:24.760
the hydrogen protons and
electrons associated with it.

00:14:24.760 --> 00:14:26.710
You end up with just water.

00:14:26.710 --> 00:14:30.420
So you start off with hydrogens
and then you end up

00:14:30.420 --> 00:14:32.320
with just O2.

00:14:32.320 --> 00:14:34.670
And then the hydrogen protons--
the electron got

00:14:34.670 --> 00:14:37.730
taken up by the chlorophyll.

00:14:37.730 --> 00:14:42.070
When you start off with that,
we've seen already that you

00:14:42.070 --> 00:14:45.220
end up with the electron
sitting in NADPH.

00:14:45.220 --> 00:14:47.400
The electron sitting
out here in NADPH.

00:14:47.400 --> 00:14:51.955
At some point you have NAD
as the final acceptor.

00:14:51.955 --> 00:14:54.300
Let me do it in the
right color.

00:14:54.300 --> 00:14:58.600
You have NAD plus as
the final acceptor.

00:14:58.600 --> 00:14:59.850
And it becomes NADPH.

00:15:03.070 --> 00:15:04.820
You can imagine it accepts
maybe a hydrogen

00:15:04.820 --> 00:15:06.030
proton from out here.

00:15:06.030 --> 00:15:09.570
It accepts the electron from
this electron transport chain

00:15:09.570 --> 00:15:10.990
in photosynthesis.

00:15:10.990 --> 00:15:14.570
And it becomes NADPH and that
travels in the stroma, which

00:15:14.570 --> 00:15:17.230
is where the dark reactions
occur that actually produce

00:15:17.230 --> 00:15:18.340
the carbohydrate.

00:15:18.340 --> 00:15:22.710
But this idea of an electron
going from water to NADPH,

00:15:22.710 --> 00:15:39.710
this is called non-cyclic
photophosphorylation And it's

00:15:39.710 --> 00:15:43.160
called non-cyclic because you're
not reusing the same

00:15:43.160 --> 00:15:44.610
electrons over and over again.

00:15:44.610 --> 00:15:46.705
The electrons start off, and
depending how you view it, in

00:15:46.705 --> 00:15:47.700
the chlorophyll or the water.

00:15:47.700 --> 00:15:50.280
And they end up in the NADPH.

00:15:50.280 --> 00:15:53.640
Now there's another type of
photophosphorylation and you

00:15:53.640 --> 00:15:55.360
might guess what it's called.

00:15:55.360 --> 00:15:57.740
It's called cyclic.

00:15:57.740 --> 00:16:02.470
Cyclic photophosphorylation
We'll see when we study the

00:16:02.470 --> 00:16:06.280
dark cycles or the Calvin cycle
or the dark reactions or

00:16:06.280 --> 00:16:08.720
the light independent
reactions, that it

00:16:08.720 --> 00:16:09.960
uses a lot of ATP.

00:16:09.960 --> 00:16:12.420
Actually ATP in disproportion
to the

00:16:12.420 --> 00:16:14.290
amount of NADPH it uses.

00:16:14.290 --> 00:16:17.140
It uses both, but it
uses a ton of ATP.

00:16:17.140 --> 00:16:24.730
So cyclic phosphorylation only
produces ATP and actually does

00:16:24.730 --> 00:16:26.370
not oxidize water.

00:16:26.370 --> 00:16:29.660
So what happens in that
situation is this electron,

00:16:29.660 --> 00:16:33.530
after it gets activated or
after it gets excited in

00:16:33.530 --> 00:16:37.550
photosystem I, it's the
electron, it eventually ends

00:16:37.550 --> 00:16:43.470
up-- instead of at NADPH, it
ends up at photosystem II.

00:16:43.470 --> 00:16:46.420
So instead of this guy having
to be replaced by electrons

00:16:46.420 --> 00:16:50.990
from water, this guy, in cyclic
photophosphorylation

00:16:50.990 --> 00:16:53.390
ends up-- well, maybe I should
do it from here-- ends up

00:16:53.390 --> 00:16:55.980
getting replaced by the
original electrons.

00:16:55.980 --> 00:16:57.155
It gets excited here.

00:16:57.155 --> 00:16:59.810
It goes from molecule to
molecule, lower energy states,

00:16:59.810 --> 00:17:02.150
hydrogen gets pumped
into the lumen.

00:17:02.150 --> 00:17:06.040
Gets excited again in
photosystem I and it enters

00:17:06.040 --> 00:17:07.490
lower and lower energy states.

00:17:07.490 --> 00:17:10.050
But then ends up again
in photosystem II.

00:17:10.050 --> 00:17:12.319
That is cyclic
photophosphorylation.

00:17:12.319 --> 00:17:15.140
So you can imagine in this
situation, since the electron

00:17:15.140 --> 00:17:20.190
never ends up at NAD plus, you
don't end up producing NADPH.

00:17:20.190 --> 00:17:24.010
And since you're replacing
this electron from the

00:17:24.010 --> 00:17:26.230
photosynthesis or from the
electron transport chain

00:17:26.230 --> 00:17:29.110
directly, you don't
have to strip the

00:17:29.110 --> 00:17:30.960
electrons off of the water.

00:17:30.960 --> 00:17:32.900
So you're not going to
produce your oxygen.

00:17:32.900 --> 00:17:37.360
So, in this situation--
so this non-cyclic

00:17:37.360 --> 00:17:39.620
phosphorylation, which
is kind of what most

00:17:39.620 --> 00:17:42.050
photophosphorylation, is what
most people associate with

00:17:42.050 --> 00:17:48.030
photophosphorylation, this
produces O2 and NADPH.

00:17:51.160 --> 00:17:53.470
And of course it produces ATP.

00:17:53.470 --> 00:17:56.320
While cyclic
photophosphorylation, because

00:17:56.320 --> 00:17:58.890
it doesn't have to strip
electrons off of water and the

00:17:58.890 --> 00:18:07.150
electrons don't end up at NADPH,
only produces ATP.

00:18:07.150 --> 00:18:10.600
So I think we now have a very
good understanding, hopefully,

00:18:10.600 --> 00:18:13.020
of the light reactions
in photosynthesis.

00:18:13.020 --> 00:18:15.190
We're now ready to take
the products of this.

00:18:15.190 --> 00:18:17.810
Now let's remember what
the products were.

00:18:17.810 --> 00:18:19.660
Well, the oxygen just
gets eliminated.

00:18:19.660 --> 00:18:21.140
We don't need the
oxygen anymore.

00:18:21.140 --> 00:18:22.960
But that goes into the
atmosphere and you and I can

00:18:22.960 --> 00:18:25.400
breathe that and we can use that
for cellular respiration.

00:18:25.400 --> 00:18:29.030
But in the photosynthesis
context, we've now generated a

00:18:29.030 --> 00:18:30.430
bunch of ATP.

00:18:30.430 --> 00:18:32.610
And now we have a
bunch of NADPH.

00:18:32.610 --> 00:18:35.750
And we can use that in
conjunction with carbon

00:18:35.750 --> 00:18:44.240
dioxide to produce actual
carbohydrates in the stroma.

00:18:44.240 --> 00:18:46.830
Outside the thylakoids, but
we're still inside of the

00:18:46.830 --> 00:18:50.500
chloroplast. And I'll cover
that in the next video.


Photosynthesis: Light Reactions 1

WEBVTT
Kind: captions
Language: en

00:00:00.380 --> 00:00:02.480
In the last video we learned
a little bit about

00:00:02.480 --> 00:00:04.130
photosynthesis.

00:00:04.130 --> 00:00:06.800
And we know in very general
terms, it's the process where

00:00:06.800 --> 00:00:12.580
we start off with photons and
water and carbon dioxide, and

00:00:12.580 --> 00:00:16.219
we use that energy in the
photons to fix the carbon.

00:00:16.219 --> 00:00:20.680
And now, this idea of carbon
fixation is essentially taking

00:00:20.680 --> 00:00:24.390
carbon in the gaseous form, in
this case carbon dioxide, and

00:00:24.390 --> 00:00:26.530
fixing it into a solid
structure.

00:00:26.530 --> 00:00:30.200
And that solid structure we fix
it into is a carbohydrate.

00:00:30.200 --> 00:00:33.390
The first end-product of
photosynthesis was this

00:00:33.390 --> 00:00:36.830
3-carbon chain, this
glyceraldehyde 3-phosphate.

00:00:36.830 --> 00:00:40.880
But then you can use that to
build up glucose or any other

00:00:40.880 --> 00:00:42.040
carbohydrate.

00:00:42.040 --> 00:00:45.630
So, with that said, let's try to
dig a little bit deeper and

00:00:45.630 --> 00:00:48.710
understand what's actually going
on in these stages of

00:00:48.710 --> 00:00:49.430
photosynthesis.

00:00:49.430 --> 00:00:50.920
Remember, we said there's
two stages.

00:00:50.920 --> 00:00:54.020
The light-dependent reactions
and then you have the light

00:00:54.020 --> 00:00:55.160
independent reactions.

00:00:55.160 --> 00:00:57.370
I don't like using the word
dark reaction because it

00:00:57.370 --> 00:00:59.600
actually occurs while
the sun is outside.

00:00:59.600 --> 00:01:01.100
It's actually occurring
simultaneously

00:01:01.100 --> 00:01:02.300
with the light reactions.

00:01:02.300 --> 00:01:05.160
It just doesn't need the
photons from the sun.

00:01:05.160 --> 00:01:08.270
But let's focus first on the
light-dependent reactions.

00:01:08.270 --> 00:01:12.660
The part that actually uses
photons from the sun.

00:01:12.660 --> 00:01:16.900
Or actually, I guess, even
photons from the heat lamp

00:01:16.900 --> 00:01:19.100
that you might have in
your greenhouse.

00:01:19.100 --> 00:01:23.500
And uses those photons in
conjunction with water to

00:01:23.500 --> 00:01:35.360
produce ATP and reduce
NADP plus to NADPH.

00:01:35.360 --> 00:01:40.070
Remember, reduction is gaining
electrons or hydrogen atoms.

00:01:40.070 --> 00:01:42.030
And it's the same thing,
because when you gain a

00:01:42.030 --> 00:01:45.270
hydrogen atom, including its
electron, since hydrogen is

00:01:45.270 --> 00:01:48.170
not too electronegative, you
get to hog its electron.

00:01:48.170 --> 00:01:51.820
So this is both gaining a
hydrogen and gaining electron.

00:01:51.820 --> 00:01:53.800
But let's study it a
little bit more.

00:01:53.800 --> 00:01:56.660
So before we dig a little
deeper, I think it's good to

00:01:56.660 --> 00:01:59.750
know a little bit about the
anatomy of a plant.

00:01:59.750 --> 00:02:03.510
So let me draw some
plant cells.

00:02:03.510 --> 00:02:06.530
So plant cells actually have
cell walls, so I can draw them

00:02:06.530 --> 00:02:08.310
a little bit rigid.

00:02:08.310 --> 00:02:11.830
So let's say that these are
plant cells right here.

00:02:11.830 --> 00:02:15.290
Each of these squares, each of
these quadrilaterals is a

00:02:15.290 --> 00:02:17.280
plant cell.

00:02:17.280 --> 00:02:19.890
And then in these plant cells
you have these organelles

00:02:19.890 --> 00:02:20.880
called chloroplasts.

00:02:20.880 --> 00:02:22.973
Remember organelles are
like organs of a cell.

00:02:22.973 --> 00:02:24.230
They are subunits,

00:02:24.230 --> 00:02:27.000
membrane-bound subunits of cells.

00:02:27.000 --> 00:02:30.300
And of course, these cells have
nucleuses and DNA and all

00:02:30.300 --> 00:02:32.440
of the other things you normally
associate with cells.

00:02:32.440 --> 00:02:33.440
But I'm not going to
draw them here.

00:02:33.440 --> 00:02:35.800
I'm just going to draw
the chloroplasts.

00:02:35.800 --> 00:02:40.970
And your average plant cell--
and there are other types of

00:02:40.970 --> 00:02:43.680
living organisms that perform
photosynthesis, but we'll

00:02:43.680 --> 00:02:44.590
focus on plants.

00:02:44.590 --> 00:02:47.950
Because that's what we tend
to associate it with.

00:02:47.950 --> 00:02:53.530
Each plant cell will contain
10 to 50 chloroplasts.

00:02:53.530 --> 00:02:56.370
I make them green on purpose
because the chloroplasts

00:02:56.370 --> 00:02:57.700
contain chlorophyll.

00:02:57.700 --> 00:02:59.560
Which to our eyes,
appear green.

00:02:59.560 --> 00:03:01.730
But remember, they're green
because they reflect green

00:03:01.730 --> 00:03:05.090
light and they absorb red
and blue and other

00:03:05.090 --> 00:03:06.040
wavelengths of light.

00:03:06.040 --> 00:03:06.860
That's why it looks green.

00:03:06.860 --> 00:03:07.600
Because it's reflecting.

00:03:07.600 --> 00:03:09.480
But it's absorbing all the
other wavelengths.

00:03:09.480 --> 00:03:11.590
But anyway, we'll talk more
about that in detail.

00:03:11.590 --> 00:03:16.090
But you'll have 10 to 50 of
these chloroplasts right here.

00:03:16.090 --> 00:03:19.930
And then let's zoom in on one
chloroplast. So if we zoom in

00:03:19.930 --> 00:03:23.320
on one chloroplast. So
let me be very clear.

00:03:23.320 --> 00:03:26.330
This thing right here
is a plant cell.

00:03:26.330 --> 00:03:29.880
That is a plant cell.

00:03:29.880 --> 00:03:32.420
And then each of these green
things right here is an

00:03:32.420 --> 00:03:38.540
organelle called the
chloroplast. And let's zoom in

00:03:38.540 --> 00:03:41.990
on the chloroplast itself.

00:03:41.990 --> 00:03:46.200
If we zoom in on one
chloroplast, it has a

00:03:46.200 --> 00:03:48.580
membrane like that.

00:03:52.840 --> 00:03:57.170
And then the fluid inside of the
chloroplast, inside of its

00:03:57.170 --> 00:03:59.610
membrane, so this fluid
right here.

00:03:59.610 --> 00:04:00.360
All of this fluid.

00:04:00.360 --> 00:04:02.430
That's called the stroma.

00:04:02.430 --> 00:04:06.000
The stroma of the chloroplast.
And then within the

00:04:06.000 --> 00:04:09.520
chloroplast itself, you have
these little stacks of these

00:04:09.520 --> 00:04:13.530
folded membranes, These
little folded stacks.

00:04:13.530 --> 00:04:16.459
Let me see if I can
do justice here.

00:04:16.459 --> 00:04:22.510
So maybe that's one, two,
doing these stacks.

00:04:22.510 --> 00:04:25.070
Each of these membrane-bound--
you can almost view them as

00:04:25.070 --> 00:04:28.570
pancakes-- let me draw
a couple more.

00:04:28.570 --> 00:04:31.970
Maybe we have some over here,
just so you-- maybe you have

00:04:31.970 --> 00:04:35.650
some over here, maybe
some over here.

00:04:35.650 --> 00:04:38.790
So each of these flattish
looking pancakes right here,

00:04:38.790 --> 00:04:40.220
these are called thylakoids.

00:04:40.220 --> 00:04:43.320
So this right here
is a thylakoid.

00:04:43.320 --> 00:04:44.570
That is a thylakoid.

00:04:48.000 --> 00:04:50.090
The thylakoid has a membrane.

00:04:50.090 --> 00:04:51.660
And this membrane is especially
important.

00:04:51.660 --> 00:04:53.460
We're going to zoom in
on that in a second.

00:04:53.460 --> 00:04:57.540
So it has a membrane, I'll color
that in a little bit.

00:04:57.540 --> 00:05:01.720
The inside of the thylakoid, so
the space, the fluid inside

00:05:01.720 --> 00:05:04.620
of the thylakoid, right
there that area.

00:05:04.620 --> 00:05:06.220
This light green color
right there.

00:05:06.220 --> 00:05:10.360
That's called the thylakoid
space or the thylakoid lumen.

00:05:12.960 --> 00:05:16.530
And just to get all of our
terminology out of the way, a

00:05:16.530 --> 00:05:22.090
stack of several thylakoids
just like that,

00:05:22.090 --> 00:05:24.630
that is called a grana.

00:05:24.630 --> 00:05:27.020
That's a stack of thylakoids.

00:05:27.020 --> 00:05:29.840
That is a grana.

00:05:29.840 --> 00:05:31.190
And this is an organelle.

00:05:31.190 --> 00:05:35.170
And evolutionary biologists,
they believe that organelles

00:05:35.170 --> 00:05:38.250
were once independent
organisms that then,

00:05:38.250 --> 00:05:41.090
essentially, teamed up with
other organisms and started

00:05:41.090 --> 00:05:42.350
living inside of their cells.

00:05:42.350 --> 00:05:45.450
So there's actually, they
have their own DNA.

00:05:45.450 --> 00:05:48.460
So mitochondria is another
example of an organelle that

00:05:48.460 --> 00:05:51.480
people believe that one time
mitochondria, or the ancestors

00:05:51.480 --> 00:05:53.960
of mitochondria, were
independent organisms. That

00:05:53.960 --> 00:05:56.320
then teamed up with other cells
and said, hey, if I

00:05:56.320 --> 00:05:58.230
produce your energy maybe
you'll give me

00:05:58.230 --> 00:06:00.510
some food or whatnot.

00:06:00.510 --> 00:06:02.150
And so they started
evolving together.

00:06:02.150 --> 00:06:03.740
And they turned into
one organism.

00:06:03.740 --> 00:06:06.460
Which makes you wonder what we
might evolve-- well anyway,

00:06:06.460 --> 00:06:07.330
that's a separate thing.

00:06:07.330 --> 00:06:09.510
So there's actually ribosomes
out here.

00:06:09.510 --> 00:06:10.450
That's good to think about.

00:06:10.450 --> 00:06:16.280
Just realize that at one point
in the evolutionary past, this

00:06:16.280 --> 00:06:18.510
organelle's ancestor
might have been

00:06:18.510 --> 00:06:20.370
an independent organism.

00:06:20.370 --> 00:06:22.690
But anyway, enough about
that speculation.

00:06:22.690 --> 00:06:29.760
Let's zoom in again on one of
these thylakoid membranes.

00:06:29.760 --> 00:06:31.010
So I'm going to zoom in.

00:06:31.010 --> 00:06:32.260
Let me make a box.

00:06:34.570 --> 00:06:37.070
Let me zoom in right there.

00:06:37.070 --> 00:06:38.990
So that's going to be
my zoom-in box.

00:06:38.990 --> 00:06:41.770
So let me make it really big.

00:06:41.770 --> 00:06:42.340
Just like this.

00:06:42.340 --> 00:06:45.560
So this is my zoom-in box.

00:06:45.560 --> 00:06:49.190
So that little box is the same
thing as this whole box.

00:06:49.190 --> 00:06:53.180
So we're zoomed in on the
thylakoid membrane.

00:06:53.180 --> 00:06:57.510
So this is the thylakoid
membrane right there.

00:06:57.510 --> 00:06:59.810
That's actually a phospho-bilipd
layer.

00:06:59.810 --> 00:07:04.020
It has your hydrophilic,
hydrophobic tails.

00:07:04.020 --> 00:07:05.660
I mean, I could draw it
like that if you like.

00:07:05.660 --> 00:07:07.810
The important thing from the
photosynthesis point of view

00:07:07.810 --> 00:07:09.550
is that it's this membrane.

00:07:09.550 --> 00:07:13.290
And on the outside of the
membrane, right here on the

00:07:13.290 --> 00:07:16.940
outside, you have the fluid
that fills up the entire

00:07:16.940 --> 00:07:20.540
chloroplast. So here you
have the stroma.

00:07:20.540 --> 00:07:23.210
And then this space right here,
this is the inside of

00:07:23.210 --> 00:07:24.620
your thylakoid.

00:07:24.620 --> 00:07:25.780
So this is the lumen.

00:07:25.780 --> 00:07:27.880
So if I were to color it
pink, right there.

00:07:27.880 --> 00:07:29.020
This is your lumen.

00:07:29.020 --> 00:07:30.780
Your thylakoid space.

00:07:30.780 --> 00:07:33.390
And in this membrane, and this
might look a little bit

00:07:33.390 --> 00:07:36.190
familiar if you think about
mitochondria and the electron

00:07:36.190 --> 00:07:37.040
transport chain.

00:07:37.040 --> 00:07:39.940
What I'm going to describe in
this video actually is an

00:07:39.940 --> 00:07:41.770
electron transport chain.

00:07:41.770 --> 00:07:44.260
Many people might not consider
it the electron transport

00:07:44.260 --> 00:07:46.610
chain, but it's the same idea.

00:07:46.610 --> 00:07:48.660
Same general idea.

00:07:48.660 --> 00:07:51.280
So on this membrane you have
these proteins and these

00:07:51.280 --> 00:07:53.235
complexes of proteins
and molecules

00:07:53.235 --> 00:07:55.470
that span this membrane.

00:07:55.470 --> 00:07:58.140
So let me draw a
couple of them.

00:07:58.140 --> 00:08:01.596
So maybe I'll call this
one, photosystem II.

00:08:01.596 --> 00:08:04.500
And I'm calling it that because
that's what it is.

00:08:04.500 --> 00:08:05.820
Photosystem II.

00:08:05.820 --> 00:08:07.310
You have maybe another
complex.

00:08:07.310 --> 00:08:08.570
And these are hugely
complicated.

00:08:08.570 --> 00:08:11.040
I'll do a sneak peek of
what photosystem II

00:08:11.040 --> 00:08:12.670
actually looks like.

00:08:12.670 --> 00:08:15.180
This is actually what
photosystem II looks like.

00:08:15.180 --> 00:08:17.890
So, as you can see, it
truly is a complex.

00:08:17.890 --> 00:08:20.460
These cylindrical things,
these are proteins.

00:08:20.460 --> 00:08:23.580
These green things are
chlorophyll molecules.

00:08:23.580 --> 00:08:25.020
I mean, there's all sorts
of things going here.

00:08:25.020 --> 00:08:27.090
And they're all jumbled
together.

00:08:27.090 --> 00:08:29.300
I think a complex probably
is the best word.

00:08:29.300 --> 00:08:32.320
It's a bunch of proteins, a
bunch of molecules just

00:08:32.320 --> 00:08:35.460
jumbled together to perform a
very particular function.

00:08:35.460 --> 00:08:38.440
We're going to describe
that in a few seconds.

00:08:38.440 --> 00:08:40.580
So that's what photosystem
II looks like.

00:08:40.580 --> 00:08:43.850
Then you also have
photosystem I.

00:08:43.850 --> 00:08:47.510
And then you have other
molecules, other complexes.

00:08:47.510 --> 00:08:52.130
You have the cytochrome B6F
complex and I'll draw this in

00:08:52.130 --> 00:08:55.120
a different color right here.

00:08:55.120 --> 00:08:57.340
I don't want to get too
much into the weeds.

00:08:57.340 --> 00:08:59.030
Because the most important thing
is just to understand.

00:08:59.030 --> 00:09:02.790
So you have other protein
complexes, protein molecular

00:09:02.790 --> 00:09:06.150
complexes here that also
span the membrane.

00:09:06.150 --> 00:09:08.840
But the general idea-- I'll tell
you the general idea and

00:09:08.840 --> 00:09:11.850
then we'll go into the
specifics-- of what happens

00:09:11.850 --> 00:09:23.360
during the light reaction, or
the light dependent reaction,

00:09:23.360 --> 00:09:25.950
is you have some photons.

00:09:25.950 --> 00:09:27.150
Photons from the sun.

00:09:27.150 --> 00:09:29.470
They've traveled 93
million miles.

00:09:29.470 --> 00:09:34.320
so you have some photons that
go here and they excite

00:09:34.320 --> 00:09:37.950
electrons in a chlorophyll
molecule, in a

00:09:37.950 --> 00:09:39.770
chlorophyll A molecule.

00:09:39.770 --> 00:09:42.920
And actually in photosystem II--
well, I won't go into the

00:09:42.920 --> 00:09:45.720
details just yet-- but they
excite a chlorophyll molecule

00:09:45.720 --> 00:09:49.060
so those electrons enter into
a high energy state.

00:09:49.060 --> 00:09:50.160
Maybe I shouldn't draw
it like that.

00:09:50.160 --> 00:09:51.690
They enter into a high
energy state.

00:09:51.690 --> 00:09:56.080
And then as they go from
molecule to molecule they keep

00:09:56.080 --> 00:09:57.760
going down in energy state.

00:09:57.760 --> 00:10:01.780
But as they go down in energy
state, you have hydrogen

00:10:01.780 --> 00:10:04.470
atoms, or actually I should say
hydrogen protons without

00:10:04.470 --> 00:10:05.070
the electrons.

00:10:05.070 --> 00:10:08.380
So you have all of these
hydrogen protons.

00:10:08.380 --> 00:10:12.580
Hydrogen protons get pumped
into the lumen.

00:10:12.580 --> 00:10:15.730
They get pumped into the lumen
and so you might remember this

00:10:15.730 --> 00:10:17.210
from the electron
transport chain.

00:10:17.210 --> 00:10:19.570
In the electron transport chain,
as electrons went from

00:10:19.570 --> 00:10:24.160
a high potential, a high energy
state, to a low energy

00:10:24.160 --> 00:10:27.110
state, that energy
was used to pump

00:10:27.110 --> 00:10:29.240
hydrogens through a membrane.

00:10:29.240 --> 00:10:33.290
And in that case it was in the
mitochondria, here the

00:10:33.290 --> 00:10:35.750
membrane is the thylakoid
membrane.

00:10:35.750 --> 00:10:39.750
But either case, you're creating
this gradient where--

00:10:39.750 --> 00:10:43.650
because of the energy from,
essentially the photons-- the

00:10:43.650 --> 00:10:46.330
electrons enter a high energy
state, they keep going into a

00:10:46.330 --> 00:10:47.370
lower energy state.

00:10:47.370 --> 00:10:49.870
And then they actually go to
photosystem I and they get hit

00:10:49.870 --> 00:10:50.910
by another photon.

00:10:50.910 --> 00:10:52.640
Well, that's a simplification,
but that's how you

00:10:52.640 --> 00:10:53.190
can think of it.

00:10:53.190 --> 00:10:55.350
Enter another high energy
state, then they go to a

00:10:55.350 --> 00:10:56.905
lower, lower and lower
energy state.

00:10:56.905 --> 00:10:59.540
But the whole time, that energy
from the electrons

00:10:59.540 --> 00:11:02.760
going from a high energy state
to a low energy state is used

00:11:02.760 --> 00:11:07.120
to pump hydrogen protons
into the lumen.

00:11:07.120 --> 00:11:08.400
So you have this huge

00:11:08.400 --> 00:11:11.440
concentration of hydrogen protons.

00:11:11.440 --> 00:11:17.680
And just like what we saw in the
electron transport chain,

00:11:17.680 --> 00:11:21.980
that concentration is then-- of
hydrogen protons-- is then

00:11:21.980 --> 00:11:24.970
used to drive ATP synthase.

00:11:24.970 --> 00:11:28.320
So the exact same-- let me see
if I can draw that ATP

00:11:28.320 --> 00:11:29.310
synthase here.

00:11:29.310 --> 00:11:30.880
You might remember ATP
synthase looks

00:11:30.880 --> 00:11:33.160
something like this.

00:11:33.160 --> 00:11:36.500
Where literally, so here you
have a huge concentration of

00:11:36.500 --> 00:11:37.850
hydrogen protons.

00:11:37.850 --> 00:11:41.010
So they'll want to
go back into the

00:11:41.010 --> 00:11:42.230
stroma from the lumen.

00:11:42.230 --> 00:11:42.900
And they do.

00:11:42.900 --> 00:11:45.670
And they go through
the ATP synthase.

00:11:45.670 --> 00:11:47.200
Let me do it in a new color.

00:11:47.200 --> 00:11:51.820
So these hydrogen protons are
going to make their way back.

00:11:51.820 --> 00:11:53.210
Go back down the gradient.

00:11:53.210 --> 00:11:55.900
And as they go down the
gradient, they literally--

00:11:55.900 --> 00:11:56.710
it's like an engine.

00:11:56.710 --> 00:11:59.710
And I go into detail on this
when I talk about respiration.

00:11:59.710 --> 00:12:04.020
And that turns, literally
mechanically turns, this top

00:12:04.020 --> 00:12:06.850
part-- the way I drew it--
of the ATP synthase.

00:12:06.850 --> 00:12:11.760
And it puts ADP and phosphate
groups together.

00:12:11.760 --> 00:12:15.560
It puts ADP plus phosphate
groups

00:12:15.560 --> 00:12:20.260
together to produce ATP.

00:12:20.260 --> 00:12:22.740
So that's the general,
very high overview.

00:12:22.740 --> 00:12:25.110
And I'm going to go into more
detail in a second.

00:12:25.110 --> 00:12:29.500
But this process that I just
described is called

00:12:29.500 --> 00:12:32.200
photophosphorylation.

00:12:32.200 --> 00:12:33.450
Let me do it in a nice color.

00:12:40.440 --> 00:12:41.850
Why is it called that?

00:12:41.850 --> 00:12:43.390
Well, because we're
using photons.

00:12:43.390 --> 00:12:44.460
That's the photo part.

00:12:44.460 --> 00:12:45.270
We're using light.

00:12:45.270 --> 00:12:49.190
We're using photons to excite
electrons in chlorophyll.

00:12:49.190 --> 00:12:52.430
As those electrons get passed
from one molecule, from one

00:12:52.430 --> 00:12:55.960
electron acceptor to another,
they enter into lower and

00:12:55.960 --> 00:12:57.480
lower energy states.

00:12:57.480 --> 00:13:01.170
As they go into lower energy
states, that's used to drive,

00:13:01.170 --> 00:13:05.270
literally, pumps that allow
hydrogen protons to go from

00:13:05.270 --> 00:13:07.240
the stroma to the lumen.

00:13:07.240 --> 00:13:09.900
Then the hydrogen protons
want to go back.

00:13:09.900 --> 00:13:13.100
They want to-- I guess you could
call it-- chemiosmosis.

00:13:13.100 --> 00:13:16.770
They want to go back into the
stroma and then that drives

00:13:16.770 --> 00:13:19.180
ATP synthase.

00:13:19.180 --> 00:13:21.940
Right here, this is
ATP synthase.

00:13:21.940 --> 00:13:26.270
ATP synthase to essentially
jam together ADPs and

00:13:26.270 --> 00:13:29.810
phosphate groups
to produce ATP.

00:13:29.810 --> 00:13:34.450
Now, when I originally talked
about the light reactions and

00:13:34.450 --> 00:13:37.010
dark reactions I said, well the
light reactions have two

00:13:37.010 --> 00:13:38.050
byproducts.

00:13:38.050 --> 00:13:42.770
It has ATP and it also has--
actually it has three.

00:13:42.770 --> 00:13:46.620
It has ATP, and it
also has NADPH.

00:13:46.620 --> 00:13:49.340
NADP is reduced.

00:13:49.340 --> 00:13:51.800
It gains these electrons
and these hydrogens.

00:13:51.800 --> 00:13:53.060
So where does that show up?

00:13:53.060 --> 00:13:56.670
Well, if we're talking about
non-cyclic oxidative

00:13:56.670 --> 00:14:00.300
photophosphorylation, or
non-cyclic light reactions,

00:14:00.300 --> 00:14:01.960
the final electron acceptor.

00:14:01.960 --> 00:14:05.330
So after that electron keeps
entering lower and lower

00:14:05.330 --> 00:14:07.445
energy states, the
final electron

00:14:07.445 --> 00:14:12.090
acceptor is NADP plus.

00:14:12.090 --> 00:14:14.340
So once it accepts the electrons
and a hydrogen

00:14:14.340 --> 00:14:17.450
proton with it, it
becomes NADPH.

00:14:20.480 --> 00:14:25.590
Now, I also said that part of
this process, water-- and this

00:14:25.590 --> 00:14:30.010
is actually a very interesting
thing-- water gets oxidized to

00:14:30.010 --> 00:14:31.270
molecular oxygen.

00:14:31.270 --> 00:14:32.980
So where does that happen?

00:14:32.980 --> 00:14:37.060
So when I said, up here in
photosystem I, that we have a

00:14:37.060 --> 00:14:43.220
chlorophyll molecule that has
an electron excited, and it

00:14:43.220 --> 00:14:44.510
goes into a higher
energy state.

00:14:44.510 --> 00:14:46.410
And then that electron
essentially gets passed from

00:14:46.410 --> 00:14:50.470
one guy to the next, that begs
the question, what can we use

00:14:50.470 --> 00:14:52.550
to replace that electron?

00:14:52.550 --> 00:14:56.340
And it turns out that we use,
we literally use, the

00:14:56.340 --> 00:14:57.690
electrons in water.

00:14:57.690 --> 00:15:03.870
So over here you literally
have H2O.

00:15:03.870 --> 00:15:08.000
And H2O donates the hydrogens
and the electrons with it.

00:15:08.000 --> 00:15:10.860
So you can kind of imagine it
donates two hydrogen protons

00:15:10.860 --> 00:15:14.930
and two electrons to replace the
electron that got excited

00:15:14.930 --> 00:15:15.860
by the photons.

00:15:15.860 --> 00:15:17.780
Because that electron got passed
all the way over to

00:15:17.780 --> 00:15:21.010
photosystem I and eventually
ends up in NADPH.

00:15:21.010 --> 00:15:26.630
So, you're literally stripping
electrons off of water.

00:15:26.630 --> 00:15:28.650
And when you strip off the
electrons and the hydrogens,

00:15:28.650 --> 00:15:30.470
you're just left with
molecular oxygen.

00:15:30.470 --> 00:15:33.300
Now, the reason why I want to
really focus on this is that

00:15:33.300 --> 00:15:34.970
there's something profound
happening here.

00:15:34.970 --> 00:15:36.500
Or at least on a chemistry
level,

00:15:36.500 --> 00:15:37.600
something profound is happening.

00:15:37.600 --> 00:15:43.510
You're oxidizing water.

00:15:43.510 --> 00:15:46.480
And in the entire biological
kingdom, the only place where

00:15:46.480 --> 00:15:49.290
we know something that is strong
enough of an oxidizing

00:15:49.290 --> 00:15:52.400
agent to oxidize water, to
literally take away electrons

00:15:52.400 --> 00:15:53.210
from water.

00:15:53.210 --> 00:15:54.640
Which means you're
really taking

00:15:54.640 --> 00:15:56.680
electrons away from oxygen.

00:15:56.680 --> 00:15:58.830
So you're oxidizing oxygen.

00:15:58.830 --> 00:16:02.120
The only place that we know
that an oxidation agent is

00:16:02.120 --> 00:16:06.560
strong enough to do this
is in photosystem II.

00:16:06.560 --> 00:16:09.560
So it's a very profound idea,
that normally electrons are

00:16:09.560 --> 00:16:10.960
very happy in water.

00:16:10.960 --> 00:16:14.660
They're very happy circulating
around oxygens.

00:16:14.660 --> 00:16:17.310
Oxygen is a very electronegative
atom.

00:16:17.310 --> 00:16:20.830
That's why we even call it
oxidizing, because oxygen is

00:16:20.830 --> 00:16:23.230
very good at oxidizing things.

00:16:23.230 --> 00:16:25.360
But all of a sudden we've
found something that can

00:16:25.360 --> 00:16:29.480
oxidize oxygen, that can strip
electrons off of oxygen and

00:16:29.480 --> 00:16:32.680
then give those electrons
to the chlorophyll.

00:16:32.680 --> 00:16:35.140
The electron gets excited
by photons.

00:16:35.140 --> 00:16:36.980
Then those photons enter
lower and lower and

00:16:36.980 --> 00:16:37.940
lower energy states.

00:16:37.940 --> 00:16:40.245
Get excited again in photosystem
I by another set

00:16:40.245 --> 00:16:42.870
of photons and then enter lower
and lower and lower

00:16:42.870 --> 00:16:43.750
energy states.

00:16:43.750 --> 00:16:47.500
And then finally,
end up at NADPH.

00:16:47.500 --> 00:16:50.140
And the whole time it entered
lower and lower energy states,

00:16:50.140 --> 00:16:54.170
that energy was being used to
pump hydrogen across this

00:16:54.170 --> 00:16:56.350
membrane from the
stroma to lumen.

00:16:56.350 --> 00:17:01.250
And then that gradient is used
to actually produce ATP.

00:17:01.250 --> 00:17:03.500
So in the next video I'm going
to give a little bit more

00:17:03.500 --> 00:17:07.050
context about what this means
in terms of energy states of

00:17:07.050 --> 00:17:11.040
electrons and what's at a higher
or lower energy state.

00:17:11.040 --> 00:17:13.200
But this is essentially
all that's happening.

00:17:13.200 --> 00:17:14.450
Electrons get excited.

00:17:17.119 --> 00:17:19.859
Those electrons eventually
end up at NADPH.

00:17:19.859 --> 00:17:23.650
And as the electron gets excited
and goes into lower

00:17:23.650 --> 00:17:25.160
and lower energy states,
it pumps

00:17:25.160 --> 00:17:27.290
hydrogen across the gradient.

00:17:27.290 --> 00:17:30.900
And then that gradient is used
to drive ATP synthase, to

00:17:30.900 --> 00:17:32.110
generate ATP.

00:17:32.110 --> 00:17:34.270
And then that original electron
that got excited, it

00:17:34.270 --> 00:17:35.480
had to be replaced.

00:17:35.480 --> 00:17:39.200
And that replaced electron is
actually stripped off of H2O.

00:17:39.200 --> 00:17:42.680
So the hydrogen protons and
the electrons of H2O are

00:17:42.680 --> 00:17:46.300
stripped away and you're just
left with molecular oxygen.

00:17:46.300 --> 00:17:49.070
And just to get a nice
appreciation of the complexity

00:17:49.070 --> 00:17:51.970
of all of this-- I showed you
this earlier in the video--

00:17:51.970 --> 00:17:55.250
but this is literally a-- I mean
this isn't a picture of

00:17:55.250 --> 00:17:56.010
photosystem II.

00:17:56.010 --> 00:17:57.830
You actually don't have
cylinders like this.

00:17:57.830 --> 00:18:01.110
But these cylinders represent
proteins.

00:18:01.110 --> 00:18:07.490
Right here, these green kind
of scaffold-like molecules,

00:18:07.490 --> 00:18:09.390
that's chlorophyll A.

00:18:09.390 --> 00:18:15.390
And what literally happens, is
you have photons hitting--

00:18:15.390 --> 00:18:17.535
actually it doesn't always have
to hit chlorophyll A.

00:18:17.535 --> 00:18:20.290
It can also hit what's called
antenna molecules.

00:18:20.290 --> 00:18:22.550
So antenna molecules are other
types of chlorophyll, and

00:18:22.550 --> 00:18:24.980
actually other types
of molecules.

00:18:24.980 --> 00:18:31.510
And so a photon, or a set of
photons, comes here and maybe

00:18:31.510 --> 00:18:35.440
it excites some electrons,
it doesn't have to be in

00:18:35.440 --> 00:18:35.870
chlorophyll A.

00:18:35.870 --> 00:18:38.220
It could be in some of these
other types of chlorophyll.

00:18:38.220 --> 00:18:41.200
Or in some of these other I
guess you could call them,

00:18:41.200 --> 00:18:44.670
pigment molecules that will
absorb these photons.

00:18:44.670 --> 00:18:46.500
And then their electrons
get excited.

00:18:46.500 --> 00:18:48.690
And you can almost imagine
it as a vibration.

00:18:48.690 --> 00:18:50.870
But when you're talking about
things on the quantum level,

00:18:50.870 --> 00:18:52.230
vibrations really don't
make sense.

00:18:52.230 --> 00:18:53.410
But it's a good analogy.

00:18:53.410 --> 00:18:55.970
They kind of vibrate their
way to chlorophyll A.

00:18:55.970 --> 00:18:58.535
And this is called
resonance energy.

00:19:04.660 --> 00:19:07.440
They vibrate their way,
eventually, to chlorophyll A.

00:19:07.440 --> 00:19:11.950
And then in chlorophyll A, you
have the electron get excited.

00:19:11.950 --> 00:19:15.620
The primary electron acceptor
is actually this molecule

00:19:15.620 --> 00:19:16.460
right here.

00:19:16.460 --> 00:19:17.470
Pheophytin.

00:19:17.470 --> 00:19:19.100
Some people call it pheo.

00:19:19.100 --> 00:19:22.380
And then from there, it keeps
getting passed on from one

00:19:22.380 --> 00:19:23.200
molecule to another.

00:19:23.200 --> 00:19:25.690
I'll talk a little bit more
about that in the next video.

00:19:25.690 --> 00:19:26.730
But this is fascinating.

00:19:26.730 --> 00:19:28.390
Look how complicated this is.

00:19:28.390 --> 00:19:32.210
In order to essentially excite
electrons and then use those

00:19:32.210 --> 00:19:36.060
electrons to start the
process of pumping

00:19:36.060 --> 00:19:37.200
hydrogens across a membrane.

00:19:37.200 --> 00:19:39.300
And this is an interesting
place right here.

00:19:39.300 --> 00:19:40.950
This is the water
oxidation site.

00:19:40.950 --> 00:19:44.370
So I got very excited about the
idea of oxidizing water.

00:19:44.370 --> 00:19:48.140
And so this is actually where
it occurs in the photosystem

00:19:48.140 --> 00:19:49.240
II complex.

00:19:49.240 --> 00:19:51.220
And you actually have this very
complicated mechanism.

00:19:51.220 --> 00:19:54.880
Because it's no joke to actually
strip away electrons

00:19:54.880 --> 00:19:58.110
and hydrogens from an actual
water molecule.

00:19:58.110 --> 00:19:59.010
I'll leave you there.

00:19:59.010 --> 00:20:01.870
And in the next video I'll talk
a little bit more about

00:20:01.870 --> 00:20:02.890
these energy states.

00:20:02.890 --> 00:20:05.380
And I'll fill in a little bit of
the gaps about what some of

00:20:05.380 --> 00:20:10.990
these other molecules that act
as hydrogen acceptors.

00:20:10.990 --> 00:20:12.470
Or you can also view
them as electron

00:20:12.470 --> 00:20:15.350
acceptors along the way.


Photosynthesis

WEBVTT
Kind: captions
Language: en

00:00:00.600 --> 00:00:01.990
Let's talk about
one of the most

00:00:01.990 --> 00:00:04.560
important biological processes.

00:00:04.560 --> 00:00:07.220
Frankly, if this process didn't
occur, we probably

00:00:07.220 --> 00:00:09.470
wouldn't have life on Earth, and
I wouldn't be making this

00:00:09.470 --> 00:00:11.900
video for you, because there'd
be no place for me to

00:00:11.900 --> 00:00:13.360
actually get food.

00:00:13.360 --> 00:00:15.185
And the process is called
photosynthesis.

00:00:20.670 --> 00:00:23.410
And you're probably reasonably
familiar with the idea.

00:00:23.410 --> 00:00:28.090
The whole idea is plants, and
actually bacteria and algae

00:00:28.090 --> 00:00:29.830
and other things,
but we normally

00:00:29.830 --> 00:00:31.430
associate it with plants.

00:00:31.430 --> 00:00:33.600
Let me make it in very simple
terms. So we normally

00:00:33.600 --> 00:00:36.190
associate it with plants.

00:00:36.190 --> 00:00:38.520
And it's a process that plants
use, and we might have learned

00:00:38.520 --> 00:00:39.730
this when we were very young.

00:00:39.730 --> 00:00:43.580
It's the process that plants
use to take carbon dioxide

00:00:43.580 --> 00:00:56.490
plus some water plus some
sunlight and turn it into some

00:00:56.490 --> 00:01:00.010
sugars or some maybe
carbohydrates.

00:01:00.010 --> 00:01:04.150
Carbohydrates or sugars
plus oxygen.

00:01:04.150 --> 00:01:09.360
Obviously, this has two very
profound pieces to it for us

00:01:09.360 --> 00:01:10.790
as a living species.

00:01:10.790 --> 00:01:14.200
One, we need carbohydrates or
we need sugars in order to

00:01:14.200 --> 00:01:15.240
fuel our bodies.

00:01:15.240 --> 00:01:17.680
You saw that in the cellular
respiration videos.

00:01:17.680 --> 00:01:21.540
We generate all of our ATP
by performing cellular

00:01:21.540 --> 00:01:24.990
respiration on glucose, which is
essentially a byproduct, or

00:01:24.990 --> 00:01:27.015
a broken down carbohydrate.

00:01:27.015 --> 00:01:30.530
It's the simplest one for us
to process in cellular

00:01:30.530 --> 00:01:31.250
respiration.

00:01:31.250 --> 00:01:33.240
And the second hugely
important part

00:01:33.240 --> 00:01:34.330
is getting the oxygen.

00:01:34.330 --> 00:01:38.490
Once again, we need to breathe
oxygen in order for us to

00:01:38.490 --> 00:01:42.500
break down glucose, in order
to respire, in order to

00:01:42.500 --> 00:01:44.100
perform cellar respiration.

00:01:44.100 --> 00:01:50.740
So these two things are key for
life, especially for life

00:01:50.740 --> 00:01:51.990
that breathes oxygen.

00:01:54.360 --> 00:01:56.220
So this process, other than
the fact that it's

00:01:56.220 --> 00:02:01.200
interesting, that there are
organisms around us, mostly

00:02:01.200 --> 00:02:05.130
plants, that are able to harness
actual sunlight.

00:02:05.130 --> 00:02:07.900
You have these fusion reactions
in the sun 93

00:02:07.900 --> 00:02:11.110
million miles away, and it's
releasing these photons, and

00:02:11.110 --> 00:02:13.570
some small subset of
those photons reach

00:02:13.570 --> 00:02:14.790
the surface of Earth.

00:02:14.790 --> 00:02:17.690
They make their way through
clouds and whatever else.

00:02:17.690 --> 00:02:21.560
And then these plants and
bacteria and algae are able to

00:02:21.560 --> 00:02:25.010
harness that somehow and turn
them into sugars that we can

00:02:25.010 --> 00:02:29.120
then eat or maybe the cow eats
them and we eat the cow if

00:02:29.120 --> 00:02:33.280
we're not vegetarians, and we
can then use that for energy.

00:02:33.280 --> 00:02:35.960
Not that the cow is all
carbohydrates, but this is

00:02:35.960 --> 00:02:39.200
essentially what is used as the
fuel or the energy for all

00:02:39.200 --> 00:02:41.365
of the other important compounds
that we eat.

00:02:41.365 --> 00:02:43.930
This is where we get
all of our fuel.

00:02:43.930 --> 00:02:45.450
So this is fuel for animals.

00:02:48.250 --> 00:02:50.620
Or you know, if you eat a
potato directly, you are

00:02:50.620 --> 00:02:52.380
directly getting your
carbohydrates.

00:02:52.380 --> 00:02:55.160
But anyway, this is a very
simple notion of

00:02:55.160 --> 00:02:56.760
photosynthesis, but it's
not incorrect.

00:02:56.760 --> 00:02:58.550
I mean, if you had to know one
thing about photosynthesis,

00:02:58.550 --> 00:03:00.290
this would be it.

00:03:00.290 --> 00:03:03.640
But let's delve a little bit
deeper and try to get into the

00:03:03.640 --> 00:03:05.930
guts of it and see if we can
understand a little bit better

00:03:05.930 --> 00:03:06.970
how this actually happens.

00:03:06.970 --> 00:03:10.780
I find it amazing that somehow
photons of sunlight are used

00:03:10.780 --> 00:03:13.540
to create these sugar
molecules or these

00:03:13.540 --> 00:03:14.226
carbohydrates.

00:03:14.226 --> 00:03:16.850
So let's delve a little
bit deeper.

00:03:16.850 --> 00:03:21.350
So we can write the general
equation for photosynthesis.

00:03:21.350 --> 00:03:23.530
Well, I've almost
written it here.

00:03:23.530 --> 00:03:27.720
But I'll write it a little bit
more scientifically specific.

00:03:27.720 --> 00:03:30.640
You start off with some
carbon dioxide.

00:03:30.640 --> 00:03:34.890
You add to that some water, and
you add to that-- instead

00:03:34.890 --> 00:03:38.030
of sunlight, I'm going to say
photons because these are what

00:03:38.030 --> 00:03:42.070
really do excite the electrons
in the chlorophyll that go

00:03:42.070 --> 00:03:44.970
down, and you'll see this
process probably in this

00:03:44.970 --> 00:03:47.530
video, and we'll go in more
detail in the next few videos.

00:03:47.530 --> 00:03:50.100
But that excited electron goes
to a high energy state, and as

00:03:50.100 --> 00:03:52.290
it goes to a lower energy state,
we're able to harness

00:03:52.290 --> 00:03:57.180
that energy to produce ATPs,
and you'll see NADPHs, and

00:03:57.180 --> 00:03:59.180
those are used to produce
carbohydrates.

00:03:59.180 --> 00:04:01.080
But we'll see that
in a little bit.

00:04:01.080 --> 00:04:03.590
But the overview of
photosynthesis, you start off

00:04:03.590 --> 00:04:10.300
with these constituents, And
then you end up with a

00:04:10.300 --> 00:04:11.210
carbohydrate.

00:04:11.210 --> 00:04:12.610
And a carbohydrate
could be glucose,

00:04:12.610 --> 00:04:14.080
doesn't have to be glucose.

00:04:14.080 --> 00:04:18.110
So the general way we can write
a carbohydrates is CH2O.

00:04:20.670 --> 00:04:24.210
And we'll put an n over here,
that we could have n multiples

00:04:24.210 --> 00:04:27.140
of these, and normally, n
will be at least three.

00:04:27.140 --> 00:04:29.990
In the case of glucose,
n is 6.

00:04:29.990 --> 00:04:34.420
You have 6 carbons, 12 hydrogens
and 6 oxygens.

00:04:34.420 --> 00:04:36.610
So this is a general term for
carbohydrates, but you could

00:04:36.610 --> 00:04:38.100
have many multiples of that.

00:04:38.100 --> 00:04:40.810
You could have these
long-chained carbohydrates, so

00:04:40.810 --> 00:04:42.620
you end up with a carbohydrate
and then you end

00:04:42.620 --> 00:04:45.920
up with some oxygen.

00:04:45.920 --> 00:04:48.730
So this right here isn't so
different than what I wrote up

00:04:48.730 --> 00:04:52.100
here in my first overview of
how we always imagined

00:04:52.100 --> 00:04:53.720
photosynthesis in our heads.

00:04:53.720 --> 00:04:56.190
In order to make this equation
balance-- let's see, I have n

00:04:56.190 --> 00:04:59.780
carbons so I need
n carbons there.

00:04:59.780 --> 00:05:02.860
Let's see, I have two
n hydrogens here.

00:05:02.860 --> 00:05:05.470
Two hydrogens and I have n
there, so I need two n

00:05:05.470 --> 00:05:06.020
hydrogens here.

00:05:06.020 --> 00:05:07.150
So I'll put an n out there.

00:05:07.150 --> 00:05:10.400
And lets see how many oxygens.

00:05:10.400 --> 00:05:12.635
I have two n oxygens,
plus another n, so I

00:05:12.635 --> 00:05:13.880
have three n oxygens.

00:05:13.880 --> 00:05:18.455
So let's see, I have one n,
and you put an n here, and

00:05:18.455 --> 00:05:21.890
then I have two n, and I think
this equation balances out.

00:05:21.890 --> 00:05:26.850
So this is a 30,000-foot view
of what's going on in

00:05:26.850 --> 00:05:27.760
photosynthesis.

00:05:27.760 --> 00:05:29.730
But when you dig a little
deeper, you'll see that this

00:05:29.730 --> 00:05:32.900
doesn't happen directly, that
this happens through a bunch

00:05:32.900 --> 00:05:37.270
of steps that eventually gets
us to the carbohydrate.

00:05:37.270 --> 00:05:40.850
So in general, we can break
down photosynthesis.

00:05:40.850 --> 00:05:42.440
I'll rewrite the word.

00:05:42.440 --> 00:05:47.830
We can break down
photosynthesis-- and we'll

00:05:47.830 --> 00:05:49.960
delve deeper into future videos,
but I want to get you

00:05:49.960 --> 00:05:54.220
the overview first--
into two stages.

00:05:54.220 --> 00:05:56.080
We can call one the
light reactions.

00:05:58.860 --> 00:06:01.900
Or sometimes they are called the
light-dependent reactions,

00:06:01.900 --> 00:06:04.010
and that actually would
probably be a better

00:06:04.010 --> 00:06:04.830
way to write it.

00:06:04.830 --> 00:06:05.840
Let me write it like that.

00:06:05.840 --> 00:06:10.880
Light dependent means that
they need light to occur.

00:06:10.880 --> 00:06:12.300
Light-dependent reactions.

00:06:12.300 --> 00:06:14.500
And then you have something
called the dark reactions, and

00:06:14.500 --> 00:06:18.140
that's actually a bad name,
because it also

00:06:18.140 --> 00:06:19.850
occurs in the light.

00:06:19.850 --> 00:06:23.300
Dark reactions, I wrote in
a slightly darker color.

00:06:23.300 --> 00:06:26.420
And the reason why I said it's a
bad name is because it still

00:06:26.420 --> 00:06:27.800
occurs in the light.

00:06:32.840 --> 00:06:34.950
But the reason why they probably
called it the dark

00:06:34.950 --> 00:06:38.100
reaction is that you don't need
light, or that part of

00:06:38.100 --> 00:06:41.620
photosynthesis isn't dependent
on photons to occur.

00:06:41.620 --> 00:06:45.130
So a better term for
it would have been

00:06:45.130 --> 00:06:55.700
light-independent reaction.

00:06:55.700 --> 00:06:58.190
So just to be clear,
the light reactions

00:06:58.190 --> 00:06:59.910
actually need sunlight.

00:06:59.910 --> 00:07:03.420
They actually need photons
for them to proceed.

00:07:03.420 --> 00:07:07.510
The dark reactions do not need
photons for them to happen,

00:07:07.510 --> 00:07:10.240
although they do occur
when the sun is out.

00:07:10.240 --> 00:07:13.010
They don't need those photons,
but they need the byproducts

00:07:13.010 --> 00:07:14.800
from the light reaction to
occur, so that's why it's

00:07:14.800 --> 00:07:17.230
called the light-independent
reaction.

00:07:17.230 --> 00:07:19.300
They occur while the sun
is out, but they

00:07:19.300 --> 00:07:20.580
don't need the sun.

00:07:20.580 --> 00:07:24.270
This needs the sun, so let
me make it very clear.

00:07:24.270 --> 00:07:26.970
So this requires sunlight.

00:07:26.970 --> 00:07:28.350
This requires photons.

00:07:31.320 --> 00:07:35.280
And let me just make a very
brief overview of this.

00:07:35.280 --> 00:07:38.880
This'll maybe let us start
building a scaffold from which

00:07:38.880 --> 00:07:40.400
we can dig deeper.

00:07:40.400 --> 00:07:42.880
So the light reactions
need photons, and

00:07:42.880 --> 00:07:46.010
then it needs water.

00:07:46.010 --> 00:07:48.600
So water goes into the light
reactions and out of the other

00:07:48.600 --> 00:07:49.820
side of the light reactions.

00:07:49.820 --> 00:07:52.850
We end up with some
molecular oxygen.

00:07:52.850 --> 00:07:54.875
So that's what happens in the
light reactions, and I'm going

00:07:54.875 --> 00:07:57.370
to go much deeper into
what actually occurs.

00:07:57.370 --> 00:08:03.030
And what the light the actions
produce is ATP, which we know

00:08:03.030 --> 00:08:07.710
is the cellular or the
biological currency of energy.

00:08:07.710 --> 00:08:10.750
It produces ATP and
it produces NADPH.

00:08:14.350 --> 00:08:16.790
Now, when we studied cellular
respiration, we saw the

00:08:16.790 --> 00:08:18.930
molecule NADH.

00:08:18.930 --> 00:08:21.310
NADPH is very similar.

00:08:21.310 --> 00:08:23.260
You just have this P there.

00:08:23.260 --> 00:08:25.710
You just have this phosphate
group there, but they really

00:08:25.710 --> 00:08:31.590
perform similar mechanisms. That
this agent right here,

00:08:31.590 --> 00:08:37.659
this molecule right here, is
able to give away-- now let's

00:08:37.659 --> 00:08:41.039
think about what this means--
it's able to give away this

00:08:41.039 --> 00:08:44.960
hydrogen and the electron
associated with this hydrogen.

00:08:44.960 --> 00:08:48.870
So if you give away an electron
to someone else or

00:08:48.870 --> 00:08:51.690
someone else gains an electron,
that something else

00:08:51.690 --> 00:08:53.310
is being reduced.

00:08:53.310 --> 00:08:54.500
Let me write that down.

00:08:54.500 --> 00:08:56.350
This is a good reminder.

00:08:56.350 --> 00:08:59.820
OIL RIG.

00:08:59.820 --> 00:09:02.750
Oxidation is losing
an electron.

00:09:02.750 --> 00:09:04.750
Reduction is gaining
an electron.

00:09:04.750 --> 00:09:07.430
Your charge is reduced when
you gain an electron.

00:09:07.430 --> 00:09:08.950
It has a negative charge.

00:09:08.950 --> 00:09:10.770
So this is a reducing agent.

00:09:10.770 --> 00:09:14.810
It gets oxidized by losing
the hydrogen and the

00:09:14.810 --> 00:09:15.620
electron with it.

00:09:15.620 --> 00:09:18.060
I have a whole discussion
on the biological versus

00:09:18.060 --> 00:09:21.670
chemistry view of oxidation,
but it's the same idea.

00:09:21.670 --> 00:09:25.530
When I lose a hydrogen, I also
lose the ability to hog that

00:09:25.530 --> 00:09:27.130
hydrogen's electron.

00:09:27.130 --> 00:09:30.270
So this right here, when it
reacts with other things, it's

00:09:30.270 --> 00:09:31.910
a reducing agent.

00:09:31.910 --> 00:09:37.030
It gives away this hydrogen and
the electron associated

00:09:37.030 --> 00:09:40.550
with it, and so the other
thing gets reduced.

00:09:40.550 --> 00:09:42.660
So this thing is a
reducing agent.

00:09:46.450 --> 00:09:49.890
And what's useful about it is
when this hydrogen, and

00:09:49.890 --> 00:09:52.230
especially the electron
associated with that hydrogen,

00:09:52.230 --> 00:09:56.950
goes from the NADPH to, say,
another molecule and goes to a

00:09:56.950 --> 00:10:01.750
lower energy state, that energy
can also be used in the

00:10:01.750 --> 00:10:02.570
dark reactions.

00:10:02.570 --> 00:10:04.800
And we saw in cellular
respiration the very similar

00:10:04.800 --> 00:10:09.570
molecule, NADH, that through
the Kreb Cycle, or actually

00:10:09.570 --> 00:10:11.950
more importantly, that through
the electron transport chain,

00:10:11.950 --> 00:10:17.350
was able to help produce ATP as
it gave away its electrons

00:10:17.350 --> 00:10:18.770
and they went to lower
energy states.

00:10:18.770 --> 00:10:20.410
But I don't want to confuse
you too much.

00:10:20.410 --> 00:10:23.280
So the light reactions, you take
in photons, you take in

00:10:23.280 --> 00:10:28.930
water, it spits out oxygen, and
it spits out ATP and NADPH

00:10:28.930 --> 00:10:32.030
that can then be used in
the dark reactions.

00:10:32.030 --> 00:10:35.520
And the dark reactions, for
most plants we talk about,

00:10:35.520 --> 00:10:36.770
it's called the Calvin Cycle.

00:10:39.610 --> 00:10:43.370
And I'll go into a lot more
detail of what actually occurs

00:10:43.370 --> 00:10:48.810
in the Calvin Cycle, but it
takes in the ATP, the NADPH,

00:10:48.810 --> 00:10:56.480
and it produces-- it doesn't
directly produce glucose.

00:10:56.480 --> 00:11:00.410
It produces-- oh, you
probably saw this.

00:11:00.410 --> 00:11:02.330
You could call it PGAL.

00:11:02.330 --> 00:11:05.990
You could call it G3P.

00:11:05.990 --> 00:11:09.410
These all stand for-- let me
write these down-- this is

00:11:09.410 --> 00:11:10.660
phosphoglyceraldehyde.

00:11:21.830 --> 00:11:23.410
My handwriting broke down.

00:11:23.410 --> 00:11:24.910
Or you could call it
glyceraldehyde 3-phosphate.

00:11:35.460 --> 00:11:38.210
Same exact molecule.

00:11:38.210 --> 00:11:41.320
You can almost imagine it as--
this is a very gross

00:11:41.320 --> 00:11:45.000
oversimplification-- as three
carbons with a phosphate group

00:11:45.000 --> 00:11:46.250
attached to it.

00:11:48.660 --> 00:11:51.750
But this can then be used to
produce other carbohydrates,

00:11:51.750 --> 00:11:52.620
including glucose.

00:11:52.620 --> 00:11:54.970
If you have two of these,
you can use those

00:11:54.970 --> 00:11:57.160
two to produce glucose.

00:11:57.160 --> 00:11:59.820
So let's just take a quick
overview again because this is

00:11:59.820 --> 00:12:00.610
super important.

00:12:00.610 --> 00:12:03.000
I'm going to make videos on the
light reactions and the

00:12:03.000 --> 00:12:04.030
dark reactions.

00:12:04.030 --> 00:12:05.850
Those will be the next
two videos I make.

00:12:05.850 --> 00:12:08.450
So photosynthesis, you
start with photons.

00:12:08.450 --> 00:12:11.500
All of these occur when the sun
is out, but only the light

00:12:11.500 --> 00:12:14.270
reactions actually
need the photons.

00:12:14.270 --> 00:12:16.090
The light reactions take
photons-- we're going to go

00:12:16.090 --> 00:12:17.740
into more detail about what
actually occurs--

00:12:17.740 --> 00:12:19.550
and it takes in water.

00:12:19.550 --> 00:12:21.130
Oxygen gets spit out.

00:12:21.130 --> 00:12:24.670
ATP and NADPH get spit out,
which are then used by the

00:12:24.670 --> 00:12:28.020
dark reaction, or the
Calvin Cycle, or the

00:12:28.020 --> 00:12:30.310
light-independent reaction,
because these still

00:12:30.310 --> 00:12:31.470
occur in the light.

00:12:31.470 --> 00:12:32.820
They just don't need photons.

00:12:32.820 --> 00:12:34.980
So they're the light-independent
reaction.

00:12:34.980 --> 00:12:37.530
And it uses that in
conjunction-- and we'll talk

00:12:37.530 --> 00:12:39.940
about other molecules that
are used in conjunction.

00:12:39.940 --> 00:12:43.950
Oh, and I forgot a very
important constituent of the

00:12:43.950 --> 00:12:45.840
dark reaction.

00:12:45.840 --> 00:12:47.690
It needs carbon dioxide.

00:12:47.690 --> 00:12:50.070
That's where you get your
carbons to keep producing

00:12:50.070 --> 00:12:54.220
these phosphoglyceraldehydes, or
glyceraldehyde 3-phosphate.

00:12:54.220 --> 00:12:56.600
So that's super important.

00:12:56.600 --> 00:13:00.830
It takes in the carbon dioxide,
the products from the

00:13:00.830 --> 00:13:05.000
light reactions, and then uses
that in the Calvin Cycle to

00:13:05.000 --> 00:13:09.070
produce this very simple
building block of other

00:13:09.070 --> 00:13:09.980
carbohydrates.

00:13:09.980 --> 00:13:12.790
And if you remember from
glycolysis, you might remember

00:13:12.790 --> 00:13:16.650
that this PGAL molecule, or this
G3P-- same thing-- this

00:13:16.650 --> 00:13:20.210
was actually the first product
when we split glucose in two

00:13:20.210 --> 00:13:21.720
when we performed
the glycolysis.

00:13:21.720 --> 00:13:23.270
So now we're going
the other way.

00:13:23.270 --> 00:13:25.540
We're building glucose
so that we can split

00:13:25.540 --> 00:13:26.910
it later for energy.

00:13:26.910 --> 00:13:29.340
So this is an overview of
photosynthesis, and in the

00:13:29.340 --> 00:13:31.090
next couple of videos, I'm
actually going to delve a

00:13:31.090 --> 00:13:33.430
little bit deeper and tell you
about the light reactions and

00:13:33.430 --> 00:13:36.630
the dark reactions and how
they actually occur.