Rational inequalities

WEBVTT
Kind: captions
Language: en

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In this video I want to do a
couple of inequality problems

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that are deceptively tricky.

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And you might be saying,
hey, aren't all inequality

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problems deceptively tricky?

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And on some level
you're probably right.

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But let's start with
the first problem.

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We have x minus 1 over x
plus 2 is greater than 0.

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And I'm actually going to show
you two ways to do this.

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The first way is, I think, on
some level, the simpler way.

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But I'll show you both methods
and whatever works for you,

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well, it works for you.

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So the first way you can think
about this, if I have just any

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number divided by any other
number and I say that they're

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going to be greater than 0.

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Well, we just have to remember
our properties of multiplying

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and dividing negative numbers.

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In what situation is
this fraction going

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to be greater than 0?

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Well, this is going to be
greater than 0 only if both a

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and-- so we could write both a
is greater than 0 and

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b is greater than 0.

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So this is one circumstance
where this'll

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definitely be true.

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We have a positive divided
by a positive; it'll

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definitely be a positive.

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It'll definitely be
greater than 0.

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Or we could have the situation
where we have a negative

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divided by a negative.

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If we have the same sign
divided by the same sign we're

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also going to be positive.

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So or-- a is less than 0
and b is less than 0.

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So whenever you have any type
of rational expression like

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this being greater than 0,
there's two situations in

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which it will be true.

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The numerator and the
denominator are both

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greater than 0, or
they're both less than 0.

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So let's remember that and
actually do this problem.

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So there's two situations
to solve this problem.

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The first is where both of
them are greater than 0.

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If that and that are both
greater than 0, we're cool.

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So we could say our first
solution-- maybe I'll draw a

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little tree like that-- is x
minus 1 greater than 0 and

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x plus 2 greater than 0.

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That's equivalent to this.

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The top and the bottom-- if
they're both greater than 0

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then when you divide them
you're going to get

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something greater than 0.

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The other option-- we just
saw that-- is if both of

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them were less than 0.

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So the other option is x
minus 1 less than 0 and

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x plus 2 less than 0.

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If both of these are less than
0 then you have a negative

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divided by a negative,
which will be positive.

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Which will be greater than 0.

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So let's actually solve in
both of these circumstances.

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So x minus 1 greater than 0.

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If we add 1 to both
sides of that we get

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x is greater than 1.

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And if we do x plus 2 greater
than 0, if we subtract 2 from

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both sides of that equation--
remember I'm doing this

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right now-- we get x is
greater than minus 2.

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So for both of these to hold
true-- so in this little brown

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or red color, whatever you want
to think of it-- in order for

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both of these to go hold true,
x has to be greater than 1 and

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x has to be greater
than minus 2.

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This statement we figured out
means that x has to be greater

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than 1 and this statement
tells us that x has to

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be greater than minus 2.

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Now, if x is greater than 1 and
x has to be greater than minus

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2, x clearly has to
be greater than 1.

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You know, 0 would not satisfy
this because 0 is greater

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than minus 2, but it's
not greater than 1.

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So for something to be greater
than 1 and minus 2 it has

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to be greater than 1.

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This whole chain of thought
where I'm saying the numerator

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and the denominator are greater
than 0, that's only going to

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happen if x is greater than 1.

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Because if x is greater than 1,
then x is definitely going to

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be greater than negative 2.

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Any number greater than
1 is definitely greater

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than negative 2.

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So that's one situation in
which this equation holds true,

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and we can even try it out.

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Let's say x was 2.

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2 minus 1 is 1 over 2 plus 2.

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It's 1/4.

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It's a positive number.

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Now let's do the situation
where both of these

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are negative.

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If the x minus 1 is less than
0, if we add 1 to both sides of

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that equation that tells us
so x minus 1 is less than 0.

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That's the same thing-- if we
add 1 to both sides of that--

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as saying that x
is less than 1.

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So that constraint boils
down to that constraint.

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Now this constraint, x
plus 2 is less than 0.

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If we subtract 2 from
both sides we get x

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is less than minus 2.

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So this constraint boils
down to that constraint.

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So in order for both of these
guys to be negative, both the

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numerator and the denominator
to be negative-- we know that

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x has to be less than 1 and x
has to be less than minus 2.

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Now if something has to be less
than 1 and it has to be less

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than minus 2, well, it just
has to be less than minus 2.

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Anything less than minus 2 is
going to satisfy both of these.

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So this boils down to
just x could also be

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less than minus 2.

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And remember, this is an or.

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Either both the numerator and
the denominator are positive,

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or they're both negative.

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So both of them being positive
boil down to x could be greater

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than 1, or both of them being
negative boils down to x

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is less than minus 2.

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So our solution is x could be
greater than 1 or-- that's

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both of these positive--
or x is less than minus 2.

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That's both of these negative.

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And if we wanted to draw it on
a number line-- let me draw a

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number line just like that.

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That could be 0 and then
we have 1, so x could

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be greater than 1.

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Not greater than or equal to.

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So we put a little circle
there to show that

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we're not including 1.

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And everything greater than 1
will satisfy this equation.

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Or anything less than minus 2.

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So we have minus 1, minus 2,
anything less than minus 2 will

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also satisfy this equation by
both making the numerator

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and denominator negative.

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And you could try it out.

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Minus 3.

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Minus 3 minus 1.

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I'll just do minus 3 minus
1 is equal to minus 4.

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And then minus 3 plus 2.

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Minus 3 plus 2 is
equal to minus 1.

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Minus 4 divided by
minus 1 is positive 4.

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So all of these negative
numbers also work.

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Now, I told you that I
would show you two ways

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of doing this problem.

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So let me show you another way
if you found this one maybe

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a little bit confusing.

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So the other way-- let
me rewrite the problem.

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You get x minus 1 over x
plus 2 is greater than 0.

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And actually, let's mix it up
a little bit and you could

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apply the same logic.

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Let's say it's greater than or
equal-- well, actually no.

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I'll just keep it the same way
and maybe in the next video

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I'll do the case where it's
greater than or equal just

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because I really don't want
to-- maybe I want to

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incrementally step up the
level of difficulty.

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We're just saying x minus
1 over x plus 2 is just

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straight up greater than 0.

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Now one thing you might say is
well, if I have a rational

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expression like this, maybe I
multiply both sides of this

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equation by x plus 2.

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So I get rid of it in the
denominator and I can

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multiply it times 0 and
get it out of the way.

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But the problem is when you
multiply both sides of an

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inequality by a number-- if
you're multiplying by a

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positive you can keep the
inequality the same.

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But if you're multiplying by a
negative you have to switch the

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inequality, and we don't
know whether x plus 2 is

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positive or negative.

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So let's do both situations.

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Let's do one situation
where x plus 2-- let

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me write it this way.

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x plus 2 is greater than 0.

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And then another situation
where-- let me do that

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in a different color.

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Where x plus 2 is less than 0.

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These are the two
possibilities for x plus 2.

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Actually, in those situations
can x plus 2 equal 0?

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If x plus 2 were to be equal to
0 than this whole expression

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would be undefined.

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And so that definitely won't
be a situation that we

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want to deal with it.

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It would an undefined
situation.

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So these are our two situations
when we're multiplying

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both sides.

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So if x plus 2 is greater
than 0 that means that x

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is greater than minus 2.

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We can just subtract 2 from
both sides of this equation.

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So if x is greater than
minus 2, then x plus

00:08:35.920 --> 00:08:37.330
2 is greater than 0.

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And then we can multiply
both sides of this

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equation times x plus 2.

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So you have x minus 1 over
x plus 2 greater than 0.

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I'm going to multiply both
sides by x plus 2, which I'm

00:08:50.200 --> 00:08:54.400
assuming is positive because
x is greater than minus 2.

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Multiply both sides
by x plus 2.

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These cancel out.

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0 times x plus 2 is it just 0.

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You're just left with x minus
1 is greater than-- this

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just simplified to 0.

00:09:07.770 --> 00:09:09.690
Solve for x, add 1 to
both sides, you get

00:09:09.690 --> 00:09:11.670
x is greater than 1.

00:09:11.670 --> 00:09:14.830
So we saw that if x plus 2 is
greater than 0, or we could

00:09:14.830 --> 00:09:18.890
say, if x is greater than
minus 2, then x also has

00:09:18.890 --> 00:09:21.390
to be greater than 1.

00:09:21.390 --> 00:09:22.860
Or you could say if x is
great-- well, you could

00:09:22.860 --> 00:09:24.180
go both ways in that.

00:09:24.180 --> 00:09:28.510
But we say, look, both of
these things have to be true.

00:09:28.510 --> 00:09:31.380
If for x to satisfy both
of these, it just has

00:09:31.380 --> 00:09:32.380
to be greater than 1.

00:09:32.380 --> 00:09:34.010
Because if it's greater
than 1 it's definitely

00:09:34.010 --> 00:09:36.670
going to satisfy this
constraint over here.

00:09:36.670 --> 00:09:39.760
So for this branch we come
up with the solution

00:09:39.760 --> 00:09:42.050
x is greater than 1.

00:09:42.050 --> 00:09:45.300
So this is one situation where
x plus 2 is greater than 0.

00:09:45.300 --> 00:09:50.240
The other situation is x
plus 2 being less than 0.

00:09:50.240 --> 00:09:52.960
If x plus 2 is less than 0
that's equivalent to saying

00:09:52.960 --> 00:09:55.070
that x is less than minus 2.

00:09:55.070 --> 00:09:57.560
You just subtract 2
from both sides.

00:09:57.560 --> 00:10:00.510
Now, if x plus 2 is less than
0, what we'll have to do

00:10:00.510 --> 00:10:03.630
when we multiply both
sides-- let's do that.

00:10:03.630 --> 00:10:06.690
We have x minus 1
over x plus 2.

00:10:06.690 --> 00:10:10.190
We have some inequality
and then we have a 0.

00:10:10.190 --> 00:10:14.420
Now if we multiply both
sides by x plus 2, x plus

00:10:14.420 --> 00:10:15.890
2 is a negative number.

00:10:15.890 --> 00:10:17.640
When you multiply both
sides of an equation by a

00:10:17.640 --> 00:10:19.430
negative number you have
to swap the inequality.

00:10:19.430 --> 00:10:23.150
So this greater than sign will
become a less than sign because

00:10:23.150 --> 00:10:26.340
we're assuming that the
x plus 2 is negative.

00:10:26.340 --> 00:10:27.680
These cancel out.

00:10:27.680 --> 00:10:30.090
0 times anything is 0.

00:10:30.090 --> 00:10:34.220
We get that x minus
1 is less than 0.

00:10:34.220 --> 00:10:39.640
Solving for x, adding 1 to
both sides, x is less than 1.

00:10:39.640 --> 00:10:43.430
So in the case that x plus 2 is
less than 0 or x is less than

00:10:43.430 --> 00:10:46.580
minus 2, x must be less than 1.

00:10:46.580 --> 00:10:49.780
Well, I mean we know-- if you
say something has to be less

00:10:49.780 --> 00:10:53.030
than minus 2 and less than 1,
just saying that it's less

00:10:53.030 --> 00:10:54.720
than minus 2 will do the job.

00:10:54.720 --> 00:10:57.220
Anything less than minus 2 is
going to satisfy this one.

00:10:57.220 --> 00:10:58.790
But not anything that satisfies
this one is going to

00:10:58.790 --> 00:10:59.510
satisfy that one.

00:10:59.510 --> 00:11:02.520
So this is the only constraint
we have to worry about.

00:11:02.520 --> 00:11:06.160
So in the event where x plus 2
is less than 0, we can just

00:11:06.160 --> 00:11:08.470
say that x has to be
less than minus 2.

00:11:08.470 --> 00:11:10.420
That'll satisfy this equation.

00:11:10.420 --> 00:11:12.750
So our final result is x is
either going to be greater

00:11:12.750 --> 00:11:16.710
than 1 or x is going to
be less than minus 2.

00:11:16.710 --> 00:11:20.581
So once again, I can graph
it on the number line.

00:11:20.581 --> 00:11:25.120
x is greater than
1 right there.

00:11:25.120 --> 00:11:29.160
You have 0, minus 1, minus 2.

00:11:29.160 --> 00:11:32.840
And then you have x is less
than minus 2, you're not

00:11:32.840 --> 00:11:34.290
including the minus 2.

00:11:34.290 --> 00:11:35.310
And just like that.

00:11:35.310 --> 00:11:38.080
And that is the exact same
result we got up here.

00:11:38.080 --> 00:11:41.290
So whichever version
you find to be easier.

00:11:41.290 --> 00:11:43.120
But you can see, they're both a
little bit nuanced and you have

00:11:43.120 --> 00:11:45.340
to think a little bit about
what happens when you multiply

00:11:45.340 --> 00:11:48.790
or divide by positive
or negative numbers.


CAM Plants

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:03.670
A couple of videos ago, we
saw that in classic C-3

00:00:03.670 --> 00:00:06.240
photosynthesis-- and once again
it's called C-3 because

00:00:06.240 --> 00:00:13.770
the first time that carbon
dioxide is fixed, it's fixed

00:00:13.770 --> 00:00:15.470
into a 3-carbon molecule.

00:00:15.470 --> 00:00:18.590
But we saw the problem with C-3
photosynthesis is that the

00:00:18.590 --> 00:00:22.580
enzyme that does the carbon
fixation, it can also react

00:00:22.580 --> 00:00:23.430
with oxygen.

00:00:23.430 --> 00:00:27.470
And when oxygen essentially
reacts with ribulose

00:00:27.470 --> 00:00:30.110
biphosphate instead of your
carbon, you get an

00:00:30.110 --> 00:00:31.330
unproductive reaction.

00:00:31.330 --> 00:00:33.390
Not only is it unproductive,
it'll actually suck up your

00:00:33.390 --> 00:00:35.900
ATP and your NADPH and
you'll go nowhere.

00:00:35.900 --> 00:00:38.540
So every now and then, when
oxygen bonds here instead of a

00:00:38.540 --> 00:00:42.240
carbon dioxide, you get
nothing produced, net.

00:00:42.240 --> 00:00:43.910
Everything becomes
less efficient.

00:00:43.910 --> 00:00:49.740
And so in the last video, we
saw that some plants have

00:00:49.740 --> 00:00:52.660
evolved a way to get
around this.

00:00:52.660 --> 00:00:56.390
And what they do is, they fix
their carbon on the outside,

00:00:56.390 --> 00:00:58.930
on cells that are actually
exposed to the air.

00:00:58.930 --> 00:01:01.090
And then once they fix the
carbon they actually fix it

00:01:01.090 --> 00:01:05.850
into a 4-carbon molecule, into
oxaloacetate And then that

00:01:05.850 --> 00:01:08.740
gets turned into malate Then
they pump the malate deeper

00:01:08.740 --> 00:01:11.930
within the leaf, where you
aren't exposed to oxygen.

00:01:11.930 --> 00:01:14.720
And then they take the carbon
dioxide off the malate, and

00:01:14.720 --> 00:01:17.940
this is where they actually
perform the Calvin cycle.

00:01:17.940 --> 00:01:20.540
And even though you do have
your rubiscos still there,

00:01:20.540 --> 00:01:23.830
your rubisco isn't going to
have-- the photorespiration is

00:01:23.830 --> 00:01:24.980
not going to occur.

00:01:24.980 --> 00:01:26.980
Because it only has access
to carbon dioxide.

00:01:26.980 --> 00:01:29.710
It does not have access to
this oxygen out here.

00:01:29.710 --> 00:01:31.950
Now that's a very efficient
way of producing sugars.

00:01:31.950 --> 00:01:34.630
And that's why some of the
plants that we associate with

00:01:34.630 --> 00:01:38.100
being very strong sugar, or even
ethanol producers, all

00:01:38.100 --> 00:01:40.610
perform C-4 photosynthesis.

00:01:40.610 --> 00:01:49.360
Corn, sugarcane,
and crab grass.

00:01:49.360 --> 00:01:51.850
And these are all very, very
efficient sugar producers.

00:01:51.850 --> 00:01:53.260
Because they don't have to
worry too much about

00:01:53.260 --> 00:01:54.890
photorespiration.

00:01:54.890 --> 00:01:57.400
Now some plants have a slightly
different problem.

00:01:57.400 --> 00:02:00.970
They're not so worried about the
efficiency of the process.

00:02:00.970 --> 00:02:03.750
They're more worried
about losing water.

00:02:03.750 --> 00:02:05.040
And you can imagine what
plants these are.

00:02:05.040 --> 00:02:07.150
These are plants that
are in the desert.

00:02:07.150 --> 00:02:10.900
Because these stomata, these
pores that are on the leaves,

00:02:10.900 --> 00:02:15.720
they let in air, but they
can also let out water.

00:02:15.720 --> 00:02:16.970
I mean, if I'm in the
rainforest, I

00:02:16.970 --> 00:02:18.080
don't care about that.

00:02:18.080 --> 00:02:20.220
But if I'm in the middle of the
desert, I don't want to

00:02:20.220 --> 00:02:23.230
let out water vapor through
my stomata.

00:02:23.230 --> 00:02:26.460
So the ideal situation is, I
would want my stomata closed

00:02:26.460 --> 00:02:28.140
during the daytime.

00:02:28.140 --> 00:02:29.730
This is what I want.

00:02:29.730 --> 00:02:32.120
So I want-- if I'm in
the desert, let

00:02:32.120 --> 00:02:34.510
me make this clear.

00:02:34.510 --> 00:02:46.310
If I'm in the desert I want
stomata closed during the day.

00:02:46.310 --> 00:02:47.230
For obvious reasons.

00:02:47.230 --> 00:02:49.580
I don't want all my water
to vaporize out of

00:02:49.580 --> 00:02:51.090
these holes in my leaves.

00:02:51.090 --> 00:02:56.900
But at the same time, the
problem is that photosynthesis

00:02:56.900 --> 00:02:58.580
can only occur during
the daytime.

00:02:58.580 --> 00:02:59.880
And that includes the
dark reactions.

00:02:59.880 --> 00:03:02.040
Remember, I've said multiple
times, the dark reactions are

00:03:02.040 --> 00:03:02.680
badly named.

00:03:02.680 --> 00:03:04.800
They're more the light
independent reactions.

00:03:04.800 --> 00:03:07.220
But they both occur
simultaneously-- the light

00:03:07.220 --> 00:03:09.600
independent and light
dependent-- and only during

00:03:09.600 --> 00:03:10.850
the daytime.

00:03:21.410 --> 00:03:25.360
And if your stomata is closed,
you need to perform

00:03:25.360 --> 00:03:30.190
photosynthesis, especially the
Calvin cycle, you need CO2.

00:03:30.190 --> 00:03:31.650
So how can you get
around this?

00:03:31.650 --> 00:03:34.870
If I want to close my stomata
during the day, but I need CO2

00:03:34.870 --> 00:03:37.660
during the day, how can
I solve this problem?

00:03:37.660 --> 00:03:40.700
And what desert plants, or
many desert plants, have

00:03:40.700 --> 00:03:44.240
evolved to do, essentially
does photosynthesis, but

00:03:44.240 --> 00:03:49.250
instead of fixing the carbon
in outer cells and then

00:03:49.250 --> 00:03:52.070
pushing it in to inner cells and
then performing the Calvin

00:03:52.070 --> 00:03:55.370
cycle, instead of outer and
inner cells, they do it at the

00:03:55.370 --> 00:03:58.100
nighttime and in the daytime.

00:03:58.100 --> 00:04:01.940
So in CAM plants-- and these are
called CAM plants because,

00:04:01.940 --> 00:04:03.235
I could tell you what
it stands for.

00:04:03.235 --> 00:04:06.180
It stands for crassulacean
acid metabolism.

00:04:06.180 --> 00:04:09.380
And that's because it was first
observed in that species

00:04:09.380 --> 00:04:11.200
of plants, the crassulacean
plant.

00:04:11.200 --> 00:04:13.260
But these are just called,
you could call it CAM

00:04:13.260 --> 00:04:15.720
photosynthesis or CAM plants.

00:04:15.720 --> 00:04:19.480
They're essentially a subset
of C-4 plants.

00:04:19.480 --> 00:04:22.029
But instead of performing C-4
photosynthesis, kind of an

00:04:22.029 --> 00:04:23.670
outside cells and inside cells,
they do it at the

00:04:23.670 --> 00:04:24.920
nighttime and the day.

00:04:33.420 --> 00:04:40.545
And what they do is, at night
they keep their stomata open.

00:04:44.890 --> 00:04:50.190
And they perform, and they're
able to fix-- and everything

00:04:50.190 --> 00:04:51.960
occurs in the mesophyll
cells and the CAM

00:04:51.960 --> 00:04:54.290
cells, in the CAM plants.

00:04:54.290 --> 00:04:58.350
So at nighttime, when they're
not afraid of losing water--

00:04:58.350 --> 00:05:00.790
let's say this is a mesophyll
cell right here--

00:05:00.790 --> 00:05:03.160
my stomata is open.

00:05:03.160 --> 00:05:06.280
Let's say that this is my
stomata right there.

00:05:06.280 --> 00:05:08.090
And so it lets in
carbon dioxide.

00:05:08.090 --> 00:05:09.870
I'm not worried about
losing water vapor.

00:05:09.870 --> 00:05:11.190
It's night time right now.

00:05:11.190 --> 00:05:14.150
So carbon dioxide
comes in here.

00:05:14.150 --> 00:05:16.870
And then it fixes the
carbon dioxide.

00:05:16.870 --> 00:05:19.650
It fixes it the exact same way
that the C-4 plants do.

00:05:19.650 --> 00:05:23.080
So you have your CO2 come in.

00:05:23.080 --> 00:05:26.680
You have your PEP .

00:05:26.680 --> 00:05:30.910
It's all facilitated by PEP
carboxylase That's the enzyme.

00:05:30.910 --> 00:05:34.570
That can only fix CO2, that can
only react with CO2, not

00:05:34.570 --> 00:05:35.470
with oxygen.

00:05:35.470 --> 00:05:39.430
And then that is used to
produce-- and we saw it here

00:05:39.430 --> 00:05:44.460
in our CAM-4 diagram in the last
video, that is to used to

00:05:44.460 --> 00:05:45.490
produce malate.

00:05:45.490 --> 00:05:48.025
A 4-carbon molecule.

00:05:52.920 --> 00:05:55.380
And then the malate-- and then
this is what's key-- the

00:05:55.380 --> 00:05:58.640
malate get stored in other
organelles in the cell.

00:05:58.640 --> 00:06:00.775
In the vacuoles , which are, you
can kind of view them as

00:06:00.775 --> 00:06:02.920
large storage containers
in the cell.

00:06:02.920 --> 00:06:04.620
So I drew this as
the whole cell.

00:06:04.620 --> 00:06:08.990
I mean, this is actually all
occurring in your chloroplast.

00:06:08.990 --> 00:06:12.430
But you can imagine your cell
having a big storage center

00:06:12.430 --> 00:06:15.180
where the malate gets
stored at night.

00:06:15.180 --> 00:06:18.480
And you can view malate as
almost a carbon dioxide store.

00:06:18.480 --> 00:06:20.750
Because later on we can
access the malate and

00:06:20.750 --> 00:06:22.180
get the carbon dioxide.

00:06:22.180 --> 00:06:24.670
And that's exactly what these
CAM plants are going to do.

00:06:24.670 --> 00:06:26.380
So this is nighttime.

00:06:26.380 --> 00:06:28.520
Then the sun comes up.

00:06:28.520 --> 00:06:31.050
So now we're in the daytime.

00:06:31.050 --> 00:06:34.255
This desert plant, well maybe
it's a cactus, it doesn't want

00:06:34.255 --> 00:06:35.620
to lose its water vapor.

00:06:35.620 --> 00:06:36.870
So it closes its stomata.

00:06:39.290 --> 00:06:43.110
This particular stoma
now is closed.

00:06:43.110 --> 00:06:44.090
It's now closed.

00:06:44.090 --> 00:06:45.380
And you say, oh boy, how
is it going to perform

00:06:45.380 --> 00:06:46.680
photosynthesis?

00:06:46.680 --> 00:06:48.800
Well, it can perform
photosynthesis in

00:06:48.800 --> 00:06:50.085
that very same cell.

00:06:52.830 --> 00:06:56.660
Because it stored up all of
this malate at night.

00:06:56.660 --> 00:06:59.870
And so now the malate can be
pumped out of the vacuoles

00:06:59.870 --> 00:07:03.950
into the stroma of our
chloroplast. And then you can

00:07:03.950 --> 00:07:05.460
have pyruvate break off.

00:07:05.460 --> 00:07:07.990
But the more important thing
is you have CO2 break off.

00:07:07.990 --> 00:07:10.160
So you have a ready
supply of CO2.

00:07:10.160 --> 00:07:13.290
And now we can perform our
standard Calvin cycle.

00:07:13.290 --> 00:07:17.040
And in an environment only
with CO2, our stomata is

00:07:17.040 --> 00:07:18.300
closed, so we're ready to go.

00:07:18.300 --> 00:07:24.330
Our CO2 reacts with ribulose
bisphosphate It is catalyzed

00:07:24.330 --> 00:07:27.420
by rubisco It's the whole
Calvin cycle and we

00:07:27.420 --> 00:07:29.660
produced our sugar.

00:07:29.660 --> 00:07:32.340
So this is kind of a
neat adaptation.

00:07:32.340 --> 00:07:35.500
In these high, very efficient
sugar-producing plants that

00:07:35.500 --> 00:07:39.430
aren't worried about water, they
perform carbon fixation

00:07:39.430 --> 00:07:42.690
on things that are exposed to
the air and then they pump

00:07:42.690 --> 00:07:45.160
kind of a stored version of
the carbon deeper into the

00:07:45.160 --> 00:07:48.280
leaf to actually perform the
Calvin cycle so that it's not

00:07:48.280 --> 00:07:49.080
lossy, so that

00:07:49.080 --> 00:07:50.380
photorespiration doesn't occur.

00:07:50.380 --> 00:07:52.750
Because down here you
have no oxygen.

00:07:52.750 --> 00:07:56.700
The desert plants benefit from
that property as well, but

00:07:56.700 --> 00:07:58.820
their whole concern is, I don't
want to keep my stomata

00:07:58.820 --> 00:08:00.370
open in the daytime.

00:08:00.370 --> 00:08:02.650
So what I do is, I fix
my carbon at night.

00:08:02.650 --> 00:08:05.160
But I use the exact
same process.

00:08:05.160 --> 00:08:08.830
I use PEP carboxylase
And I store my

00:08:08.830 --> 00:08:10.540
carbon dioxide at night.

00:08:10.540 --> 00:08:15.940
And in the daytime, I can
actually-- when my

00:08:15.940 --> 00:08:18.310
light-dependant reactions are
occurring, they're producing

00:08:18.310 --> 00:08:22.780
my ATP and my NADH I can also
perform my dark reactions in

00:08:22.780 --> 00:08:23.410
the daytime.

00:08:23.410 --> 00:08:25.550
As I said, the dark reactions
always occur in the daytime.

00:08:25.550 --> 00:08:27.430
Or my light-independent
reactions.

00:08:27.430 --> 00:08:31.790
Because even though my stomata
is closed, I have a store of

00:08:31.790 --> 00:08:34.224
carbon dioxide in the
form of malate.


C-4 Photosynthesis

WEBVTT
Kind: captions
Language: en

00:00:00.530 --> 00:00:02.980
In the last video we discovered
what seems like a

00:00:02.980 --> 00:00:04.580
problem with the Calvin Cycle.

00:00:04.580 --> 00:00:09.350
That you have this big protein
here, enzyme, that facilitates

00:00:09.350 --> 00:00:10.290
the Calvin Cycle.

00:00:10.290 --> 00:00:13.790
All of the molecules that are
involved bond to this and then

00:00:13.790 --> 00:00:16.680
it twists and turns and it jams
things together so that

00:00:16.680 --> 00:00:17.570
they react properly.

00:00:17.570 --> 00:00:19.900
And we know what this
is, the RuBisCo.

00:00:19.900 --> 00:00:24.890
RuBisCo enzyme, or ribulose

00:00:24.890 --> 00:00:26.140
bisphosphate carboxylase oxygenase.

00:00:28.240 --> 00:00:32.770
And we know when the Calvin
Cycle operates properly you'll

00:00:32.770 --> 00:00:37.880
have some carbon dioxide
attached in one

00:00:37.880 --> 00:00:39.240
part of this enzyme.

00:00:39.240 --> 00:00:43.440
And then you'll have some RuBP,
or maybe you could call

00:00:43.440 --> 00:00:48.390
it the proper word, ribulose-1
5-bisphosphate.

00:00:48.390 --> 00:00:49.880
And they're going to react.

00:00:49.880 --> 00:00:52.650
And then after they react, if
everything with the Calvin

00:00:52.650 --> 00:00:56.040
Cycle is going properly, they're
going to react and

00:00:56.040 --> 00:00:58.830
form-- they're going to be
jammed together and then split

00:00:58.830 --> 00:01:02.000
into-- for one molecule of that
and one molecule of that,

00:01:02.000 --> 00:01:04.550
you're going to have
two molecules of

00:01:04.550 --> 00:01:05.800
3-phosphoglycerate.

00:01:10.760 --> 00:01:13.340
In the last video I started with
three of these and three

00:01:13.340 --> 00:01:15.180
of these so I ended up
with six of these.

00:01:15.180 --> 00:01:17.190
But for every one of these you
end up with two of these.

00:01:17.190 --> 00:01:19.050
This is the proper
Calvin Cycle.

00:01:19.050 --> 00:01:22.730
Then these turn into your
phosphoglyceraldehydes.

00:01:22.730 --> 00:01:23.540
These turn into two

00:01:23.540 --> 00:01:25.590
phosphoglyceraldehydes, or PGALs.

00:01:28.120 --> 00:01:30.750
And then for every six of these
that are produced-- and

00:01:30.750 --> 00:01:33.640
maybe I should write a three
here, a three here and then

00:01:33.640 --> 00:01:38.170
I'll have six of these-- and
for every six of these that

00:01:38.170 --> 00:01:41.840
are produced, five go back into
the cycle to produce.

00:01:41.840 --> 00:01:46.320
So five PGALs, or glyceraldehyde
3-phosphate go

00:01:46.320 --> 00:01:49.940
back into the cycle to produce
the ribulose bisphosphate.

00:01:49.940 --> 00:01:54.370
And one of them is kind of our
end product of photosynthesis

00:01:54.370 --> 00:01:56.530
that can be used to produce
other carbohydrates.

00:01:56.530 --> 00:01:58.060
So, one PGAL.

00:01:58.060 --> 00:02:01.280
And the whole problem, we saw
with the Calvin Cycle, is that

00:02:01.280 --> 00:02:05.420
RuBisCo does not only
fix carbon dioxide.

00:02:05.420 --> 00:02:07.320
That instead of carbon
dioxide we might

00:02:07.320 --> 00:02:09.509
have an oxygen molecule.

00:02:09.509 --> 00:02:11.890
We might have an oxygen molecule
that jumps in here.

00:02:11.890 --> 00:02:16.110
And it can also attach to
the RuBisCo enzyme.

00:02:16.110 --> 00:02:19.090
And in that situation, the
oxygen and the ribulose

00:02:19.090 --> 00:02:22.050
bisphosphate react.

00:02:22.050 --> 00:02:25.220
So if we had three ribulose
bisphosphates and three

00:02:25.220 --> 00:02:28.780
oxygens, instead of producing
six phosphoglycerates we're

00:02:28.780 --> 00:02:33.710
only going to produce five
phosphoglycerates and we're

00:02:33.710 --> 00:02:35.905
going to produce five
phosphoglycolates.

00:02:43.090 --> 00:02:46.590
Which is that byproduct that
gets processed later on.

00:02:46.590 --> 00:02:47.520
And then these five.

00:02:47.520 --> 00:02:48.520
You're going to have
five here.

00:02:48.520 --> 00:02:49.890
You can't have one left over.

00:02:49.890 --> 00:02:51.420
And then you're not going
to produce anything.

00:02:51.420 --> 00:02:53.380
In doing this whole cycle
you have to use up a

00:02:53.380 --> 00:02:54.960
bunch of ATPs and NADHs.

00:02:54.960 --> 00:02:56.440
So this is a problem.

00:02:56.440 --> 00:02:59.290
If there's a lot of oxygen
present, or even a little bit

00:02:59.290 --> 00:03:01.300
of oxygen present, it's
going to make this a

00:03:01.300 --> 00:03:03.920
little bit less efficient.

00:03:03.920 --> 00:03:06.410
Because every now and then an
oxygen's going to jump in

00:03:06.410 --> 00:03:10.770
where a carbon dioxide is needed
to actually produce an

00:03:10.770 --> 00:03:12.660
actual carbohydrate
in the end.

00:03:12.660 --> 00:03:15.800
So how do plants solve
this problem?

00:03:15.800 --> 00:03:21.140
Well, one solution would be to
operate the Calvin Cycle in an

00:03:21.140 --> 00:03:24.500
environment where there is
very, very little oxygen.

00:03:24.500 --> 00:03:26.900
Or you can almost
say no oxygen.

00:03:26.900 --> 00:03:28.780
And this is exactly what
some plants do.

00:03:28.780 --> 00:03:29.980
You're like, wait,
how do I do that?

00:03:29.980 --> 00:03:32.600
Do I have to go to a planet
where there's no oxygen?

00:03:32.600 --> 00:03:32.850
No!

00:03:32.850 --> 00:03:35.980
What they do is-- and to
understand this we'll have to

00:03:35.980 --> 00:03:40.173
understand a little bit of
the actual make-up of

00:03:40.173 --> 00:03:41.100
the leaf of a plant.

00:03:41.100 --> 00:03:42.990
And that doesn't hurt because
everything we've been doing

00:03:42.990 --> 00:03:44.190
now has been biochemistry.

00:03:44.190 --> 00:03:45.670
it's nice to see leaves.

00:03:45.670 --> 00:03:49.750
So if I draw-- let's say
that that is a leaf.

00:03:49.750 --> 00:03:50.880
That is a leaf.

00:03:50.880 --> 00:03:52.990
I can make it look
nice like a leaf.

00:03:52.990 --> 00:03:54.620
That's a nice looking leaf.

00:03:54.620 --> 00:03:57.320
On your leaf's surface, and
actually on both sides of it,

00:03:57.320 --> 00:04:01.360
you have these little pores,
these little holes on the

00:04:01.360 --> 00:04:01.930
leaf's surface.

00:04:01.930 --> 00:04:03.020
They are actually surrounded
by these things

00:04:03.020 --> 00:04:03.930
called guard cells.

00:04:03.930 --> 00:04:05.780
But the important thing is that
these little pores-- and

00:04:05.780 --> 00:04:07.940
they're actually much smaller
than that; you'd have to get a

00:04:07.940 --> 00:04:10.850
microscope to actually see
them-- they're called stomata.

00:04:10.850 --> 00:04:13.570
Or one individual of
these pores or

00:04:13.570 --> 00:04:16.579
holes is called a stoma.

00:04:16.579 --> 00:04:20.940
And this is where the oxygen
and the-- mainly the carbon

00:04:20.940 --> 00:04:23.920
dioxide-- but this is where the
air enters the cell and

00:04:23.920 --> 00:04:25.870
this is actually where
water vapor is also

00:04:25.870 --> 00:04:26.860
released from the cell.

00:04:26.860 --> 00:04:33.420
If we draw a cross section of
a leaf, so let me do my best

00:04:33.420 --> 00:04:36.810
to draw a cross section
like this.

00:04:36.810 --> 00:04:39.280
Let me draw it like that
and maybe that's the

00:04:39.280 --> 00:04:41.300
bottom of the leaf.

00:04:41.300 --> 00:04:43.820
This would be my stoma.

00:04:43.820 --> 00:04:45.640
This is the actual opening.

00:04:45.640 --> 00:04:48.410
This is the actual opening and
plants can actually open and

00:04:48.410 --> 00:04:49.660
close their stomata.

00:04:52.600 --> 00:04:53.760
The plural of stoma
is stomata.

00:04:53.760 --> 00:04:55.520
They can open and close
their stomata.

00:04:55.520 --> 00:04:57.760
But the important thing to
realize is what's going on

00:04:57.760 --> 00:04:59.390
inside the cell.

00:04:59.390 --> 00:05:03.810
So most plants, you have this
whole photosynthetic process,

00:05:03.810 --> 00:05:07.180
or photosynthesis process,
occurring in these mesophyll

00:05:07.180 --> 00:05:09.090
cells, which are really just
these middle layer cells.

00:05:09.090 --> 00:05:12.180
And I'll do a detailed video
in the future about the

00:05:12.180 --> 00:05:13.970
anatomy of a plant.

00:05:13.970 --> 00:05:17.530
But these are the
mesophyll cells.

00:05:17.530 --> 00:05:21.880
This is where photosynthesis
normally occurrs.

00:05:21.880 --> 00:05:24.890
And because they use carbon
dioxide, or they need air--

00:05:24.890 --> 00:05:26.110
actually I drew it wrong.

00:05:26.110 --> 00:05:26.990
Let me draw it a little
bit better.

00:05:26.990 --> 00:05:29.050
There's actually space
between them, so that

00:05:29.050 --> 00:05:30.520
air can get to them.

00:05:30.520 --> 00:05:34.710
There's mesophyll, I'm doing
a very rough drawing.

00:05:34.710 --> 00:05:37.950
But in this situation air can
enter through a stoma and then

00:05:37.950 --> 00:05:41.650
it can fill the space between
these mesophyll cells and it

00:05:41.650 --> 00:05:44.010
can provide air to the
mesophyll cells.

00:05:44.010 --> 00:05:46.740
And when I say air, that air is
made up of carbon dioxide

00:05:46.740 --> 00:05:48.440
and oxygen and nitrogen
and all the things

00:05:48.440 --> 00:05:50.170
that are in our air.

00:05:50.170 --> 00:05:53.320
And of course, it needs the
carbon dioxide to actually

00:05:53.320 --> 00:05:55.560
perform the Calvin Cycle.

00:05:55.560 --> 00:05:58.810
Now, we just said that it's
not just getting CO2.

00:05:58.810 --> 00:06:00.940
If it was just getting CO2
you wouldn't worry about

00:06:00.940 --> 00:06:02.430
photorespiration.

00:06:02.430 --> 00:06:05.170
It's also getting oxygen.

00:06:05.170 --> 00:06:07.280
It's also getting molecular
oxygen.

00:06:07.280 --> 00:06:10.430
So what can the plant to
do to prevent this?

00:06:10.430 --> 00:06:12.770
And not all plants-- you know,
most plants just deal with

00:06:12.770 --> 00:06:13.570
photorespiration.

00:06:13.570 --> 00:06:16.540
It's just a little less
efficient than the ideal.

00:06:16.540 --> 00:06:20.440
But some plants have-- I guess
we could say-- evolved past

00:06:20.440 --> 00:06:22.120
the photorespiration problem.

00:06:22.120 --> 00:06:24.490
And these are called
C-4 plants.

00:06:24.490 --> 00:06:28.290
Or they perform C-4
photosynthesis.

00:06:31.300 --> 00:06:33.500
And we'll understand, hopefully
in a few minutes,

00:06:33.500 --> 00:06:35.000
why it's called C-4.

00:06:35.000 --> 00:06:37.980
Just as a reminder, when we go
up to the mechanism up here.

00:06:37.980 --> 00:06:42.020
The classic Calvin Cycle, the
first byproduct is this

00:06:42.020 --> 00:06:45.850
phosphoglycerate, this
is a 3-carbon chain.

00:06:45.850 --> 00:06:48.870
So it's saying that the first
time that you fix carbon

00:06:48.870 --> 00:06:51.230
dioxide, or actually the first
time you fix carbon dioxide or

00:06:51.230 --> 00:06:53.220
oxygen, but let's say the first
time you fix carbon

00:06:53.220 --> 00:06:56.810
dioxide you end up with a
3-carbon chain molecule.

00:06:56.810 --> 00:07:02.680
That's why this is called
C-3 photosynthesis.

00:07:02.680 --> 00:07:04.310
So, that's a clue.

00:07:04.310 --> 00:07:08.340
On C-4 plants, the very first
time that they fix carbon

00:07:08.340 --> 00:07:11.870
dioxide they must end up with
a 4-carbon molecule.

00:07:11.870 --> 00:07:15.150
And what they do-- and this is
the interesting thing-- is you

00:07:15.150 --> 00:07:19.350
have all your mesophyll cells
that are out here.

00:07:19.350 --> 00:07:20.110
They're getting air.

00:07:20.110 --> 00:07:22.030
They're getting carbon
dioxide.

00:07:22.030 --> 00:07:24.480
You know, they're getting carbon
dioxide and they're

00:07:24.480 --> 00:07:26.590
getting oxygen and
whatever else.

00:07:26.590 --> 00:07:28.550
So you have all your mesophyll
cells that are getting air.

00:07:28.550 --> 00:07:31.555
But you also have cells that
are deeper within, more

00:07:31.555 --> 00:07:35.150
embedded within the leaf that
aren't being exposed directly

00:07:35.150 --> 00:07:38.060
to the air coming through
the stomata.

00:07:38.060 --> 00:07:39.700
So you have these bundle-sheath
cells.

00:07:39.700 --> 00:07:43.450
And these are actually the cells
that surround the actual

00:07:43.450 --> 00:07:46.300
pipes in the plant that
distribute the fluid up and

00:07:46.300 --> 00:07:46.760
down the plant.

00:07:46.760 --> 00:07:48.880
And we'll do a whole video on
the anatomy of the plant.

00:07:48.880 --> 00:07:51.560
I really just want you to
understand what's going on in

00:07:51.560 --> 00:07:53.420
C-4 photosynthesis.

00:07:53.420 --> 00:07:56.700
So you have these other cells
that are more embedded.

00:07:56.700 --> 00:07:59.250
They don't have direct
access to the air.

00:07:59.250 --> 00:08:01.930
So these are bundle-sheath
cells.

00:08:05.950 --> 00:08:09.460
And what these plants do is,
the carbon dioxide comes in

00:08:09.460 --> 00:08:12.030
and-- so in the standard Calvin
Cycle, everything

00:08:12.030 --> 00:08:14.040
happens in the mesophyll cells
and you have to deal with

00:08:14.040 --> 00:08:15.360
photorespiration.

00:08:15.360 --> 00:08:20.100
In your C-4 plant, or your
plants performing C-4

00:08:20.100 --> 00:08:23.950
photosynthesis, what happens
is the carbon dioxide comes

00:08:23.950 --> 00:08:26.820
in-- so this is in the
mesophyll cell.

00:08:26.820 --> 00:08:28.020
Let me be neat about it.

00:08:28.020 --> 00:08:37.150
So in our mesophyll cell, that's
that right there, you

00:08:37.150 --> 00:08:41.350
have CO2 coming in
and it reacts.

00:08:41.350 --> 00:08:45.930
Instead of reacting with RuBP
or ribulose bisphosphate, it

00:08:45.930 --> 00:08:50.580
reacts with another very hairy
sounding compound, we'll just

00:08:50.580 --> 00:08:51.610
call it PEP.

00:08:51.610 --> 00:08:57.580
But it's phosphoenolpyruvate
So that's PEP.

00:08:57.580 --> 00:09:02.880
You just have to remember,
it is a 3-carbon chain.

00:09:02.880 --> 00:09:05.140
Now let me write down the word,
because sometimes you

00:09:05.140 --> 00:09:07.640
might want to know, what
does PEP stand for?

00:09:07.640 --> 00:09:14.470
It's phosphoenolpyruvate or

00:09:14.470 --> 00:09:16.910
phosphoenolpyruvic acid Either way.

00:09:19.440 --> 00:09:22.270
3-carbon molecule, it's got
other stuff hanging off of

00:09:22.270 --> 00:09:23.930
this but we just have to
remember the carbons.

00:09:23.930 --> 00:09:26.620
So when these two react what are
you going to end up with?

00:09:26.620 --> 00:09:27.870
Well you could guess.

00:09:27.870 --> 00:09:29.930
You have one carbon, you
have three carbons.

00:09:29.930 --> 00:09:32.965
You're going to end up with
a 4-carbon molecule.

00:09:37.750 --> 00:09:42.980
And this reaction right here
is facilitated, not by

00:09:42.980 --> 00:09:44.460
RuBisCo, ribulose bisphosphate
carboxylase oxygenase.

00:09:47.840 --> 00:09:50.720
It's facilitated by a
different enzyme.

00:09:50.720 --> 00:09:51.920
And this is the key.

00:09:51.920 --> 00:09:53.310
This is the key for C-4.

00:09:53.310 --> 00:09:54.970
This is a different enzyme.

00:09:54.970 --> 00:09:58.360
This is called PEP
carboxylase.

00:09:58.360 --> 00:09:59.740
Let me write it down.

00:09:59.740 --> 00:10:00.990
PEP carboxylase.

00:10:03.330 --> 00:10:04.710
And that's a fitting name.

00:10:04.710 --> 00:10:09.880
Remember, RuBisCo or ribulose
bisphosphate carboxylase, it

00:10:09.880 --> 00:10:15.480
reacted ribulose bisphosphate
with carbon or oxygen.

00:10:15.480 --> 00:10:17.570
That's where that oxygenase
comes from.

00:10:17.570 --> 00:10:20.410
But now we have something
that reacts PEP, our

00:10:20.410 --> 00:10:23.580
phosphoenolpyruvate with
carbon dioxide.

00:10:23.580 --> 00:10:26.780
So it's called PEP
carboxylase.

00:10:26.780 --> 00:10:28.800
Actually this is carboxylase,
not carboxylate.

00:10:31.320 --> 00:10:32.330
It's an enzyme.

00:10:32.330 --> 00:10:33.900
This is PEP carboxylase.

00:10:33.900 --> 00:10:36.370
And what's special about PEP
carboxylase, and why it's

00:10:36.370 --> 00:10:39.920
useful in preventing
photorespiration, is that it

00:10:39.920 --> 00:10:41.405
can only fix carbon.

00:10:46.820 --> 00:10:49.420
Not oxygen.

00:10:49.420 --> 00:10:50.950
So you can imagine, this
is occurring in

00:10:50.950 --> 00:10:51.830
the mesophyll cell.

00:10:51.830 --> 00:10:55.020
You have oxygen and carbon
dioxide running around here.

00:10:55.020 --> 00:11:00.305
But only carbon dioxide can
react with the PEP via the PEP

00:11:00.305 --> 00:11:01.960
carboxylase.

00:11:01.960 --> 00:11:04.760
So then they react, they
actually produce oxaloacetic

00:11:04.760 --> 00:11:15.210
acid or oxaloacetate And you
might remember this from the

00:11:15.210 --> 00:11:15.960
Kreb Cycle.

00:11:15.960 --> 00:11:19.510
This was the thing that was the
first reactive species in

00:11:19.510 --> 00:11:20.330
our Kreb Cycle.

00:11:20.330 --> 00:11:24.470
So all of these molecules, they
keep showing up in our

00:11:24.470 --> 00:11:25.370
chemical pathways.

00:11:25.370 --> 00:11:27.980
And that's interesting if you
find that type of thing

00:11:27.980 --> 00:11:28.680
interesting.

00:11:28.680 --> 00:11:32.290
But the important thing is,
they form oxaloacetate and

00:11:32.290 --> 00:11:35.650
then oxaloacetate
gets converted.

00:11:35.650 --> 00:11:37.970
Let me make it, I made
this not as neatly as

00:11:37.970 --> 00:11:39.870
I would like to.

00:11:39.870 --> 00:11:44.620
But then that gets converted to
either malate or aspartate

00:11:44.620 --> 00:11:47.090
but these are all 4-carbon
molecules.

00:11:47.090 --> 00:11:48.330
They're a little different;
they're going to have

00:11:48.330 --> 00:11:51.050
different oxygens and hydrogens
hanging off of them.

00:11:51.050 --> 00:12:04.990
But this is either malate
or aspartate

00:12:04.990 --> 00:12:09.210
Most books will just say, oh it
will eventually just form

00:12:09.210 --> 00:12:10.940
into malate only.

00:12:10.940 --> 00:12:17.740
And then this, this malate, will
then essentially react to

00:12:17.740 --> 00:12:18.965
produce carbon dioxide.

00:12:18.965 --> 00:12:19.730
And you're like, wait.

00:12:19.730 --> 00:12:20.720
That doesn't make sense.

00:12:20.720 --> 00:12:24.800
I have carbon dioxide, it gets
fixed on oxaloacetic acid and

00:12:24.800 --> 00:12:27.610
then it gets turned into malate
or aspartate And then

00:12:27.610 --> 00:12:29.350
later I'm going to turn into
carbon dioxide again.

00:12:29.350 --> 00:12:30.610
What's the whole point?

00:12:30.610 --> 00:12:32.070
And this is the key.

00:12:32.070 --> 00:12:34.440
This is the whole crux
of the issue.

00:12:34.440 --> 00:12:40.620
So now this malate is going to
be converted back into PEP and

00:12:40.620 --> 00:12:42.370
carbon dioxide.

00:12:42.370 --> 00:12:43.960
You're like, what was
the whole point

00:12:43.960 --> 00:12:45.080
of this whole reaction?

00:12:45.080 --> 00:12:47.380
I just ended up with carbon
dioxide and PEP again.

00:12:47.380 --> 00:12:48.690
I'm just going in circles.

00:12:48.690 --> 00:12:50.830
But the neat thing about this,
and the reason why this

00:12:50.830 --> 00:12:55.540
prevents photorespiration is
that this part, this part of

00:12:55.540 --> 00:13:01.030
the reaction right here-- maybe
I should do it like

00:13:01.030 --> 00:13:03.630
this-- this part of the
reaction occurs in the

00:13:03.630 --> 00:13:05.000
mesophyll cell.

00:13:05.000 --> 00:13:07.980
It occurs up here.

00:13:07.980 --> 00:13:10.440
It occurs over here in
the mesophyll cell.

00:13:10.440 --> 00:13:12.250
So you have this malate.

00:13:12.250 --> 00:13:16.120
And then the malate actually
gets transferred into these

00:13:16.120 --> 00:13:18.090
bundle-sheath cells.

00:13:18.090 --> 00:13:23.090
So the malate gets transferred
into the bundle-sheath cells

00:13:23.090 --> 00:13:28.340
via little tubes that connect
the cells called plasmodesmata

00:13:28.340 --> 00:13:30.110
Sounds like the name
of a horror movie.

00:13:30.110 --> 00:13:34.290
So let me draw this a
little bit neater.

00:13:34.290 --> 00:13:40.080
So over here you have, with
exposure to the air, you have

00:13:40.080 --> 00:13:41.910
your mesophyll cell.

00:13:41.910 --> 00:13:46.240
Air is coming in, O2, CO2,
everything is coming in.

00:13:46.240 --> 00:13:50.580
But only CO2 can be
fixed with PEP.

00:13:50.580 --> 00:13:54.860
So you have the PEP here, the
phosphoenolpyruvate So you

00:13:54.860 --> 00:13:56.810
have your Pep here.

00:13:56.810 --> 00:14:00.880
Only CO2 can be reacted with
PEP because of the PEP

00:14:00.880 --> 00:14:02.130
carboxylase.

00:14:05.990 --> 00:14:07.490
This is the enzyme
that's operating.

00:14:07.490 --> 00:14:09.570
So this is much more specific
than the ribulose bisphosphate

00:14:09.570 --> 00:14:11.250
carboxylase, or the RuBisCo.

00:14:16.560 --> 00:14:18.080
So the oxygen just
gets ignored.

00:14:18.080 --> 00:14:20.810
Even though it's hanging around
these mesophyll cells.

00:14:20.810 --> 00:14:23.950
And then these, this
gets converted.

00:14:23.950 --> 00:14:25.500
Oxaloacetic acid.

00:14:25.500 --> 00:14:27.720
And then to malate.

00:14:27.720 --> 00:14:32.200
But once it gets converted to
malate, the malate gets

00:14:32.200 --> 00:14:37.660
transferred deeper into the
actual cell via these

00:14:37.660 --> 00:14:43.860
plasmodesmata So this might be
butting right up against

00:14:43.860 --> 00:14:46.410
bundle-sheath cell, that's
deeper in the cell.

00:14:46.410 --> 00:14:50.280
This bundle-sheath cell has
no access to oxygen.

00:14:50.280 --> 00:14:52.180
So the malate comes in.

00:14:52.180 --> 00:14:54.970
You have these little pipes
that connect the cell.

00:14:54.970 --> 00:14:57.450
Maybe I'll just draw one pipe.

00:14:57.450 --> 00:14:59.340
So let's say there's one pipe.

00:14:59.340 --> 00:15:01.910
So the malate can come here.

00:15:01.910 --> 00:15:05.040
And then, within this deeper
cell, within this

00:15:05.040 --> 00:15:12.260
bundle-sheath cell, it can
then react to form carbon

00:15:12.260 --> 00:15:15.600
dioxide and pyruvate.

00:15:15.600 --> 00:15:17.146
And then the pyruvate--
so let's just

00:15:17.146 --> 00:15:19.750
say this is the pyruvate.

00:15:19.750 --> 00:15:23.040
That pyruvate can then later
go back to actually

00:15:23.040 --> 00:15:24.170
form the PEP again.

00:15:24.170 --> 00:15:26.920
So this can go back through
your plasmodesmata

00:15:26.920 --> 00:15:28.810
and form the PEP.

00:15:28.810 --> 00:15:31.880
So the whole value here is, now
in the bundle-sheath cell,

00:15:31.880 --> 00:15:34.690
I have an environment that
only has carbon dioxide.

00:15:34.690 --> 00:15:36.210
I have no oxygen here.

00:15:36.210 --> 00:15:38.150
I was able to essentially
select for the

00:15:38.150 --> 00:15:40.160
carbon dioxide outside.

00:15:40.160 --> 00:15:43.090
Or closer to the air in
the mesophyll cell.

00:15:43.090 --> 00:15:47.360
And now that I'm deeper within
the plant, I'm in an

00:15:47.360 --> 00:15:49.520
environment that only has CO2,
because I've already

00:15:49.520 --> 00:15:50.240
selected for it.

00:15:50.240 --> 00:15:53.900
And now I can perform
the Calvin Cycle.

00:15:53.900 --> 00:15:56.960
So now, inside this deeper
cell, inside this

00:15:56.960 --> 00:16:00.940
bundle-sheath cell, I can fix
the carbon dioxide with the

00:16:00.940 --> 00:16:04.410
ribulose bisphosphate using
RuBisCo, just like we learned

00:16:04.410 --> 00:16:05.630
in the original Calvin Cycle.

00:16:05.630 --> 00:16:09.560
And we have the whole cycle
and we produce our sugars.

00:16:09.560 --> 00:16:12.670
We produce our
phosphoglyceraldehydees or our

00:16:12.670 --> 00:16:14.350
PGALs, whatever you
want to call them.

00:16:14.350 --> 00:16:17.930
And the whole value this is,
that we were able to avoid the

00:16:17.930 --> 00:16:19.210
photorespiration problem.

00:16:19.210 --> 00:16:21.760
Because now the Calvin Cycle is
occurring in an environment

00:16:21.760 --> 00:16:23.640
that only has carbon dioxide.

00:16:23.640 --> 00:16:25.580
And I think I already mentioned
it, but this is

00:16:25.580 --> 00:16:27.050
called C-4 photosynthesis.

00:16:30.420 --> 00:16:34.120
And it's an adaption to make
sure that you don't waste

00:16:34.120 --> 00:16:38.045
cycles of your Calvin Cycle
through photorespiration.

00:16:38.045 --> 00:16:41.130
And of course it was called C-4,
because the first time

00:16:41.130 --> 00:16:43.350
that carbon gets fixed
it doesn't happen

00:16:43.350 --> 00:16:44.390
in the Calvin Cycle.

00:16:44.390 --> 00:16:47.440
It happens up here, with
PEP carboxylase.

00:16:47.440 --> 00:16:51.060
And it gets fixed with PEP
into a 4-carbon chain.

00:16:51.060 --> 00:16:55.730
And that's why it's called
C-4 photosynthesis.


Photorespiration

WEBVTT
Kind: captions
Language: en

00:00:00.460 --> 00:00:03.410
Let's review the Calvin
cycle a little bit.

00:00:03.410 --> 00:00:05.410
Maybe I'm going to go into a
little bit more detail than

00:00:05.410 --> 00:00:06.470
the last video.

00:00:06.470 --> 00:00:10.110
And by doing that, we'll be
able to appreciate an

00:00:10.110 --> 00:00:13.480
inefficiency, or what we think
is an inefficiency that occurs

00:00:13.480 --> 00:00:15.640
in many photosynthetic plants.

00:00:15.640 --> 00:00:18.350
So just to review the Calvin
cycle, you start

00:00:18.350 --> 00:00:20.500
off with some CO2.

00:00:20.500 --> 00:00:23.950
In the last video, I had six
molecules of CO2 and six

00:00:23.950 --> 00:00:27.150
molecules of ribulose
bisphosphate.

00:00:27.150 --> 00:00:28.590
But in this one, just to show
you that I could have

00:00:28.590 --> 00:00:32.430
literally just divided that last
cycle by two, I'm going

00:00:32.430 --> 00:00:34.210
to start with three and three.

00:00:34.210 --> 00:00:36.640
Three molecules of
carbon dioxide.

00:00:36.640 --> 00:00:40.150
Three molecules of ribulose
bisphosphate.

00:00:40.150 --> 00:00:43.020
If you don't remember what
ribulose bisphosphate was, it

00:00:43.020 --> 00:00:48.410
was just a 5-carbon molecule.

00:00:48.410 --> 00:00:50.090
It had two phosphates on it.

00:00:50.090 --> 00:00:51.800
I can draw the phosphates.

00:00:51.800 --> 00:00:54.510
So you have a phosphate there.

00:00:54.510 --> 00:00:57.070
And you have a phosphate
there.

00:00:57.070 --> 00:00:59.680
And we saw on the last video--
and here I'm going to do a

00:00:59.680 --> 00:01:01.420
little bit more detail.

00:01:01.420 --> 00:01:03.740
Of course carbon dioxide,
there's only one carbon.

00:01:03.740 --> 00:01:05.319
I can just draw a carbon
like that.

00:01:05.319 --> 00:01:07.300
I mean, I don't want to draw
the oxygens right now.

00:01:07.300 --> 00:01:09.520
I just want to focus on just
the carbons and maybe the

00:01:09.520 --> 00:01:10.450
phosphates.

00:01:10.450 --> 00:01:13.210
But in the last video we saw
that these two merged.

00:01:13.210 --> 00:01:16.070
Or these two reacted.

00:01:16.070 --> 00:01:21.950
They reacted in the presence
of ribulose bisphosphate

00:01:21.950 --> 00:01:24.180
carbocylase, which
is this enzyme.

00:01:24.180 --> 00:01:27.230
It was that big protein that I
showed you in the last video.

00:01:27.230 --> 00:01:29.420
And it's called RuBisCo
for short.

00:01:29.420 --> 00:01:32.390
So I'm going to write RuBisCo
in the middle

00:01:32.390 --> 00:01:33.360
of this whole cycle.

00:01:33.360 --> 00:01:37.690
Because all of this is going
on with the RuBisCo enzyme.

00:01:37.690 --> 00:01:40.735
These molecules are joining on
to the RuBisCo enzyme and then

00:01:40.735 --> 00:01:43.860
the ATP and the NADHs are
reacting on different parts.

00:01:43.860 --> 00:01:46.810
And that's essentially what's
driving the whole reaction.

00:01:46.810 --> 00:01:49.640
So you have all of this is
happening with that big

00:01:49.640 --> 00:01:55.370
RuBisCo protein, or enzyme, or
whatever you want to call it.

00:01:55.370 --> 00:01:56.380
And this just stands for
ribulose bisphosphate

00:01:56.380 --> 00:01:58.740
carboxylate.

00:01:58.740 --> 00:02:02.220
So it essentially telling you
that it merges a carbon onto

00:02:02.220 --> 00:02:04.200
ribulose bisphosphate.

00:02:04.200 --> 00:02:07.450
And the last video, I just did
a very hand-wavey, I didn't

00:02:07.450 --> 00:02:08.460
show you all of the
intermediates.

00:02:08.460 --> 00:02:11.880
But I showed you, look, if in
this situation you end up

00:02:11.880 --> 00:02:15.570
using six ATPs and six NADHs,
you'll end up with

00:02:15.570 --> 00:02:17.580
glyceraldehyde 3-phosphate.

00:02:17.580 --> 00:02:21.440
Or G3P.

00:02:21.440 --> 00:02:24.030
Or that's also called
phosphoglyceraldehyde so you

00:02:24.030 --> 00:02:26.330
can also write it as PGAL.

00:02:26.330 --> 00:02:28.790
But a very easy way to visualize
it as just a

00:02:28.790 --> 00:02:32.930
3-carbon chain with
a phosphate.

00:02:32.930 --> 00:02:34.930
With a phosphate group.

00:02:34.930 --> 00:02:40.630
And the last video when I
started with six carbon

00:02:40.630 --> 00:02:43.600
dioxides and six ribulose
bisphosphates.

00:02:43.600 --> 00:02:46.070
And I ended up with
12 of these.

00:02:46.070 --> 00:02:47.770
Now I have three and three.

00:02:47.770 --> 00:02:49.320
Now I'm going to end up
with six of these.

00:02:49.320 --> 00:02:51.810
So I'm going to end
up with six G3Ps.

00:02:51.810 --> 00:02:53.110
And the math works out, right?

00:02:53.110 --> 00:02:56.720
I have three carbons plus three
times five carbons.

00:02:56.720 --> 00:02:59.100
That's a total of 18 carbons.

00:02:59.100 --> 00:03:01.250
And here I have six times
three carbons.

00:03:01.250 --> 00:03:02.700
Or 18 carbons.

00:03:02.700 --> 00:03:04.115
And that's what I showed you in
the last video, but there

00:03:04.115 --> 00:03:08.120
are actually some intermediary
steps.

00:03:08.120 --> 00:03:11.720
So the very first thing that's
actually formed is

00:03:11.720 --> 00:03:12.970
3-phosphoglycerate.

00:03:20.630 --> 00:03:23.190
And the whole reason why I'm
showing you this detail, and

00:03:23.190 --> 00:03:25.380
maybe you're taking a biology
class where they actually go

00:03:25.380 --> 00:03:28.400
into this level of details, is
I want to show that this

00:03:28.400 --> 00:03:32.270
compound also gets produced in
that inefficient cycle that

00:03:32.270 --> 00:03:33.970
we're going to explore
in this video.

00:03:33.970 --> 00:03:36.210
So you get 3-phosphoglycerate,
which is

00:03:36.210 --> 00:03:37.390
also a 3-carbon chain.

00:03:37.390 --> 00:03:38.640
But it's a different
one than this.

00:03:38.640 --> 00:03:41.530
And this isn't ready to produce
glucose just yet.

00:03:41.530 --> 00:03:44.790
But you could also imagine that
as a 3-carbon chain with

00:03:44.790 --> 00:03:46.360
a phosphate group.

00:03:46.360 --> 00:03:50.260
And then, and course you're
going to produce six of these.

00:03:50.260 --> 00:03:51.900
Just to make sure all the carbon
gets accounted for.

00:03:51.900 --> 00:03:55.205
You have 18 carbons here, you
have 18 carbons here.

00:03:55.205 --> 00:03:56.830
So you're going to
have six of these

00:03:56.830 --> 00:04:00.580
3-phosphoglycerates get produced.

00:04:00.580 --> 00:04:04.600
And then each of those gets
a phosphate from ATP.

00:04:04.600 --> 00:04:07.990
So you're going to have
six ATPs come in.

00:04:07.990 --> 00:04:10.460
Remember these ATPs got
produced in the light

00:04:10.460 --> 00:04:14.470
dependent reactions that occur
in the membrane of our

00:04:14.470 --> 00:04:16.130
thylakoids.

00:04:16.130 --> 00:04:19.180
Anyway, we have these ATPs.

00:04:19.180 --> 00:04:22.000
You have six ATPs.

00:04:22.000 --> 00:04:24.820
And then they become six ADPs.

00:04:24.820 --> 00:04:26.780
So essentially they have given
away their phosphate groups.

00:04:26.780 --> 00:04:29.040
So they're going to give one
phosphate group to each of

00:04:29.040 --> 00:04:34.100
these six 3-phosphoglycerates.

00:04:34.100 --> 00:04:36.870
And then you're going to end
up with this, right here.

00:04:36.870 --> 00:04:37.920
I'm running out of space.

00:04:37.920 --> 00:04:40.640
Maybe I didn't do it as neatly
as I should have.

00:04:40.640 --> 00:04:44.240
But you're going to end up with
this three chain carbon,

00:04:44.240 --> 00:04:45.580
or 3-carbon compound.

00:04:45.580 --> 00:04:47.790
It's going to have two
phosphates then.

00:04:47.790 --> 00:04:49.920
And this, you could
know the name.

00:04:49.920 --> 00:04:51.170
This is 1 3-bisphosphoglycerate.

00:04:53.870 --> 00:04:55.890
This was just
3-phosphoglycerate, which

00:04:55.890 --> 00:04:58.220
means that the phosphate
is on the third carbon.

00:04:58.220 --> 00:05:01.190
Now we have 1
3-bisphophoglycerate.

00:05:01.190 --> 00:05:04.160
We have a phosphate on the
1-carbon and the 3-carbon.

00:05:04.160 --> 00:05:05.020
We have two of them.

00:05:05.020 --> 00:05:06.270
So it's bisphosphoglycerate.

00:05:09.080 --> 00:05:11.050
So that's to get
us right there.

00:05:11.050 --> 00:05:14.020
And then to get from the 1
3-biposphoglycerate, and of

00:05:14.020 --> 00:05:16.110
course we have six
of these as well.

00:05:16.110 --> 00:05:17.830
Six of these turn into
six of these.

00:05:17.830 --> 00:05:22.430
And to turn these six 1
3-phosphoglycerate into our

00:05:22.430 --> 00:05:25.690
three glyceraldehyde
3 phosphates, or

00:05:25.690 --> 00:05:28.430
phosphoglyceraldehyde the
names are very daunting.

00:05:28.430 --> 00:05:35.920
This is where we're actually
going to oxidize our NADPH.

00:05:35.920 --> 00:05:39.030
Remember oxidizing is losing
electrons or losing hydrogens

00:05:39.030 --> 00:05:41.280
with their electrons,
either way.

00:05:41.280 --> 00:05:46.170
So here, on this step right
here, two things are going to

00:05:46.170 --> 00:05:47.560
happen, actually.

00:05:47.560 --> 00:05:57.760
You're going to have six NADPHs
become six NAD pluses.

00:05:57.760 --> 00:06:00.220
So they're losing that hydrogen
and the electrons.

00:06:00.220 --> 00:06:03.450
And also one phosphate is going
to be lost from each of

00:06:03.450 --> 00:06:04.930
these molecules.

00:06:04.930 --> 00:06:07.290
And then you're going to
also have plus six of

00:06:07.290 --> 00:06:08.140
the phosphate groups.

00:06:08.140 --> 00:06:09.440
The phosphate groups don't
get lost here.

00:06:09.440 --> 00:06:12.190
They get added onto these
molecules right there.

00:06:12.190 --> 00:06:16.360
And then we end up with our
sixth glyceraldehyde

00:06:16.360 --> 00:06:19.310
3-phosphates, which we learned
in the last video, we can then

00:06:19.310 --> 00:06:22.720
use to produce fuel or
glucose or other

00:06:22.720 --> 00:06:24.370
carbohydrates in the cell.

00:06:24.370 --> 00:06:27.740
But we also learned that this
is called the Calvin cycle.

00:06:27.740 --> 00:06:32.380
So all of it doesn't get used
to produce actual glucose or

00:06:32.380 --> 00:06:34.850
other carbohydrates.

00:06:34.850 --> 00:06:43.310
Most of it, out of the six, five
of them are going to be

00:06:43.310 --> 00:06:47.190
used to produce our three
ribulose bisphosphates.

00:06:47.190 --> 00:06:49.870
So let me do it like this.

00:06:49.870 --> 00:06:55.440
So you're going to have five
G3Ps go in that direction.

00:06:55.440 --> 00:06:59.030
So for every six G3Ps you
produce, five go back into the

00:06:59.030 --> 00:07:01.670
cycle and then one
exits the cycle.

00:07:01.670 --> 00:07:02.580
We saw this in the last video.

00:07:02.580 --> 00:07:04.822
In the last video I had 12
here, so 10 went in this

00:07:04.822 --> 00:07:06.420
direction and two in
this direction.

00:07:06.420 --> 00:07:08.240
I'm just dividing everything
by two in this video.

00:07:08.240 --> 00:07:10.970
And the whole reason why I
multiplied by two in the last

00:07:10.970 --> 00:07:13.360
video is so that we end up
with two going into this

00:07:13.360 --> 00:07:15.170
direction because two
is enough to produce

00:07:15.170 --> 00:07:16.310
at least one glucose.

00:07:16.310 --> 00:07:17.270
But we don't have to have two.

00:07:17.270 --> 00:07:20.260
We could just say we do this
whole cycle twice to get two.

00:07:20.260 --> 00:07:24.760
But anyway, I have one
glyceraldehyde 3-phosphate

00:07:24.760 --> 00:07:26.250
coming out of this end.

00:07:26.250 --> 00:07:29.550
And then these five
glyceraldehyde 3-phosphates

00:07:29.550 --> 00:07:31.410
that stay in the cycle.

00:07:31.410 --> 00:07:33.950
Remember those are
just 3-carbons

00:07:33.950 --> 00:07:34.890
with a phosphate group.

00:07:34.890 --> 00:07:36.560
I drew it over here already.

00:07:36.560 --> 00:07:42.430
They will then use another
three ATP.

00:07:51.600 --> 00:07:54.630
So we end up with three ADPs.

00:07:54.630 --> 00:07:56.490
And the phosphate groups
actually end up on the

00:07:56.490 --> 00:08:00.130
ribulose bisphosphate and they
get essentially recycled into

00:08:00.130 --> 00:08:03.600
ribulose bisphosphate for the
whole cycle to happen again.

00:08:03.600 --> 00:08:05.830
Remember when all this is
happening you can kind of

00:08:05.830 --> 00:08:09.150
imagine in or on the surface
of this big enzyme.

00:08:09.150 --> 00:08:10.400
This ribulose bisphosphate
carboxylase.

00:08:12.980 --> 00:08:16.940
And actually, just so you know
a piece of terminology, this

00:08:16.940 --> 00:08:20.020
type of photosynthesis is called
C3 photosynthesis.

00:08:26.650 --> 00:08:30.250
And the whole reason why it's
called C3 photosynthesis is

00:08:30.250 --> 00:08:35.409
because the very first product
that you get when you react

00:08:35.409 --> 00:08:38.840
with carbon dioxide, when the
first time you fix your carbon

00:08:38.840 --> 00:08:41.850
dioxide-- remember carbon
dioxide fixation is when you

00:08:41.850 --> 00:08:43.450
take it in its gaseous
form and you actually

00:08:43.450 --> 00:08:45.600
put it into a molecule.

00:08:45.600 --> 00:08:48.470
The very first carbon molecule
you get once the carbon

00:08:48.470 --> 00:08:51.330
dioxide gets fixed is
a 3-carbon molecule.

00:08:51.330 --> 00:08:53.020
It's phosphoglycerate.

00:08:53.020 --> 00:08:54.170
That's why I showed
it to you there.

00:08:54.170 --> 00:08:57.810
Because that's where
the three in the C3

00:08:57.810 --> 00:08:59.920
photosynthesis comes from.

00:08:59.920 --> 00:09:03.840
Now, everything I've just done
is a review of what we did in

00:09:03.840 --> 00:09:04.740
the last video.

00:09:04.740 --> 00:09:06.250
Maybe with a little
bit more detail.

00:09:06.250 --> 00:09:07.470
And I showed you that you
don't have to start

00:09:07.470 --> 00:09:08.250
with six and six.

00:09:08.250 --> 00:09:10.960
You could start with three and
three and just have five go

00:09:10.960 --> 00:09:13.630
down here and one go down
here instead of doubling

00:09:13.630 --> 00:09:14.065
everything.

00:09:14.065 --> 00:09:17.230
But what I want to show you now
is an inefficiency that

00:09:17.230 --> 00:09:18.560
goes on in plants.

00:09:18.560 --> 00:09:22.780
This RuBisCo doesn't
necessarily

00:09:22.780 --> 00:09:25.640
just fix carbon dioxide.

00:09:25.640 --> 00:09:29.300
It can also react with oxygen.

00:09:29.300 --> 00:09:31.140
And actually, its name is
ribulose bisphosphate

00:09:31.140 --> 00:09:36.570
carboxylase oxygenase, which
means it can react

00:09:36.570 --> 00:09:39.030
with carbon or oxygen.

00:09:39.030 --> 00:09:41.825
And this mechanism-- so let's
say you have your ribulose

00:09:41.825 --> 00:09:43.740
bisphosphate hanging around.

00:09:43.740 --> 00:09:50.380
You know we're in the stroma
of our chloroplast.

00:09:50.380 --> 00:09:53.610
So let's say we have our five
ribulose bisphosphates.

00:10:03.070 --> 00:10:07.810
And instead of reacting with
carbon it can actually react

00:10:07.810 --> 00:10:10.350
with oxygen.

00:10:10.350 --> 00:10:12.820
So instead of having carbon
dioxide here, I can have O2

00:10:12.820 --> 00:10:14.180
coming in here.

00:10:14.180 --> 00:10:15.940
We have oxygen coming in here.

00:10:15.940 --> 00:10:19.560
And all of this, once again, is
occurring on the surface or

00:10:19.560 --> 00:10:23.510
with the assistance
of the RuBisCo.

00:10:23.510 --> 00:10:27.290
The exact same, the
RuBisCo enzyme.

00:10:27.290 --> 00:10:28.540
Ribulose bisphosphate
carboxylase oxygenase.

00:10:31.710 --> 00:10:32.210
That's why.

00:10:32.210 --> 00:10:34.530
Because it can also
fix oxygen.

00:10:34.530 --> 00:10:38.800
And when these two things react,
you don't get useful

00:10:38.800 --> 00:10:42.810
things that can be used for
fuel and all of that.

00:10:42.810 --> 00:10:47.930
You end up with-- and my drawing
is a little messy--

00:10:47.930 --> 00:10:52.240
you end up with-- well when
these two guys react-- you'll

00:10:52.240 --> 00:10:59.330
end up with five molecules
of this guy right here.

00:10:59.330 --> 00:11:03.975
Five molecules of this

00:11:03.975 --> 00:11:07.310
3-phosphoglycerate right there.

00:11:07.310 --> 00:11:08.800
Remember here we ended
up with six of them.

00:11:08.800 --> 00:11:10.380
But now we're only going
to end up with five

00:11:10.380 --> 00:11:11.630
phosphoglycerates.

00:11:17.770 --> 00:11:20.070
I know the names are
all very confusing.

00:11:20.070 --> 00:11:22.550
But the basic idea is to
remember that what's happening

00:11:22.550 --> 00:11:25.950
here is if carbon dioxide isn't
getting fixed, oxygen

00:11:25.950 --> 00:11:26.630
can get fixed.

00:11:26.630 --> 00:11:28.880
And you end up with five
phosphoglycerates.

00:11:28.880 --> 00:11:32.380
And you actually also end up
with five-- this is called

00:11:32.380 --> 00:11:34.630
phosphoglycolate.

00:11:34.630 --> 00:11:37.400
I know these are very
daunting names.

00:11:37.400 --> 00:11:44.300
Phosphoglycolate, which is
a 2-carbon molecule.

00:11:44.300 --> 00:11:46.380
Which makes sense because we
only have five carbons to

00:11:46.380 --> 00:11:47.350
start with.

00:11:47.350 --> 00:11:51.650
This thing right here is
a 3-carbon molecule.

00:11:51.650 --> 00:11:55.080
That's a 3-carbon molecule
and this right here

00:11:55.080 --> 00:11:56.760
is a 2-carbon molecule.

00:11:56.760 --> 00:12:00.010
So this right here is going to
be a 2-carbon molecule and it

00:12:00.010 --> 00:12:01.880
has a phosphate group.

00:12:01.880 --> 00:12:04.760
So as you can imagine, in this
situation we're not going to

00:12:04.760 --> 00:12:07.710
be able to keep going forward
and produce our glyceraldehyde

00:12:07.710 --> 00:12:11.100
3-phosphate, which we can then
use to make up carbohydrates.

00:12:11.100 --> 00:12:11.950
We're stuck.

00:12:11.950 --> 00:12:13.925
We just have these five
phosphoglycerates.

00:12:16.960 --> 00:12:20.420
These can go on and some
percentage of them, but the

00:12:20.420 --> 00:12:21.750
ratios are all getting
messed up.

00:12:21.750 --> 00:12:23.690
But everything doesn't
necessarily happen this

00:12:23.690 --> 00:12:24.760
cleanly in the cell.

00:12:24.760 --> 00:12:28.600
These things can go on, but we
have one less being produced.

00:12:28.600 --> 00:12:31.850
And this thing right here, it's
actually using up some of

00:12:31.850 --> 00:12:35.750
our carbon from our ribulose
bisphosphate.

00:12:35.750 --> 00:12:37.785
And if this thing kept going
up, all of our ribulose

00:12:37.785 --> 00:12:39.910
bisphosphate is going
to get eaten up.

00:12:39.910 --> 00:12:43.400
We're not going to be able to
continue on into this cycle

00:12:43.400 --> 00:12:44.450
over and over again.

00:12:44.450 --> 00:12:47.800
And actually this is kind
of a waste product.

00:12:47.800 --> 00:12:51.700
Right here this is
a waste product.

00:12:51.700 --> 00:12:53.410
Or we think it's a
waste product.

00:12:53.410 --> 00:12:56.770
And this actually has to exit
your chloroplast and actually

00:12:56.770 --> 00:12:59.770
get processed by other
organelles in the plant cells.

00:12:59.770 --> 00:13:04.060
And these are called, these
waste processing organelles in

00:13:04.060 --> 00:13:08.350
your cells, these are
peroxisomes I know there's a

00:13:08.350 --> 00:13:11.060
lot of complicated
terminology here.

00:13:11.060 --> 00:13:15.690
But the important thing to
remember, when RuBisCo fixes

00:13:15.690 --> 00:13:19.200
oxygen, this is actually called
photorespiration.

00:13:19.200 --> 00:13:21.220
That's an important
word to know.

00:13:21.220 --> 00:13:22.470
Photorespiration.

00:13:24.860 --> 00:13:28.670
All of a sudden, instead of
being able to carry forward

00:13:28.670 --> 00:13:32.040
with your Calvin cycle and
produce a lot of sugar,

00:13:32.040 --> 00:13:34.300
instead you are depleting
your RuBisCo.

00:13:34.300 --> 00:13:36.060
So this is a very bad process.

00:13:36.060 --> 00:13:38.000
This is going to get in the
way of your Calvin cycle.

00:13:38.000 --> 00:13:44.220
And remember how important
each of these G3Ps are.

00:13:44.220 --> 00:13:46.500
Because for every turn of the
Calvin cycle, or at least the

00:13:46.500 --> 00:13:49.640
way I did it, we only produce
one G3P that actually gets

00:13:49.640 --> 00:13:51.220
used for something useful.

00:13:51.220 --> 00:13:58.530
The other five G3Ps have to go
back to produce ribulose

00:13:58.530 --> 00:14:00.480
bisphosphate.

00:14:00.480 --> 00:14:02.320
RuBisCo of course
is the enzyme.

00:14:02.320 --> 00:14:05.610
So in this situation we only
have five phosphoglycerates to

00:14:05.610 --> 00:14:06.230
begin with.

00:14:06.230 --> 00:14:10.650
Maybe if we have our ATP or an
NADH, we can convert these

00:14:10.650 --> 00:14:13.770
five-- and it doesn't have to
go in this direction-- but

00:14:13.770 --> 00:14:17.870
maybe we can convert these five
phosphoglycerates into

00:14:17.870 --> 00:14:20.610
five glyceraldehyde
3-phosphates.

00:14:20.610 --> 00:14:21.930
G3Ps.

00:14:21.930 --> 00:14:26.300
Then all of these are going to
be used back to produce our

00:14:26.300 --> 00:14:28.160
ribulose bisphosphate.

00:14:28.160 --> 00:14:30.690
So to go from here to here,
we actually had to

00:14:30.690 --> 00:14:33.160
use up ATP and NADH.

00:14:33.160 --> 00:14:36.090
And then to go from here to
here, we have to use more ATP.

00:14:36.090 --> 00:14:38.350
But we went through this whole
cycle and we didn't produce

00:14:38.350 --> 00:14:41.820
anything, in terms of useful
things that can be used to--

00:14:41.820 --> 00:14:44.050
essentially sugars or
carbohydrates that can be used

00:14:44.050 --> 00:14:47.940
to fuel or provide structure
for the plant in any way.

00:14:47.940 --> 00:14:50.490
So this is a completely--
or we think-- this

00:14:50.490 --> 00:14:51.765
is a wasteful process.

00:14:55.990 --> 00:15:00.040
So people wonder, when you
look a lot of biological

00:15:00.040 --> 00:15:02.560
systems you don't see wasteful
processes all the time.

00:15:02.560 --> 00:15:05.240
You would say, well wouldn't
natural selection have

00:15:05.240 --> 00:15:06.480
selected against this?

00:15:06.480 --> 00:15:09.490
And some people believe that
this is just a remnant from

00:15:09.490 --> 00:15:13.510
our evolutionary past, or our
plants' evolutionary past,

00:15:13.510 --> 00:15:15.860
where there wasn't a lot of
oxygen in the atmosphere.

00:15:15.860 --> 00:15:21.040
And if there wasn't a lot of
oxygen in the atmosphere, this

00:15:21.040 --> 00:15:23.030
was not that likely
of an occurrence.

00:15:23.030 --> 00:15:25.630
But there's some people who
actually believe that no, this

00:15:25.630 --> 00:15:27.770
might actually have
been selected

00:15:27.770 --> 00:15:29.100
for in natural selection.

00:15:29.100 --> 00:15:31.830
Because if there is a lot of
oxygen hanging around in the

00:15:31.830 --> 00:15:37.710
cell, more than you need, that
oxygen might actually react

00:15:37.710 --> 00:15:41.190
with your ATP and create other
harmful compounds in the cell.

00:15:41.190 --> 00:15:45.030
And this might be a way of
sopping up the harmful oxygen

00:15:45.030 --> 00:15:47.110
that's actually hanging
around the cell.

00:15:47.110 --> 00:15:48.050
So who knows?

00:15:48.050 --> 00:15:49.390
But it's interesting idea.

00:15:49.390 --> 00:15:53.235
That you have this one enzyme
that can react with ribulose

00:15:53.235 --> 00:15:56.210
bisphosphate and
carbon dioxide.

00:15:56.210 --> 00:15:59.320
And if that happens, we just get
our regular Calvin cycle.

00:15:59.320 --> 00:16:04.250
Or it can react with ribulose
bisphosphate and oxygen and

00:16:04.250 --> 00:16:07.310
actually fix oxygen into
these two molecules.

00:16:07.310 --> 00:16:09.610
Especially the phosphoglycolate
which we

00:16:09.610 --> 00:16:11.130
think is a waste compound.

00:16:11.130 --> 00:16:14.320
And if you just went on and on
in the Calvin cycle in this

00:16:14.320 --> 00:16:19.460
way, you will not produce
any useful sugars.

00:16:19.460 --> 00:16:21.960
So in the next video, what I
want to do is study some

00:16:21.960 --> 00:16:25.310
plants that have been
able to get by this

00:16:25.310 --> 00:16:26.850
photorespiration problem.

00:16:26.850 --> 00:16:28.850
And you could imagine,
photorespiration could be a

00:16:28.850 --> 00:16:33.200
really big deal or it could be
very harmful in situations

00:16:33.200 --> 00:16:37.040
where, one, it's very important
for a plant to be

00:16:37.040 --> 00:16:38.730
very productive of sugars.

00:16:38.730 --> 00:16:41.890
And it can also, we'll see,
it can be a problem-- it's

00:16:41.890 --> 00:16:43.550
definitely a problem if there's
a huge amount of

00:16:43.550 --> 00:16:44.440
oxygen content.

00:16:44.440 --> 00:16:46.890
But in the next video I'm going
to show you plants that

00:16:46.890 --> 00:16:50.600
have gotten around this
problem by, instead of

00:16:50.600 --> 00:16:55.030
performing C-3 photosynthesis,
which is the classic Calvin

00:16:55.030 --> 00:16:58.320
cycle that I just showed
you, they perform C-4

00:16:58.320 --> 00:16:59.130
photosynthesis.

00:16:59.130 --> 00:17:01.840
And I'll show you what that
means in the next video.