Impact on median and mean when increasing highest value

WEBVTT
Kind: captions
Language: en

00:00:00.188 --> 00:00:01.325
- [Voiceover] Let's think about what happens

00:00:01.325 --> 00:00:04.181
to the median and mean of a set of numbers

00:00:04.181 --> 00:00:06.225
when I change one of the numbers.

00:00:06.225 --> 00:00:08.035
And so let's look at this example.

00:00:08.035 --> 00:00:11.101
A group of four friends likes to bowl together,

00:00:11.101 --> 00:00:15.186
and each friend keeps track of his all-time highest score

00:00:15.186 --> 00:00:16.906
in a single game.

00:00:16.906 --> 00:00:21.201
Their high scores are all between 180 and 220,

00:00:21.201 --> 00:00:25.357
except for Adam, whose high score is 250.

00:00:25.357 --> 00:00:27.725
Adam then bowls a great game

00:00:27.725 --> 00:00:31.092
and has a new high score of 290.

00:00:31.092 --> 00:00:35.110
How will increasing Adam's high score affect the mean

00:00:35.110 --> 00:00:37.199
and median?

00:00:37.199 --> 00:00:39.149
Now, like always, pause this video

00:00:39.149 --> 00:00:41.257
and see if you can figure this out yourself.

00:00:42.307 --> 00:00:43.956
All right, so let's just think about what they're saying.

00:00:43.956 --> 00:00:45.698
We have four friends,

00:00:45.698 --> 00:00:50.063
and they each keep track of their all-time high score,

00:00:50.063 --> 00:00:51.548
so we're gonna have four data points,

00:00:51.548 --> 00:00:55.100
an all-time high score for each of the friends.

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So let's see, this is the lowest score of the friends,

00:00:57.549 --> 00:00:59.917
this is the second lowest, second to highest,

00:00:59.917 --> 00:01:02.447
and this is the highest scoring of the friends.

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So let's see.

00:01:03.423 --> 00:01:06.070
Their high scores are all between 100 and 220,

00:01:06.070 --> 00:01:08.973
except for Adam, whose high score is 250.

00:01:08.973 --> 00:01:12.525
So before Adam bowls this super-awesome game,

00:01:12.525 --> 00:01:14.220
the scores look something like this.

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The lowest score is 180.

00:01:16.976 --> 00:01:21.202
Adam scores 250.

00:01:21.202 --> 00:01:22.711
And if you take Adam out of the picture,

00:01:22.711 --> 00:01:25.056
the highest score is a 220.

00:01:25.056 --> 00:01:26.821
And we actually don't know what this score right

00:01:26.821 --> 00:01:28.422
over there is.

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Now, after Adam bowls a great new game

00:01:30.628 --> 00:01:32.903
and has a new high score of 290,

00:01:32.903 --> 00:01:34.691
what does the data set look like?

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Well, this guy's high score hasn't changed.

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This guy's high score hasn't changed.

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This guy's high score hasn't changed.

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But now Adam has a new high score.

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Instead of 250, it is now 290.

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So my question is,

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well, the first question is, does this change the median?

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Well, remember the median is the middle number.

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And if we're looking at four numbers here,

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the median is going to be the average

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of the middle two numbers.

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So we're gonna take the average

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of whatever this question mark is and 220.

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That's going to be the median.

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Now, over here, after Adam has scored a new high score,

00:02:15.396 --> 00:02:17.137
how do we calculate the median?

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Well, we still have four numbers,

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and the middle two are still the same two middle numbers.

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Whatever this friend's highest score was, it hasn't changed,

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and so we're gonna have the same median.

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It's gonna be 220 plus question mark, divided by two.

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It's gonna be halfway between question mark and 220.

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So our median won't change.

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So median, no change.

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Median, no change.

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So let's think about the mean now.

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Well, the mean, you take the sum of all these numbers

00:02:48.356 --> 00:02:49.889
and then you divide by four,

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and then you take the sum

00:02:50.775 --> 00:02:52.930
of all these numbers and divide by four.

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So which sum is going to be higher?

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Well, the first three numbers are the same,

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but in the second list you have a higher number.

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290 is higher than 250.

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So if you take these four and divide by four,

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you're going to have a larger value

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than if you take these four and divide by four,

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because their sum is gonna be larger.

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And so the mean is going to go up.

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The mean will increase.

00:03:19.194 --> 00:03:21.563
The mean will increase.

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So median, no change, and mean, increase.

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All right, so this says both increase. No, that's not right.

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The median will increase. No, median doesn't change.

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The mean will increase. Yep.

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And the median will stay the same.

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Yep, that's exactly what we're talking about.

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And if you wanna make it a little bit more tangible,

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you can replace question mark with some number.

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You could replace it, maybe this question mark is 200.

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And if you try it out with 200,

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just to make things tangible, you're going to see that

00:03:50.634 --> 00:03:52.329
that is indeed going to be the case.

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The median would be halfway between these two numbers.

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And I just arbitrarily picked 200.

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It could be any number some place

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between 180 and 220,

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but you see, for this example,

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it's very tangibly that the median does not change.

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But the mean increases, 'cause we increase the sum.

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We increase the sum by 40,

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because we've increased this last number,

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and only the last number, by 40.


Exocytosis

WEBVTT
Kind: captions
Language: en

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- Let's talk a little bit about

00:00:01.850 --> 00:00:03.580
exocytosis,

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which is how the cell is able to

00:00:05.490 --> 00:00:07.690
release larger molecules

00:00:07.690 --> 00:00:09.650
that might be used by the rest of the body.

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Other cells, or maybe it's going to be part

00:00:11.420 --> 00:00:13.720
of the extracellular matrix.

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And to understand how this works,

00:00:15.090 --> 00:00:17.090
it's really the reverse of endocytosis,

00:00:17.090 --> 00:00:18.490
we're going to go

00:00:18.490 --> 00:00:20.090
and produce some proteins

00:00:20.090 --> 00:00:21.690
in the endoplasmic reticulum,

00:00:21.690 --> 00:00:23.120
this is our classic example.

00:00:23.120 --> 00:00:25.220
Those proteins are then going to bud off,

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in their own little vesicles,

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which then merge with the golgi apparatus

00:00:29.390 --> 00:00:31.390
where they are further processed.

00:00:31.390 --> 00:00:33.990
So they're processed in the golgi apparatus,

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right over here,

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and then eventually they're going to bud off

00:00:36.490 --> 00:00:39.150
of the golgi apparatus in new vesicles,

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and those vesicles are going to make their way

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over to the cell's outer membrane,

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the plasma membrane

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and the membranes of the vesicles

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are going to merge with the membrane

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of the cell

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and in doing so, they're going to release

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their contents.

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And this is classic exocytosis,

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there are other cases

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where maybe it

00:00:56.120 --> 00:00:57.790
merges partially, releases the contents

00:00:57.790 --> 00:00:59.050
and then buds back

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is called the "kiss and run" method

00:01:00.820 --> 00:01:01.890
of exocytosis,

00:01:01.890 --> 00:01:03.020
but the classic one is

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it merges with the membrane.

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We can look at this,

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this membrane

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after the vesicle's membrane has merged

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with the plasma membrane,

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the membrane might look like this.

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So if the vesicle,

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let me do the vesicle's membrane in orange,

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if the vesicle's membrane is at an orange,

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well now it has merged like this

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and it has released its contents.

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It has released the protein

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to be used someplace else,

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someplace else in the body.

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And I want to be clear,

00:01:32.920 --> 00:01:33.590
this membrane,

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and I talk about it many times

00:01:35.350 --> 00:01:36.190
in many other videos,

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even though I've drawn it as one line

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right over here,

00:01:40.790 --> 00:01:44.190
this is going to be a phospholipid bilayer,

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so if we were to zoom in,

00:01:47.430 --> 00:01:48.990
it would look like this.

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It's a phospholipid bilayer.

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So these are some of the phospholipids

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that were part of the original

00:01:57.620 --> 00:01:59.090
part of the original membrane

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and then we also, in my little box,

00:02:00.750 --> 00:02:03.020
I get some of the ones that are part of,

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or that were part of the vesicle

00:02:04.890 --> 00:02:07.250
that was holding that protein.

00:02:07.250 --> 00:02:09.120
That were part of the vesicle

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that was holding the protein.

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So I really want to stress

00:02:12.020 --> 00:02:13.290
I want to really stress these lines

00:02:13.290 --> 00:02:14.110
that I'm drawing

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these are bilayers of phospholilpids.

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And all of these lines,

00:02:18.090 --> 00:02:18.920
these membranes that you see,

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these are all bilayers of phospholipids.

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Just to make sure

00:02:23.220 --> 00:02:25.750
that we are visualizing this correct.

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And that's what exocytosis is

00:02:27.450 --> 00:02:29.490
and one thing that I find interesting

00:02:29.490 --> 00:02:30.690
is when you first learn about it

00:02:30.690 --> 00:02:32.120
you see it diagrammed like this

00:02:32.120 --> 00:02:32.750
and you just assume,

00:02:32.750 --> 00:02:35.320
okay, well, these bubbles of these

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with these membranes

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they just randomly must float

00:02:37.750 --> 00:02:39.590
eventually to the membrane

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where they get merged

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and then they release their contents.

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But actually it isn't that chaotic.

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They actually can sit on tracks

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so they actually can sit on tracks.

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Remember we talked about the cytoskeleton

00:02:49.650 --> 00:02:50.650
which isn't drawn enough

00:02:50.650 --> 00:02:51.690
probably because it makes drawings

00:02:51.690 --> 00:02:52.590
really messy

00:02:52.590 --> 00:02:53.590
but whenever we think about a cell

00:02:53.590 --> 00:02:55.390
there's all this structure to it.

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There's all this structure to it,

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microtubules, microfilaments,

00:02:59.190 --> 00:03:00.520
intermediate filaments,

00:03:00.520 --> 00:03:02.250
all of these things over here

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that not only provide structure to the cell,

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but they can be used to transport.

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And these vesicles

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these vesicles can actually ride

00:03:10.550 --> 00:03:11.750
on these structures

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and you could actually have

00:03:14.960 --> 00:03:17.550
motor proteins that are using ATP

00:03:17.550 --> 00:03:19.740
to actively push the vesicle

00:03:19.740 --> 00:03:20.890
containing its contents

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so this is a kind of transportation.

00:03:22.520 --> 00:03:25.020
It's really like a factory

00:03:25.020 --> 00:03:27.450
to push them towards the membrane

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so that they can be released.

00:03:29.150 --> 00:03:30.720
So whenever I think about it, it's fascinating

00:03:30.720 --> 00:03:31.890
'cause I always talk about these cells

00:03:31.890 --> 00:03:34.150
being a universe unto themselves.

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And they aren't just these blobs,

00:03:35.890 --> 00:03:36.890
they have all of these structures,

00:03:36.890 --> 00:03:38.090
there's all of these proteins

00:03:38.090 --> 00:03:40.250
that are really on an unbelievably small scale

00:03:40.250 --> 00:03:43.620
able to do these fairly intricate processes.

00:03:43.620 --> 00:03:45.420
So what I just showed you, once again,

00:03:45.420 --> 00:03:47.350
this is classic exocytosis,

00:03:47.350 --> 00:03:49.680
you'll see it when you have proteins, lipids,

00:03:49.680 --> 00:03:51.020
being produced by the cell

00:03:51.020 --> 00:03:52.690
that need to be released somehow.

00:03:52.690 --> 00:03:54.980
They're also famously used in

00:03:54.980 --> 00:03:57.710
neurons,

00:03:57.710 --> 00:03:58.790
the chemical signal,

00:03:58.790 --> 00:04:00.720
when you go from one neuron to another,

00:04:00.720 --> 00:04:03.890
you have exocytosis of neurotransmitters

00:04:03.890 --> 00:04:06.290
that will trigger the next neuron.

00:04:06.290 --> 00:04:08.920
So these are very, very important processes.

00:04:08.920 --> 00:04:11.410
Our bodies would not function properly

00:04:11.410 --> 00:04:12.890
or, wouldn't function at all,

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if we did not have exocytosis.


Uniporters, symporters and antiporters

WEBVTT
Kind: captions
Language: en

00:00:00.083 --> 00:00:02.289
- [Voiceover] So let's talk about all of the different ways

00:00:02.289 --> 00:00:04.030
that molecules can be transported

00:00:04.030 --> 00:00:06.282
across a cellular membrane.

00:00:06.282 --> 00:00:07.954
So perhaps the most basic,

00:00:07.954 --> 00:00:10.346
the most passive of the passive transports

00:00:10.346 --> 00:00:11.971
would be straight up diffusion.

00:00:11.971 --> 00:00:13.550
And if you have a small enough molecule,

00:00:13.550 --> 00:00:15.203
let's say this is molecular oxygen.

00:00:15.203 --> 00:00:18.639
It's small, it doesn't have any charge, it has no polarity.

00:00:18.639 --> 00:00:20.779
That will be able to diffuse down

00:00:20.779 --> 00:00:24.007
its concentration gradient through the cellular membrane.

00:00:24.007 --> 00:00:26.398
But as we start to talk about things with more charge

00:00:26.398 --> 00:00:27.768
or things that might be larger

00:00:27.768 --> 00:00:29.631
then we're going to need some help.

00:00:29.631 --> 00:00:32.346
Now the first type of help is just help to allow things

00:00:32.346 --> 00:00:34.990
to flow down their concentration gradient.

00:00:34.990 --> 00:00:37.038
And that we call facilitated diffusion.

00:00:37.038 --> 00:00:39.847
We have a whole video on facilitated diffusion.

00:00:39.847 --> 00:00:41.217
And one form of facilitated diffusion,

00:00:41.217 --> 00:00:43.214
Hey, just open up a tunnel and let things

00:00:43.214 --> 00:00:44.844
flow down their concentration gradient.

00:00:44.844 --> 00:00:48.354
We saw that with the potassium ion channels,

00:00:48.354 --> 00:00:51.424
where potassium builds up on the inside of the cell

00:00:51.424 --> 00:00:53.217
because of the sodium potassium pump,

00:00:53.217 --> 00:00:55.748
let me just be clear, down here this is the inside,

00:00:55.748 --> 00:00:58.627
this is the outside of the cell,

00:00:58.627 --> 00:01:00.814
and then but then these channels allow the potassium

00:01:00.814 --> 00:01:02.398
to flow down their concentration gradient.

00:01:02.398 --> 00:01:04.395
It's gonna be put in check because of its charge

00:01:04.395 --> 00:01:05.375
and its more positive outside

00:01:05.375 --> 00:01:06.977
and we'll talk about in other videos,

00:01:06.977 --> 00:01:08.401
but it's just a simple tunnel.

00:01:08.401 --> 00:01:10.537
Now sometimes that tunnel is gated.

00:01:10.537 --> 00:01:12.975
It's only going to be open if a certain trigger is hit.

00:01:12.975 --> 00:01:15.251
And we see that when we talk about signals

00:01:15.251 --> 00:01:19.086
going down a neuron voltage gated channel.

00:01:19.086 --> 00:01:21.060
Once the voltage hits a certain amount,

00:01:21.060 --> 00:01:22.871
then the channel opens, and then the sodium

00:01:22.871 --> 00:01:24.705
that has a higher concentration outside,

00:01:24.705 --> 00:01:27.930
can flow down its concentration gradient inside.

00:01:27.930 --> 00:01:31.235
But both of these, this is considered passive transport,

00:01:31.235 --> 00:01:33.835
it's facilitated diffusion, passive transport,

00:01:33.835 --> 00:01:35.437
we're allowing things to flow down

00:01:35.437 --> 00:01:37.132
their concentration gradient.

00:01:37.132 --> 00:01:39.412
You can see here, the sodium is going in,

00:01:39.412 --> 00:01:41.479
I'm sorry, the potassium is going in the direction

00:01:41.479 --> 00:01:42.621
of its concentration gradient,

00:01:42.621 --> 00:01:45.059
it's high concentration inside,

00:01:45.059 --> 00:01:46.434
low concentration outside,

00:01:46.434 --> 00:01:47.711
so we're allowing it to flow down

00:01:47.711 --> 00:01:49.266
the concentration gradient.

00:01:49.266 --> 00:01:52.383
Here the sodium is high concentration outside,

00:01:52.383 --> 00:01:54.496
low concentration inside, and this happened

00:01:54.496 --> 00:01:56.799
because of the sodium potassium pump,

00:01:56.799 --> 00:01:58.889
but we're allowing it to now flow down

00:01:58.889 --> 00:02:00.700
its concentration gradient.

00:02:00.700 --> 00:02:02.822
Now let's talk about active transport.

00:02:02.822 --> 00:02:05.701
So passive transport doesn't require any energy

00:02:05.701 --> 00:02:06.797
to make this stuff happen.

00:02:06.797 --> 00:02:09.403
It's just about things flowing down their gradient.

00:02:09.403 --> 00:02:13.470
In active transport, we're either directly using energy

00:02:13.470 --> 00:02:16.396
to make something go against its gradient,

00:02:16.396 --> 00:02:19.995
or we're using some energy from a previous active transport

00:02:19.995 --> 00:02:22.572
to help facilitate something else

00:02:22.572 --> 00:02:24.151
going against this gradient.

00:02:24.151 --> 00:02:26.199
So first let's talk about primary active transport,

00:02:26.199 --> 00:02:29.427
'cause this might be a little bit more easy to think about.

00:02:29.427 --> 00:02:31.799
None of them are that daunting.

00:02:31.799 --> 00:02:33.336
And the best case of this,

00:02:33.336 --> 00:02:35.078
if we're talking about animal cells,

00:02:35.078 --> 00:02:37.678
is the sodium potassium pump.

00:02:37.678 --> 00:02:39.838
The sodium potassium pump, super important

00:02:39.838 --> 00:02:43.343
for establishing resting membrane voltage,

00:02:43.343 --> 00:02:45.559
I guess you could say resting membrane potential,

00:02:45.559 --> 00:02:48.414
but the concentration gradients it establishes

00:02:48.414 --> 00:02:49.668
are also very important.

00:02:49.668 --> 00:02:51.828
The sodium having a high, it establishes,

00:02:51.828 --> 00:02:55.543
it pumps sodium ions out of the cell

00:02:55.543 --> 00:02:57.749
against its concentration gradient.

00:02:57.749 --> 00:02:59.583
So we say that sodium ions already have

00:02:59.583 --> 00:03:01.278
a higher concentration outside,

00:03:01.278 --> 00:03:02.996
but it keeps pumping them out.

00:03:02.996 --> 00:03:05.945
And to do that, it needs to use ATP.

00:03:05.945 --> 00:03:10.403
It breaks up ATP into ADP and a phosphate group.

00:03:10.403 --> 00:03:11.726
It hydrolizes it.

00:03:11.726 --> 00:03:14.076
And so that's why it's sometimes called an ATPase.

00:03:14.076 --> 00:03:17.141
It's an enzyme that helps break up ATP.

00:03:17.141 --> 00:03:20.401
But it uses that, and it uses that energy to pump sodium

00:03:20.401 --> 00:03:24.604
out of the cell and potassium into the cell.

00:03:24.604 --> 00:03:27.297
And then, as we'll see, that sodium that's pumped out of it,

00:03:27.297 --> 00:03:28.900
that kind of forms a potential energy

00:03:28.900 --> 00:03:33.381
because it starts to build a chemo-electrochemical gradient

00:03:33.381 --> 00:03:36.841
which can later be used to power secondary active transport.

00:03:36.841 --> 00:03:39.140
We'll talk about that in a few seconds.

00:03:39.140 --> 00:03:40.840
Now this is in animal cells,

00:03:40.840 --> 00:03:46.110
the analog in plant cells, fungi, protus prokaryotes

00:03:46.110 --> 00:03:50.522
is the proton ATPase, or the proton pump,

00:03:50.522 --> 00:03:52.240
which does the same thing, but it does it,

00:03:52.240 --> 00:03:53.982
instead of doing it in two directions,

00:03:53.982 --> 00:03:56.350
it does it for protons, it pumps the protons

00:03:56.350 --> 00:03:59.732
out of the cell against their concentration gradient.

00:03:59.732 --> 00:04:01.468
So even though you have a higher concentration

00:04:01.468 --> 00:04:04.161
outside than inside, it'll continue to pump them out,

00:04:04.161 --> 00:04:07.853
but to do, to power it, it uses ATP to change

00:04:07.853 --> 00:04:10.105
its conformation in the right way.

00:04:10.105 --> 00:04:11.823
And so that's why it's of, this is often called

00:04:11.823 --> 00:04:16.908
the proton ATPase, this is called sodium potassium ATPase,

00:04:16.908 --> 00:04:18.905
that's our friend the sodium potassium pump,

00:04:18.905 --> 00:04:21.046
this is called proton ATPase,

00:04:21.046 --> 00:04:24.436
and you wouldn't see these in the same cell.

00:04:24.436 --> 00:04:25.880
So maybe I'll draw a little line over here,

00:04:25.880 --> 00:04:30.408
this would be in plants, fungi, protus, things like that.

00:04:30.408 --> 00:04:32.149
This would be in animal cells.

00:04:32.149 --> 00:04:35.428
But both of them are actively using energy.

00:04:35.428 --> 00:04:38.469
They are directly using ATP to transport things

00:04:38.469 --> 00:04:40.308
against their concentration gradient,

00:04:40.308 --> 00:04:42.375
which is why we call it active transport.

00:04:42.375 --> 00:04:45.625
Now, because you have these concentration gradients,

00:04:45.625 --> 00:04:48.899
or these electrochemical gradients are established,

00:04:48.899 --> 00:04:53.596
those can be used to do other forms of active transport.

00:04:53.596 --> 00:04:56.340
And that's what we call secondary active transport.

00:04:56.340 --> 00:04:58.654
So this right over here, this is my little depiction

00:04:58.654 --> 00:05:03.030
of a symporter, and this is a sodium glucose symporter.

00:05:03.030 --> 00:05:05.723
And what it does, is it leverages the sodium

00:05:05.723 --> 00:05:08.301
flowing down its concentration gradient.

00:05:08.301 --> 00:05:09.787
And once again, that was established

00:05:09.787 --> 00:05:11.876
with the sodium potassium pump,

00:05:11.876 --> 00:05:13.599
so it's flowing down its concentration gradient,

00:05:13.599 --> 00:05:15.248
but it's leveraging that energy,

00:05:15.248 --> 00:05:18.870
you can imagine putting a little wheel

00:05:18.870 --> 00:05:20.843
under a waterfall to make it spin,

00:05:20.843 --> 00:05:24.466
to also make glucose, to transport glucose against

00:05:24.466 --> 00:05:25.933
its concentration gradient.

00:05:25.933 --> 00:05:28.069
So the glucose already has a,

00:05:28.069 --> 00:05:30.252
will have a high concentration gradient here,

00:05:30.252 --> 00:05:32.550
low over here, but it's transporting glucose

00:05:32.550 --> 00:05:34.227
against its concentration gradient.

00:05:34.227 --> 00:05:36.108
We talk about it in other videos.

00:05:36.108 --> 00:05:39.104
And then another example of secondary active transport

00:05:39.104 --> 00:05:41.982
is an antiporter, or an exchanger.

00:05:41.982 --> 00:05:44.583
In the symporter, a cotransporter, they're both going

00:05:44.583 --> 00:05:46.812
in the same direction even though one is going

00:05:46.812 --> 00:05:48.414
with its concentration gradient,

00:05:48.414 --> 00:05:50.341
that's essentially powering it,

00:05:50.341 --> 00:05:51.734
and the other one is going against

00:05:51.734 --> 00:05:52.988
its concentration gradient,

00:05:52.988 --> 00:05:54.594
that's why it's active transport.

00:05:54.594 --> 00:05:57.915
With an exchanger, they're going in opposite directions,

00:05:57.915 --> 00:06:01.193
so you have the sodium calcium ion exchanger,

00:06:01.193 --> 00:06:02.772
and here the sodium is going down

00:06:02.772 --> 00:06:04.188
its concentration gradient,

00:06:04.188 --> 00:06:06.115
and that fuel's taking the calcium ions

00:06:06.115 --> 00:06:09.927
outside of the cell against its concentration gradient.

00:06:09.927 --> 00:06:11.460
So once again, anytime something is going

00:06:11.460 --> 00:06:13.387
against its concentration gradient,

00:06:13.387 --> 00:06:15.964
and once again, in this case, it's calcium,

00:06:15.964 --> 00:06:18.008
it's going to be active transport.

00:06:18.008 --> 00:06:20.771
But since the sodium ions, the sodium ions

00:06:20.771 --> 00:06:21.824
are going in a different direction

00:06:21.824 --> 00:06:25.144
than the calcium ions, we call this an antiporter,

00:06:25.144 --> 00:06:29.185
while this right over here is a cotransporter, a symporter.

00:06:29.185 --> 00:06:31.948
Now you might say hey, isn't the sodium potassium pump,

00:06:31.948 --> 00:06:33.299
isn't this an antiporter?

00:06:33.299 --> 00:06:35.760
Things are going in different directions?

00:06:35.760 --> 00:06:37.598
And the difference is, both of these,

00:06:37.598 --> 00:06:39.201
this is primary active transport

00:06:39.201 --> 00:06:40.687
and the sodium potassium pump,

00:06:40.687 --> 00:06:42.219
both of these things are going against

00:06:42.219 --> 00:06:44.169
their concentration gradient.

00:06:44.169 --> 00:06:46.329
And a true antiporter, it's really

00:06:46.329 --> 00:06:47.931
secondary active transport.

00:06:47.931 --> 00:06:50.090
One of them is going with their concentration gradient,

00:06:50.090 --> 00:06:52.203
going down it, which is providing the energy,

00:06:52.203 --> 00:06:55.709
to take the other thing against its concentration gradient.

00:06:55.709 --> 00:06:58.865
So anyway, hopefully this gives you a high level overview

00:06:58.865 --> 00:07:00.567
of the various forms of transport,

00:07:00.567 --> 00:07:02.146
and it gives you more appreciation

00:07:02.146 --> 00:07:06.766
for how beautiful and intricate and mesmerizing

00:07:06.766 --> 00:07:08.559
a cellular membranes and all the different things

00:07:08.559 --> 00:07:11.757
that cells have to do actually are.


Electrochemical gradients and secondary active transport

WEBVTT
Kind: captions
Language: en

00:00:00.202 --> 00:00:02.466
- [Voiceover] In the video on the sodium-potassum pump,

00:00:02.466 --> 00:00:06.204
we talk about how it helps a cell establish its

00:00:06.204 --> 00:00:08.061
resting membrane potential.

00:00:08.061 --> 00:00:11.265
And it does that by pumping, actively pumping,

00:00:11.265 --> 00:00:14.423
three sodium ions out for every two potassium ions

00:00:14.423 --> 00:00:17.383
it pumps in, and that by itself, that ratio of three to two

00:00:17.383 --> 00:00:19.589
by itself doesn't establish the full

00:00:19.589 --> 00:00:22.038
resting membrane potential, but then

00:00:22.038 --> 00:00:24.581
the potassium ions are allowed to start

00:00:24.581 --> 00:00:27.762
diffusing down their concentration gradient

00:00:27.762 --> 00:00:29.863
from the inside back to the outside.

00:00:29.863 --> 00:00:32.022
And of course there's a balancing force there,

00:00:32.022 --> 00:00:34.762
or a balancing factor, and that's the charge.

00:00:34.762 --> 00:00:37.397
Because if the outside is more positive than the inside,

00:00:37.397 --> 00:00:42.157
a positively-charged ion, which the potassium ions are,

00:00:42.157 --> 00:00:44.502
well, they're not gonna want to go up here so much

00:00:44.502 --> 00:00:46.185
because of their charge.

00:00:46.185 --> 00:00:48.542
It's more positive here than it is over here.

00:00:48.542 --> 00:00:50.805
They'd actually want to go back, but their concentration

00:00:50.805 --> 00:00:52.721
gradient, they're going to be bumping into the bottom

00:00:52.721 --> 00:00:55.043
of this channel more than the top,

00:00:55.043 --> 00:00:56.401
and so you're going to have a balance.

00:00:56.401 --> 00:00:57.980
They're going to start diffusing through,

00:00:57.980 --> 00:01:00.336
but you're not going to have equal concentrations

00:01:00.336 --> 00:01:02.925
because the charge is going to keep them back here.

00:01:02.925 --> 00:01:04.620
But what about the sodium ions?

00:01:04.620 --> 00:01:06.036
The sodium ions are getting more and more

00:01:06.036 --> 00:01:08.056
concentrated up here, and up here is getting

00:01:08.056 --> 00:01:09.705
more and more positive.

00:01:09.705 --> 00:01:11.644
If the sodium ions were left to their own devices,

00:01:11.644 --> 00:01:13.501
if there was no membrane over here, they

00:01:13.501 --> 00:01:15.346
would naturally, if we just looked at the concentration

00:01:15.346 --> 00:01:18.396
gradient, they would naturally want to diffuse down.

00:01:18.396 --> 00:01:21.136
We have a high concentration over here,

00:01:21.136 --> 00:01:23.457
we have a low concentration over there.

00:01:23.457 --> 00:01:26.557
So if there was no membrane, then they would just naturally

00:01:26.557 --> 00:01:28.101
diffuse from high to low.

00:01:28.101 --> 00:01:29.877
That's their concentration gradient.

00:01:29.877 --> 00:01:32.013
And also if there was no membrane, we've

00:01:32.013 --> 00:01:34.753
already talked about it being much more positive

00:01:34.753 --> 00:01:38.131
on this side than it is on this side.

00:01:38.131 --> 00:01:40.511
Or you could say we have a positive potential difference

00:01:40.511 --> 00:01:42.693
between here and here.

00:01:42.693 --> 00:01:45.874
So the positively charged ions, like the sodiums up here,

00:01:45.874 --> 00:01:48.753
would want to go down because of their charge.

00:01:48.753 --> 00:01:50.576
And so there's two reasons why they would want

00:01:50.576 --> 00:01:53.339
to go from this side of the membrane to that side

00:01:53.339 --> 00:01:54.477
of the membrane.

00:01:54.477 --> 00:01:58.075
Their concentration gradient and their charge,

00:01:58.075 --> 00:01:59.236
the electric potential.

00:01:59.236 --> 00:02:00.595
There's this potential energy of them wanting

00:02:00.595 --> 00:02:02.974
to get away from all the positive charges.

00:02:02.974 --> 00:02:05.830
And so that combined motivation for the

00:02:05.830 --> 00:02:10.172
sodium ions to go in that direction, we call that

00:02:10.172 --> 00:02:13.105
the electrochemical gradient.

00:02:13.105 --> 00:02:16.465
Electrochemical

00:02:19.185 --> 00:02:20.285
gradient.

00:02:20.285 --> 00:02:22.414
And I already said it once, but I'll say it again.

00:02:22.414 --> 00:02:25.583
It's a combination of the electric gradient and

00:02:25.583 --> 00:02:26.419
the chemical gradient.

00:02:26.419 --> 00:02:27.951
The chemical gradient, you have higher concentration

00:02:27.951 --> 00:02:30.482
here, lower here, you would want to diffuse down,

00:02:30.482 --> 00:02:32.037
more things are going to bump on this side

00:02:32.037 --> 00:02:35.253
than on this side, so you're going to have a net flow down,

00:02:35.253 --> 00:02:37.076
if you didn't have this membrane here.

00:02:37.076 --> 00:02:39.212
And then when you think about the electric potential,

00:02:39.212 --> 00:02:40.953
more positive on this side than on this side,

00:02:40.953 --> 00:02:44.157
so positive ions would want to go down.

00:02:44.157 --> 00:02:46.630
And so you could view this gradient as a source

00:02:46.630 --> 00:02:48.418
of potential energy.

00:02:48.418 --> 00:02:50.449
And cells, in fact, use this gradient,

00:02:50.449 --> 00:02:53.537
in fact, the sodium electrochemical gradient as

00:02:53.537 --> 00:02:55.093
a source of energy.

00:02:55.093 --> 00:02:57.891
And so let's say this protein

00:02:57.891 --> 00:02:59.876
right over here, this is what we're going to

00:02:59.876 --> 00:03:01.818
call a symporter.

00:03:01.818 --> 00:03:04.116
This is a symporter.

00:03:04.116 --> 00:03:07.169
And what it does is, it uses the electrochemical

00:03:07.169 --> 00:03:11.871
gradient of one ion, in this case sodium,

00:03:11.871 --> 00:03:14.135
so it uses the fact that sodium really wants to

00:03:14.135 --> 00:03:16.851
go through the membrane, and it uses that energy.

00:03:16.851 --> 00:03:19.405
Imagine like water falling down a waterfall

00:03:19.405 --> 00:03:23.735
and it can turn a turbine or turn a water mill

00:03:23.735 --> 00:03:24.734
type of thing.

00:03:24.734 --> 00:03:27.473
And so it uses that energy of the sodium

00:03:27.473 --> 00:03:30.294
flowing down its electrochemical gradient,

00:03:30.294 --> 00:03:32.372
it wants to go in this direction for two reasons,

00:03:32.372 --> 00:03:35.809
concentration and electric potential, or I guess

00:03:35.809 --> 00:03:37.956
you'd say its electrostatic charge, and then it

00:03:37.956 --> 00:03:40.371
uses that energy to transport other things.

00:03:40.371 --> 00:03:43.215
And the most famous symporter with sodium

00:03:43.215 --> 00:03:44.748
is glucose.

00:03:44.748 --> 00:03:46.895
It's going to use that, the sodium and the glucose

00:03:46.895 --> 00:03:48.277
are going to go together.

00:03:48.277 --> 00:03:49.867
And the glucose is being transported

00:03:49.867 --> 00:03:52.433
against its concentration gradient.

00:03:52.433 --> 00:03:53.733
And so if you're going to transport something

00:03:53.733 --> 00:03:57.076
against its concentration gradient, you're going to

00:03:57.076 --> 00:03:59.630
have to use active transport.

00:03:59.630 --> 00:04:02.672
So this concentration gradient, so let me

00:04:02.672 --> 00:04:05.551
be clear on glucose's concentration gradient,

00:04:05.551 --> 00:04:06.549
it looks like this.

00:04:06.549 --> 00:04:09.486
You have high concentration over here and you have

00:04:09.486 --> 00:04:10.589
low right over here.

00:04:10.589 --> 00:04:11.971
And the cell might not want to waste all this glucose,

00:04:11.971 --> 00:04:14.571
it wants to get as much glucose into the cell,

00:04:14.571 --> 00:04:16.986
or across the membrane, as possible.

00:04:16.986 --> 00:04:18.855
And so it's going to have to do some active transport

00:04:18.855 --> 00:04:21.130
to go against its concentration gradient.

00:04:21.130 --> 00:04:23.487
To go in this direction.

00:04:23.487 --> 00:04:25.205
And over here, the source of energy

00:04:25.205 --> 00:04:27.109
to go against the concentration gradient,

00:04:27.109 --> 00:04:30.684
is the stored potential energy from the

00:04:30.684 --> 00:04:33.343
electrochemical gradient of the sodium.

00:04:33.343 --> 00:04:36.106
And so this type of active transport, where you're

00:04:36.106 --> 00:04:40.053
using the energy that was stored up through another

00:04:40.053 --> 00:04:42.723
form of active transport, the sodium-potassium pump,

00:04:42.723 --> 00:04:45.625
we call this Secondary Active Transport.

00:04:45.625 --> 00:04:48.144
So what's going over here, this sodium-glucose

00:04:48.144 --> 00:04:51.800
symporter, this is Secondary Active Transport.

00:04:51.800 --> 00:04:56.339
Secondary Active Transport.

00:04:56.339 --> 00:05:01.049
It's using the stored energy from the

00:05:01.423 --> 00:05:04.941
electrochemical gradient of one molecule, it's

00:05:04.941 --> 00:05:09.503
using that stored energy to drive the active transport

00:05:09.503 --> 00:05:12.440
of another molecule, glucose, going against

00:05:12.440 --> 00:05:15.047
its concentration gradient.


Sodium potassium pump

WEBVTT
Kind: captions
Language: en

00:00:00.260 --> 00:00:01.480
- [Voiceover] What I hope to do in this video

00:00:01.480 --> 00:00:03.210
is give ourselves an appreciation

00:00:03.210 --> 00:00:05.150
for the sodium-potassium pump,

00:00:05.150 --> 00:00:06.860
and as the name implies, it pumps

00:00:06.860 --> 00:00:08.800
sodium and potassium, but it does it

00:00:08.800 --> 00:00:10.450
in different directions.

00:00:10.450 --> 00:00:12.570
So this little depiction right over here,

00:00:12.570 --> 00:00:14.835
this is my drawing, my rendition

00:00:14.835 --> 00:00:16.170
of the sodium-potassium pump,

00:00:16.170 --> 00:00:18.620
it's a trans-membrane, I guess you could say

00:00:18.620 --> 00:00:21.035
protein complex right over here.

00:00:21.035 --> 00:00:22.790
And in this resting state,

00:00:22.790 --> 00:00:25.550
it is open to the inside of the cell

00:00:25.550 --> 00:00:29.450
and it has an affinity for sodium ions.

00:00:29.450 --> 00:00:30.830
And so the sodium ions,

00:00:30.830 --> 00:00:32.350
you see three sodium ions

00:00:32.350 --> 00:00:33.400
depicted here in blue.

00:00:33.400 --> 00:00:36.600
They're going to bind to the pump.

00:00:36.600 --> 00:00:38.530
And once they bind to it,

00:00:38.530 --> 00:00:40.820
then it's going to want to be

00:00:40.820 --> 00:00:42.960
phosphorylated by an ATP,

00:00:42.960 --> 00:00:43.820
and we see that right over here.

00:00:43.820 --> 00:00:44.950
This is ATP,

00:00:44.950 --> 00:00:47.630
adenosine triphosphate.

00:00:47.630 --> 00:00:49.260
And when it gets phosphorylated,

00:00:49.260 --> 00:00:50.781
it's a release of energy

00:00:50.781 --> 00:00:52.330
and it allows the confirmation

00:00:52.330 --> 00:00:54.530
of the actual protein to change.

00:00:54.530 --> 00:00:57.020
So the new confirmation of the protein,

00:00:57.020 --> 00:00:58.420
it's now going to open up

00:00:58.420 --> 00:01:01.120
to the outside, close off to the inside,

00:01:01.120 --> 00:01:02.700
and now it's no longer going to

00:01:02.700 --> 00:01:05.260
have an affinity for sodium ions,

00:01:05.260 --> 00:01:07.350
but an affinity for potassium ions,

00:01:07.350 --> 00:01:08.450
and this is fascinating,

00:01:08.450 --> 00:01:10.730
that release of energy, change of confirmation,

00:01:10.730 --> 00:01:12.120
that these proteins really are

00:01:12.120 --> 00:01:13.500
these molecular machines,

00:01:13.500 --> 00:01:15.560
these fascinating molecular machines.

00:01:15.560 --> 00:01:16.900
But once that happens,

00:01:16.900 --> 00:01:18.422
this change of confirmation,

00:01:18.422 --> 00:01:19.360
the sodium ions are going to

00:01:19.360 --> 00:01:21.460
be released outside of the cell.

00:01:21.460 --> 00:01:23.050
And then you're going to have potassium ions

00:01:23.050 --> 00:01:25.190
that are going to bind from the outside.

00:01:25.190 --> 00:01:26.806
And then once that happens,

00:01:26.806 --> 00:01:28.060
the change in confirmation,

00:01:28.060 --> 00:01:28.690
we're going to have a,

00:01:28.690 --> 00:01:30.900
it's going to get dephosphorylated

00:01:30.900 --> 00:01:32.050
and then you're going to go back

00:01:32.050 --> 00:01:33.660
to your original confirmation,

00:01:33.660 --> 00:01:35.689
your original confirmation right over here.

00:01:35.689 --> 00:01:37.390
Where you no longer have an affinity

00:01:37.390 --> 00:01:39.060
for potassium ions,

00:01:39.060 --> 00:01:40.435
they're going to be released,

00:01:40.435 --> 00:01:41.000
and then you're going to be

00:01:41.000 --> 00:01:43.076
back in the original phase.

00:01:43.076 --> 00:01:44.150
So this is fascinating.

00:01:44.150 --> 00:01:46.450
By using ATP, by using energy,

00:01:46.450 --> 00:01:48.020
this is active transport,

00:01:48.020 --> 00:01:49.384
it takes energy to do this.

00:01:49.384 --> 00:01:51.020
Let me write this down.

00:01:51.020 --> 00:01:52.750
This is active,

00:01:52.750 --> 00:01:54.690
this is active transport

00:01:54.690 --> 00:01:56.420
that we are talking about

00:01:56.420 --> 00:01:57.520
right over here.

00:01:57.520 --> 00:01:58.820
We're able to pump,

00:01:58.820 --> 00:02:01.237
using an ATP, we're able to pump

00:02:01.237 --> 00:02:03.700
three sodium ions out,

00:02:03.700 --> 00:02:07.330
three sodium ions out,

00:02:07.330 --> 00:02:08.620
so let me write that down.

00:02:08.620 --> 00:02:09.692
Three sodium ions out.

00:02:09.692 --> 00:02:11.530
And in the process, we pump

00:02:11.530 --> 00:02:14.520
two potassium ions in.

00:02:14.520 --> 00:02:16.760
So we pump two

00:02:16.760 --> 00:02:19.420
potassium ions in.

00:02:19.420 --> 00:02:20.360
Now you might say, okay,

00:02:20.360 --> 00:02:23.020
the outside, since these both have positive charge,

00:02:23.020 --> 00:02:24.590
but I have three sodium going out,

00:02:24.590 --> 00:02:26.150
two potassium going in.

00:02:26.150 --> 00:02:27.500
That must make the outside

00:02:27.500 --> 00:02:29.850
more positive than the inside,

00:02:29.850 --> 00:02:32.361
and that actually is true.

00:02:32.361 --> 00:02:34.620
But that by itself isn't fully responsible.

00:02:34.620 --> 00:02:36.000
It's actually only partially responsible

00:02:36.000 --> 00:02:40.370
for the electric potential difference

00:02:40.370 --> 00:02:41.920
between the inside of the membrane

00:02:41.920 --> 00:02:43.496
and the outside of the membrane.

00:02:43.496 --> 00:02:45.290
What really sets that up is that

00:02:45.290 --> 00:02:47.432
you actually have channel proteins

00:02:47.432 --> 00:02:49.220
that allow potassium ions

00:02:49.220 --> 00:02:52.300
to move down, to diffuse down

00:02:52.300 --> 00:02:53.730
their concentration gradient.

00:02:53.730 --> 00:02:55.130
So let's think about what happens

00:02:55.130 --> 00:02:56.300
before I even talk about

00:02:56.300 --> 00:02:58.300
these channel proteins right over here.

00:02:58.300 --> 00:03:00.250
Because of the sodium-potassium pump,

00:03:00.250 --> 00:03:02.790
what is sodium's concentration gradient?

00:03:02.790 --> 00:03:04.412
Well, it has a higher concentration

00:03:04.412 --> 00:03:05.760
on the outside,

00:03:05.760 --> 00:03:08.220
it has a higher concentration on the outside

00:03:08.220 --> 00:03:09.970
and it has a lower concentration

00:03:09.970 --> 00:03:11.260
on the inside.

00:03:11.260 --> 00:03:13.566
This is sodium's concentration gradient.

00:03:13.566 --> 00:03:16.160
What is potassium's concentration gradient?

00:03:16.160 --> 00:03:17.930
Well, potassium is getting pumped in

00:03:17.930 --> 00:03:21.200
from the outside into the cell.

00:03:21.200 --> 00:03:24.290
So potassium has the opposite concentration gradient.

00:03:24.290 --> 00:03:27.060
It has a high concentration inside

00:03:27.060 --> 00:03:30.660
and it has a low concentration outside.

00:03:30.660 --> 00:03:34.880
Now if we let potassium go through,

00:03:34.880 --> 00:03:36.503
we've talked in previous videos

00:03:36.503 --> 00:03:38.670
about ions just not being that,

00:03:38.670 --> 00:03:40.266
that just the general membrane,

00:03:40.266 --> 00:03:42.256
if it's not facilitated in some way,

00:03:42.256 --> 00:03:45.156
isn't that permeable to things like ions,

00:03:45.156 --> 00:03:46.600
like sodium and potassium ions.

00:03:46.600 --> 00:03:49.259
But if you have channel proteins

00:03:49.259 --> 00:03:51.386
right over here that let the potassium

00:03:51.386 --> 00:03:53.326
get out, what's going to happen?

00:03:53.326 --> 00:03:55.266
Well, you might have one of two answers.

00:03:55.266 --> 00:03:55.966
You might say, oh, well, look.

00:03:55.966 --> 00:03:58.426
Hey, you know, things diffuse down

00:03:58.426 --> 00:03:59.836
their concentration gradient,

00:03:59.836 --> 00:04:00.956
you have a higher probability since you have

00:04:00.956 --> 00:04:03.196
more potassium here than up here,

00:04:03.196 --> 00:04:04.526
higher probability of them

00:04:04.526 --> 00:04:06.426
going in the right direction on this side

00:04:06.426 --> 00:04:07.926
and moving from this side to that side

00:04:07.926 --> 00:04:10.266
than you have them going from that side to that side.

00:04:10.266 --> 00:04:14.056
And so you would have a net outflow of potassium.

00:04:14.056 --> 00:04:15.466
And some of you might say, well, okay,

00:04:15.466 --> 00:04:16.896
that makes sense if you only care

00:04:16.896 --> 00:04:19.075
about the concentration gradient.

00:04:19.075 --> 00:04:21.086
But what happens if we look at the charge?

00:04:21.086 --> 00:04:23.730
Because we're saying that the inside of the cell

00:04:23.730 --> 00:04:25.766
is going to be less positive

00:04:27.956 --> 00:04:28.896
and the outside of the cell

00:04:28.896 --> 00:04:30.376
is going to be more positive

00:04:30.376 --> 00:04:31.119
because it has more,

00:04:31.119 --> 00:04:33.156
that we have the net ion change,

00:04:33.156 --> 00:04:37.556
so it's gonna be more positive out here.

00:04:37.556 --> 00:04:39.786
So positive ions, they don't like,

00:04:39.786 --> 00:04:41.616
you want to move away from charges

00:04:41.616 --> 00:04:42.826
that are the same as you.

00:04:42.826 --> 00:04:44.856
You want to move to the places

00:04:44.856 --> 00:04:46.526
that are a more negative,

00:04:46.526 --> 00:04:48.666
so you'd say, well, these potassium ions

00:04:48.666 --> 00:04:50.086
are positively charged,

00:04:50.086 --> 00:04:51.216
why would they wanna go from

00:04:51.216 --> 00:04:54.886
a less positive place to a more positive place?

00:04:54.886 --> 00:04:57.026
And if you are saying either one of these things,

00:04:57.026 --> 00:04:58.576
talking about the concentration gradient

00:04:58.576 --> 00:05:00.986
or talking about the electric potential difference,

00:05:00.986 --> 00:05:03.946
you are actually going to be right in both cases.

00:05:03.946 --> 00:05:06.086
These are going to be balancing forces.

00:05:06.086 --> 00:05:07.226
The concentration gradient

00:05:07.226 --> 00:05:08.286
is going to allow some of

00:05:08.286 --> 00:05:10.826
these potassium ions to pour out,

00:05:10.826 --> 00:05:14.579
but the concentrations of potassium ions

00:05:14.579 --> 00:05:16.442
aren't going to fully equalize

00:05:16.442 --> 00:05:19.688
because of the electric potential difference,

00:05:19.688 --> 00:05:22.136
because, hey, it's more positive out here,

00:05:22.136 --> 00:05:23.323
it's less positive here.

00:05:23.323 --> 00:05:23.986
When they're moving out,

00:05:23.986 --> 00:05:25.236
they're going against what

00:05:25.236 --> 00:05:26.566
their charge wants to do.

00:05:26.566 --> 00:05:28.416
They're going with the concentration gradient,

00:05:28.416 --> 00:05:31.666
but at some point, that is going to balance out.

00:05:31.666 --> 00:05:33.396
And by going through this process,

00:05:33.396 --> 00:05:36.576
by pumping sodium out

00:05:36.576 --> 00:05:37.796
and with that larger ratio

00:05:37.796 --> 00:05:39.616
than what you're pumping potassium in,

00:05:39.616 --> 00:05:40.716
and then you're further allowing

00:05:40.716 --> 00:05:42.766
more positive charge to go out.

00:05:42.766 --> 00:05:44.146
You're establishing what's called

00:05:44.146 --> 00:05:46.786
the resting membrane potential for a cell.

00:05:46.786 --> 00:05:48.486
And this is super important for all cells,

00:05:48.486 --> 00:05:49.986
but especially neuron cells

00:05:49.986 --> 00:05:51.856
or neural cells or neurons.

00:05:51.856 --> 00:05:53.446
And those are gonna spend

00:05:53.446 --> 00:05:54.790
two-thirds of their energy

00:05:54.790 --> 00:05:56.866
just to establish or to keep

00:05:56.866 --> 00:05:58.436
their resting membrane potential.

00:05:58.436 --> 00:05:59.986
And as we'll see in the videos on neurons,

00:05:59.986 --> 00:06:02.086
that's because they keep leveraging that potential

00:06:02.086 --> 00:06:04.696
to send signals down the neuron.

00:06:04.696 --> 00:06:06.346
But the resting membrane potential,

00:06:06.346 --> 00:06:08.116
it's less positive here and more positive there.

00:06:08.116 --> 00:06:11.436
If you measure this relative to,

00:06:11.436 --> 00:06:12.826
let me make that a little bit neater,

00:06:12.826 --> 00:06:15.286
relative to this right over here,

00:06:15.286 --> 00:06:19.026
this difference is, depending on

00:06:19.026 --> 00:06:20.236
what estimates you look at,

00:06:20.236 --> 00:06:24.386
approximately negative 70 millivolts.

00:06:24.386 --> 00:06:26.996
I've seen estimates negative 60, negative 80.

00:06:26.996 --> 00:06:30.266
Negative 70 millivolts.

00:06:30.266 --> 00:06:32.003
And this is key for neurons,

00:06:32.003 --> 00:06:34.273
but it's key for all cells.

00:06:34.273 --> 00:06:35.593
Now the sodium-potassium pump

00:06:35.593 --> 00:06:37.853
isn't just about establishing

00:06:37.853 --> 00:06:39.823
the resting membrane potential.

00:06:39.823 --> 00:06:41.953
Having this higher sodium concentration

00:06:41.953 --> 00:06:44.793
on the outside can also be used later on

00:06:44.793 --> 00:06:47.223
for other forms of active transport.

00:06:47.223 --> 00:06:49.113
When they move down their gradient,

00:06:49.113 --> 00:06:50.223
you can do things like

00:06:50.223 --> 00:06:52.443
co-transport glucose molecules.

00:06:52.443 --> 00:06:54.453
So these biological systems

00:06:54.453 --> 00:06:55.733
are far more complicated than

00:06:55.733 --> 00:06:58.563
I often give credit for in these videos,

00:06:58.563 --> 00:07:00.433
but I want to give you a full appreciation for this.

00:07:00.433 --> 00:07:01.993
And just so you know how big of a deal

00:07:01.993 --> 00:07:03.493
the sodium-potassium pump was,

00:07:03.493 --> 00:07:05.193
it was discovered in the 1950s,

00:07:05.193 --> 00:07:08.773
but the 1997 Nobel Prize was awarded

00:07:08.773 --> 00:07:12.053
for the discovery of

00:07:12.053 --> 00:07:14.793
the sodium-potassium pump

00:07:14.793 --> 00:07:16.333
and how it works.


Endocytosis, phagocytosis, and pinocytosis

WEBVTT
Kind: captions
Language: en

00:00:00.354 --> 00:00:01.526
- [Voiceover] We have other videos where we talk

00:00:01.526 --> 00:00:03.883
about how small molecules, or ions,

00:00:03.883 --> 00:00:07.238
might be able to go through a cell's membrane

00:00:07.238 --> 00:00:09.990
in different ways, whether actively or passively,

00:00:09.990 --> 00:00:11.952
maybe facilitated in some way.

00:00:11.952 --> 00:00:13.392
What we wanna talk about in this video

00:00:13.392 --> 00:00:16.050
is how we can do this for larger things.

00:00:16.050 --> 00:00:18.790
We're gonna focus on here is bulk,

00:00:18.790 --> 00:00:21.019
bulk transport.

00:00:21.019 --> 00:00:22.320
Transport.

00:00:22.320 --> 00:00:23.492
So this first example,

00:00:23.492 --> 00:00:26.917
you could imagine this cell with this mauve

00:00:26.917 --> 00:00:29.553
or purple-colored membrane is engulfing

00:00:29.553 --> 00:00:30.992
this big green thing which is

00:00:30.992 --> 00:00:33.082
maybe a bacteria or something.

00:00:33.082 --> 00:00:34.951
And so you see that the membrane...

00:00:34.951 --> 00:00:35.892
Let me make it very clear.

00:00:35.892 --> 00:00:38.539
This is inside.

00:00:38.539 --> 00:00:40.188
This is inside the cell.

00:00:40.188 --> 00:00:42.022
This is outside.

00:00:42.022 --> 00:00:43.195
Outside the cell.

00:00:43.195 --> 00:00:44.553
And you can see the cellular membrane

00:00:44.553 --> 00:00:46.561
starts to wrap around this,

00:00:46.561 --> 00:00:49.081
I guess we can think of this a bacteria,

00:00:49.081 --> 00:00:51.217
then it fully wraps around and then

00:00:51.217 --> 00:00:53.226
that membrane that was wrapping around

00:00:53.226 --> 00:00:55.826
the bacteria pinches off and now the bacteria

00:00:55.826 --> 00:00:58.450
is inside of the cell, and it's wrapped

00:00:58.450 --> 00:00:59.959
by this membrane.

00:00:59.959 --> 00:01:03.953
And this process where you're engulfing these large things,

00:01:03.953 --> 00:01:06.623
we call this Phagocytosis.

00:01:06.623 --> 00:01:09.387
So this is phago...

00:01:09.387 --> 00:01:12.556
Phagocytosis.

00:01:12.556 --> 00:01:15.679
And the prefix I guess you'd say, phago

00:01:15.679 --> 00:01:17.920
comes from the Greek for "to eat".

00:01:17.920 --> 00:01:20.927
So this is literally about cell eating.

00:01:20.927 --> 00:01:23.516
And in many cases, this thing that is now in here,

00:01:23.516 --> 00:01:25.327
you could view this as the cell's food,

00:01:25.327 --> 00:01:29.114
this compartment that is holding this,

00:01:29.114 --> 00:01:31.295
in this case bacteria, is gonna transport it

00:01:31.295 --> 00:01:33.489
maybe to a lysosome, so it can be processed

00:01:33.489 --> 00:01:35.172
and digested in some way.

00:01:35.172 --> 00:01:37.285
We would call this big compartment,

00:01:37.285 --> 00:01:38.922
this membrane-bound compartment,

00:01:38.922 --> 00:01:42.382
we would call this a food vacuole.

00:01:42.382 --> 00:01:43.752
Food.

00:01:43.752 --> 00:01:46.806
Food vacuole.

00:01:46.806 --> 00:01:51.055
Now this scenario down here is similar but different.

00:01:51.055 --> 00:01:52.785
Over here I have the cell

00:01:52.785 --> 00:01:55.084
which is I see part of its membrane

00:01:55.084 --> 00:01:56.488
and it's in magenta right over here.

00:01:56.488 --> 00:01:58.056
We can see The Phospholipid Bilayer .

00:01:58.056 --> 00:02:02.084
That's why I drew two lines for the membrane.

00:02:02.084 --> 00:02:05.451
And instead of engulfing a large particle or bacteria,

00:02:05.451 --> 00:02:08.017
it's just engulfing some fluid.

00:02:08.017 --> 00:02:09.561
It's engulfing some fluid, so you see it's starting

00:02:09.561 --> 00:02:11.686
to wrap around this section of fluid,

00:02:11.686 --> 00:02:13.694
wraps even more around this section of fluid,

00:02:13.694 --> 00:02:15.656
and then the membrane that was around it

00:02:15.656 --> 00:02:18.118
completely pinches off and goes into the cell.

00:02:18.118 --> 00:02:20.556
And I'm drawing all of these things in two dimension,

00:02:20.556 --> 00:02:22.286
but this would actually be happening in three dimension.

00:02:22.286 --> 00:02:23.725
So this wouldn't just be a circle,

00:02:23.725 --> 00:02:25.850
this would right over here would be a sphere.

00:02:25.850 --> 00:02:28.358
And this thing that has been pinched off

00:02:28.358 --> 00:02:31.690
and is now inside the cell, we call this a vesicle,

00:02:31.690 --> 00:02:34.290
which is just a general term for these membrane-bound

00:02:34.290 --> 00:02:37.019
compartments inside of cells.

00:02:37.019 --> 00:02:39.190
And this process where the cell has essentially drunk

00:02:39.190 --> 00:02:40.757
a bunch of fluid and the stuff

00:02:40.757 --> 00:02:42.487
that happens to be in the fluid,

00:02:42.487 --> 00:02:44.995
we call this Pinocytosis.

00:02:44.995 --> 00:02:46.292
Pinocytosis.

00:02:46.852 --> 00:02:50.126
Pinocytosis.

00:02:50.126 --> 00:02:52.924
And pino comes from the Greek word "to drink".

00:02:52.924 --> 00:02:54.852
And I'm always fascinated by word roots.

00:02:54.852 --> 00:02:57.288
And I'm not a linguistic expert here,

00:02:57.288 --> 00:02:59.424
but it's neat because even in languages

00:02:59.424 --> 00:03:02.024
I'm familiar with like Hindi and Urdu,

00:03:02.024 --> 00:03:04.360
the word pina means "to drink",

00:03:04.360 --> 00:03:06.624
so maybe it's even related to the word pani

00:03:06.624 --> 00:03:08.691
which is in those words in those languages.

00:03:08.691 --> 00:03:10.723
I know all of these have a shared linguistic root,

00:03:10.723 --> 00:03:12.655
so it's always fascinating to see

00:03:12.655 --> 00:03:14.484
these linguistic connections.

00:03:14.484 --> 00:03:17.886
So this is Pinocytosis where the cell is drinking

00:03:17.886 --> 00:03:19.891
so to speak, but it's also getting the other stuff

00:03:19.891 --> 00:03:21.427
that's in that fluid.

00:03:21.427 --> 00:03:23.923
This is Phagocytosis, the cell is eating.

00:03:23.923 --> 00:03:26.489
And these are both special cases

00:03:26.489 --> 00:03:28.590
of I guess the more general term

00:03:28.590 --> 00:03:30.158
of engulfing in this way

00:03:30.158 --> 00:03:32.851
which is called Endocytosis.

00:03:32.851 --> 00:03:38.110
Endocytosis.

00:03:38.110 --> 00:03:41.280
So Phagocytosis is a form of Endocytosis,

00:03:41.280 --> 00:03:45.158
and Pinocytosis is a form of Endocytosis.

00:03:45.158 --> 00:03:46.481
Now the next question you might say is:

00:03:46.481 --> 00:03:48.757
OK I can get that this happens,

00:03:48.757 --> 00:03:51.114
this can be observed under a microscope,

00:03:51.114 --> 00:03:52.460
but how does it happen?

00:03:52.460 --> 00:03:55.119
How does the cell wrap around and pinch around?

00:03:55.119 --> 00:03:57.151
And like I say in a lot of videos,

00:03:57.151 --> 00:03:59.159
people think that we understand some of it,

00:03:59.159 --> 00:04:01.923
but this is not fully fully understood.

00:04:01.923 --> 00:04:05.539
There is views that well the cytoskeleton

00:04:05.539 --> 00:04:07.774
must be involved in some way.

00:04:07.774 --> 00:04:11.454
It has to create space here for this thing

00:04:11.454 --> 00:04:15.587
to be able to pinch off and move in that direction.

00:04:15.587 --> 00:04:20.545
It maybe will help actually the cell's membrane wrap around

00:04:20.545 --> 00:04:23.424
in some way, but these are all areas of active research.

00:04:23.424 --> 00:04:26.187
How does this Endocytosis actually occur?

00:04:26.187 --> 00:04:30.286
How does the cell know what to consume

00:04:30.286 --> 00:04:32.050
I guess you could say, and then how does the membrane

00:04:32.050 --> 00:04:34.625
actually behave in this way to do it,

00:04:34.625 --> 00:04:37.084
to do it well?


Facilitated diffusion

WEBVTT
Kind: captions
Language: en

00:00:00.446 --> 00:00:02.860
- In the first video on passive transport

00:00:02.860 --> 00:00:04.381
we talked about the most passive

00:00:04.381 --> 00:00:07.686
of passive transports and that is simple diffusion.

00:00:07.686 --> 00:00:11.420
And we talked about how small non-charged,

00:00:11.420 --> 00:00:14.342
non-polar molecules would actually have the easiest time.

00:00:14.342 --> 00:00:16.867
Things like carbon dioxide or molecular oxygen,

00:00:16.867 --> 00:00:19.073
would have the easiest time diffusing

00:00:19.073 --> 00:00:21.035
through the cellular membrane.

00:00:21.035 --> 00:00:23.844
They're small enough to kind of get through the little gaps

00:00:23.844 --> 00:00:26.688
here and then since they have no charge or polarity

00:00:26.688 --> 00:00:29.957
they're gonna be fairly indifferent as they pass through.

00:00:29.957 --> 00:00:31.477
And then we talked about in-between you have things

00:00:31.477 --> 00:00:33.496
like water molecules which are small enough to pass

00:00:33.496 --> 00:00:35.578
through the gaps but they have some polarity

00:00:35.578 --> 00:00:37.750
so they're not going to be able to get through super easily,

00:00:37.750 --> 00:00:39.766
but they will be able to seep through.

00:00:39.766 --> 00:00:40.797
And then we talked about things

00:00:40.797 --> 00:00:42.150
that would have a tough time.

00:00:42.150 --> 00:00:44.877
And that's charged particles because charged particles,

00:00:44.877 --> 00:00:47.094
and we have some ions right over here, a sodium ion,

00:00:47.094 --> 00:00:48.278
a potassium ion.

00:00:48.278 --> 00:00:51.077
Even though these are fairly small they're going to interact

00:00:51.077 --> 00:00:54.676
a lot with the phosphate heads right over here

00:00:54.676 --> 00:00:57.810
with this charge, which is gonna make it hard for them

00:00:57.810 --> 00:01:00.742
to actually penetrate through the membrane.

00:01:00.742 --> 00:01:02.925
What I wanna talk about in this video

00:01:02.925 --> 00:01:04.631
is still passive transport.

00:01:04.631 --> 00:01:07.976
And remember passive transport is about not using energy.

00:01:07.976 --> 00:01:10.743
It's about moving down the concentration gradient.

00:01:10.743 --> 00:01:13.047
But we're gonna talk about ways that passive transport

00:01:13.047 --> 00:01:14.927
can happen a little bit easier for some

00:01:14.927 --> 00:01:16.926
of these molecules over here.

00:01:16.926 --> 00:01:18.834
And that's because their transport,

00:01:18.834 --> 00:01:22.270
their passive transport, is going to be facilitated.

00:01:22.270 --> 00:01:24.049
So what we're going to talk about in this video.

00:01:24.049 --> 00:01:25.103
Let me just figure out a place

00:01:25.103 --> 00:01:29.343
where I can write it, is facilitated, faciliated diffusion.

00:01:30.457 --> 00:01:32.162
So let me write that down.

00:01:32.162 --> 00:01:34.330
"Facilitated,

00:01:34.330 --> 00:01:37.788
"Facilitated Diffusion"

00:01:37.788 --> 00:01:40.895
So last video was just straight up diffusion,

00:01:40.895 --> 00:01:42.915
now we're gonna talk about facilitating it.

00:01:42.915 --> 00:01:44.257
So what do you think,

00:01:44.257 --> 00:01:46.677
if you were trying to engineer something,

00:01:46.677 --> 00:01:49.939
that would make it easy for these types of molecules,

00:01:49.939 --> 00:01:52.033
either a water molecule or an ion,

00:01:52.033 --> 00:01:53.983
to move down its concentration gradient?

00:01:53.983 --> 00:01:55.343
What would you do?

00:01:55.343 --> 00:01:57.388
Well you might say, "Well if I didn't have

00:01:57.388 --> 00:01:59.048
"to mess with all of this, you know,

00:01:59.048 --> 00:02:01.962
"all the hydrophilic heads and then the hyrophobic tails

00:02:01.962 --> 00:02:03.744
"and then the hydrophilic heads here."

00:02:03.744 --> 00:02:05.263
Well that would make it pretty easy

00:02:05.263 --> 00:02:06.877
to move down your diffusion gradient.

00:02:06.877 --> 00:02:09.672
And that's exactly what has emerged in nature.

00:02:09.672 --> 00:02:13.852
Essentially just tunnels through the membrane.

00:02:13.852 --> 00:02:17.291
And so one form of facilitated diffusion can happen

00:02:17.291 --> 00:02:19.730
through what we call channel proteins.

00:02:20.228 --> 00:02:23.913
Let me write this in orange, for no good reason.

00:02:23.913 --> 00:02:27.906
"Channel, channel, channel proteins."

00:02:29.356 --> 00:02:30.806
"Channel proteins."

00:02:31.408 --> 00:02:33.266
And an example of a channel protein

00:02:33.266 --> 00:02:34.927
might be this one right over here.

00:02:34.927 --> 00:02:37.840
And maybe this one is specialized for being

00:02:37.840 --> 00:02:40.634
a channel for water and so we would call this,

00:02:40.634 --> 00:02:44.051
this particular one, we could call an aquaporin

00:02:44.051 --> 00:02:46.365
"Aqua, aquaporin".

00:02:46.365 --> 00:02:48.467
Which is just a channel protein for water.

00:02:48.467 --> 00:02:50.057
And so you see it has this hole on top.

00:02:50.057 --> 00:02:52.250
And let's say you had more water molecules

00:02:52.250 --> 00:02:56.094
outside the cell than you have inside the cell.

00:02:56.094 --> 00:02:58.950
And you wanted to move down its concentration gradient,

00:02:58.950 --> 00:03:02.630
or maybe you have a higher concentration of solute here,

00:03:02.630 --> 00:03:04.754
and so we're going to have osmosis occurring.

00:03:04.754 --> 00:03:07.532
So the water molecules they're more likely to come,

00:03:07.532 --> 00:03:10.000
they're more like to come from the outside to the inside

00:03:10.000 --> 00:03:11.811
than from the inside to the outside,

00:03:11.811 --> 00:03:14.493
and so you could have water molecules going there.

00:03:14.493 --> 00:03:16.147
They don't even really have to mess with the membrane,

00:03:16.147 --> 00:03:20.005
they're just gonna go through this aquaporin

00:03:20.005 --> 00:03:23.728
and then come out on the inside of the cell.

00:03:23.728 --> 00:03:26.708
And you have similar channel proteins for ions.

00:03:26.708 --> 00:03:29.033
So this might be one for ions.

00:03:29.627 --> 00:03:32.481
And so, let's say that this is a sodium,

00:03:32.481 --> 00:03:35.415
these are sodium ions right over here.

00:03:35.415 --> 00:03:38.200
They're charged, they would have trouble getting through.

00:03:38.200 --> 00:03:42.653
But this channel protein might be specific to them

00:03:42.653 --> 00:03:46.374
and allows them, it allows them, to go through.

00:03:46.374 --> 00:03:49.521
And as we'll see when you study things like neurons,

00:03:49.521 --> 00:03:52.732
we'll see that these channel proteins, especially for ions,

00:03:52.732 --> 00:03:55.228
are incredibly important for amplifying

00:03:55.228 --> 00:03:58.617
an electrical signal down, or a chemo-electrical signal,

00:03:58.617 --> 00:03:59.813
I guess I could say.

00:03:59.813 --> 00:04:02.369
And they can also be gated, they can also open and close

00:04:02.369 --> 00:04:04.110
depending on different conditions

00:04:04.110 --> 00:04:06.537
that are in different parts of the cell.

00:04:06.537 --> 00:04:08.428
So these channel proteins they could just be open.

00:04:08.428 --> 00:04:09.867
Or they could be open and closed,

00:04:09.867 --> 00:04:12.325
gated, based on different conditions.

00:04:12.325 --> 00:04:14.032
Which you can see that's actually key

00:04:14.032 --> 00:04:16.144
to what happens in nerve cells,

00:04:16.144 --> 00:04:18.173
that we'll see in future videos.

00:04:18.173 --> 00:04:21.226
Now another type of facilitated diffusion can occur

00:04:21.226 --> 00:04:25.973
through what we call carrier proteins, "carrier proteins".

00:04:26.535 --> 00:04:28.838
And I wanna be clear I'm gonna talk about carrier proteins

00:04:28.838 --> 00:04:31.571
but people are still studying exactly how they work.

00:04:31.571 --> 00:04:34.702
But the view is, okay let me just draw the membrane here.

00:04:34.702 --> 00:04:38.289
Let me draw, let me draw, a membrane.

00:04:38.289 --> 00:04:42.554
I'm gonna draw a carrier protein in the membrane.

00:04:43.191 --> 00:04:45.257
So this is a...

00:04:45.257 --> 00:04:50.112
This is a cross-section of my membrane.

00:04:50.112 --> 00:04:54.820
My phospholipid bilayer here, almost done.

00:04:55.601 --> 00:04:57.461
And then a carrier protein, and the way I'm gonna draw it

00:04:57.461 --> 00:05:00.091
isn't exactly how a carrier protein would actually look,

00:05:00.091 --> 00:05:02.404
but it will hopefully give you the right idea.

00:05:02.404 --> 00:05:06.870
So maybe it's like this, maybe it's like this.

00:05:06.870 --> 00:05:10.069
And if things wanna move down their concentration gradient.

00:05:10.069 --> 00:05:12.702
Let's say you have a higher concentration above.

00:05:12.702 --> 00:05:15.923
And I'm just gonna say some arbitrary particle

00:05:15.923 --> 00:05:19.092
has a higher concentration above than it does below.

00:05:19.092 --> 00:05:23.346
They can actually attach potentially, or kind of get into

00:05:23.346 --> 00:05:25.586
a compartment over here, and then that would trigger

00:05:25.586 --> 00:05:29.278
the carrier protein to change its shape so that,

00:05:29.278 --> 00:05:32.746
and let me see if I can draw its changed shape well.

00:05:32.746 --> 00:05:34.812
So it could change its shape.

00:05:35.763 --> 00:05:37.971
This is when it's taking stuff from above.

00:05:39.591 --> 00:05:41.912
And then when it sees that, "Hey I've got stuff here",

00:05:44.422 --> 00:05:47.318
it can change its shape to look something like this.

00:05:47.318 --> 00:05:49.125
So it could kinda flip around.

00:05:49.125 --> 00:05:51.209
Let me get the other tool.

00:05:51.882 --> 00:05:55.628
It could, whoops, really having trouble with my tools today.

00:05:57.802 --> 00:06:00.648
Alright, it could flip around like this.

00:06:01.453 --> 00:06:03.544
So before it was open to the top but now,

00:06:03.544 --> 00:06:04.897
it could flip around, and the stuff that

00:06:04.897 --> 00:06:06.326
it just collected from the top,

00:06:06.326 --> 00:06:09.769
could be dumped inside the cell.

00:06:09.769 --> 00:06:12.804
And once again this is passive transport because it's all

00:06:12.804 --> 00:06:15.439
about things moving down their concentration gradient.

00:06:15.439 --> 00:06:17.181
If there was no cellular membrane here

00:06:17.181 --> 00:06:18.841
these things would have moved in this direction.

00:06:18.841 --> 00:06:20.585
You would have had more things moving in this direction,

00:06:20.585 --> 00:06:21.648
in a given amount of time, than you would

00:06:21.648 --> 00:06:23.896
have had things going in the opposite direction.

00:06:23.896 --> 00:06:25.533
But the cellular membrane was getting in the way,

00:06:25.533 --> 00:06:26.833
but then this carrier membrane

00:06:26.833 --> 00:06:29.677
can facilitate that passive transport.

00:06:29.677 --> 00:06:32.835
It can facilitate the actual diffusion.


Passive transport and selective permeability

WEBVTT
Kind: captions
Language: en

00:00:00.379 --> 00:00:01.377
- [Voiceover] - What I wanna start thinking about

00:00:01.377 --> 00:00:03.536
in these videos are ways for molecules

00:00:03.536 --> 00:00:05.347
to go across a cellular membrane,

00:00:05.347 --> 00:00:07.325
either from the outside to the inside,

00:00:07.325 --> 00:00:09.968
or from the inside to the outside.

00:00:09.968 --> 00:00:13.545
And the first type of transport of molecule across membranes

00:00:13.545 --> 00:00:15.889
that I'm gonna talk about is transport

00:00:15.889 --> 00:00:17.584
that does not require energy.

00:00:17.584 --> 00:00:20.301
It's all about molecules moving

00:00:20.301 --> 00:00:22.414
down their concentration gradient.

00:00:22.414 --> 00:00:25.740
And that type of transport we call passive transport.

00:00:26.780 --> 00:00:28.613
Passive transport.

00:00:28.613 --> 00:00:31.005
So it does not require energy.

00:00:31.005 --> 00:00:32.560
It's really just about things moving

00:00:32.560 --> 00:00:34.618
down their concentration gradient.

00:00:36.438 --> 00:00:39.202
Let me write, "Move down concentration gradient."

00:00:39.202 --> 00:00:43.308
Move down concentration,

00:00:43.308 --> 00:00:46.718
concentration gradient.

00:00:47.938 --> 00:00:50.958
Now, if you have this cellular membrane,

00:00:50.958 --> 00:00:52.321
a lot of things might want to move

00:00:52.321 --> 00:00:54.179
down their concentration gradient,

00:00:54.179 --> 00:00:57.090
but this membrane is selectively permeable.

00:00:57.090 --> 00:00:59.099
It's going to be more or less permeable

00:00:59.099 --> 00:01:01.003
to different types of molecules.

00:01:01.003 --> 00:01:02.837
So let's think about these different types of molecules

00:01:02.837 --> 00:01:05.716
and think about how they might diffuse passively

00:01:05.716 --> 00:01:07.318
across the membrane.

00:01:07.318 --> 00:01:09.664
So if we have really small molecules, we can say,

00:01:09.664 --> 00:01:11.243
"OK, they might be able to fit in the gaps

00:01:11.243 --> 00:01:13.982
"between the hydrophilic heads,

00:01:13.982 --> 00:01:16.792
"and they might be able to fit between the gaps

00:01:16.792 --> 00:01:19.067
"of the hydrophobic tails and get through."

00:01:19.067 --> 00:01:21.043
So being small is good.

00:01:22.813 --> 00:01:27.182
If you're small, that aids transport, passive transport,

00:01:27.182 --> 00:01:29.911
aids diffusion across the membrane.

00:01:29.911 --> 00:01:31.257
And in particular, it really helps

00:01:31.257 --> 00:01:33.998
to be small and non-charged.

00:01:33.998 --> 00:01:37.783
Small and no charge.

00:01:37.783 --> 00:01:41.567
So examples of that could be things like carbon dioxide.

00:01:41.567 --> 00:01:45.932
So carbon dioxide, it's a small molecule,

00:01:45.932 --> 00:01:47.814
it doesn't have a charge.

00:01:47.814 --> 00:01:49.415
So carbon dioxide molecules,

00:01:49.415 --> 00:01:53.036
if I have a higher concentration on the outside,

00:01:53.036 --> 00:01:54.976
on the outside, actually let me do the other way around.

00:01:54.976 --> 00:01:58.099
Let's say I have a higher concentration on the inside

00:01:58.099 --> 00:02:02.116
than I have on the outside.

00:02:02.116 --> 00:02:04.183
Well, just as we learned in the diffusion video,

00:02:04.183 --> 00:02:05.528
in a given amount of time,

00:02:05.528 --> 00:02:07.914
you're gonna have more carbon dioxide molecules

00:02:07.914 --> 00:02:09.049
interacting with the bottom,

00:02:09.049 --> 00:02:11.884
going from inside of the cell and interacting

00:02:11.884 --> 00:02:14.635
with the membrane, than from the outside of the cell.

00:02:14.635 --> 00:02:17.073
And sure, they don't have any charge,

00:02:17.073 --> 00:02:19.139
and so they're not going to be particularly attracted

00:02:19.139 --> 00:02:22.152
to the hydrophilic head of our phospholipids,

00:02:22.152 --> 00:02:24.039
but they're also not going to be repelled by them.

00:02:24.039 --> 00:02:25.895
And you're gonna have more on the inside interacting

00:02:25.895 --> 00:02:27.404
with the membrane than the outside,

00:02:27.404 --> 00:02:28.913
and so, since they're small,

00:02:28.913 --> 00:02:30.982
some of them are gonna able to pass through,

00:02:30.982 --> 00:02:32.363
and they're also not going to be bothered

00:02:32.363 --> 00:02:34.458
by the hydrophobic tails,

00:02:34.458 --> 00:02:35.942
and you're gonna have things going both ways,

00:02:35.942 --> 00:02:37.553
but you're gonna have more going from the inside

00:02:37.553 --> 00:02:39.814
to the outside than from the outside to the inside.

00:02:39.820 --> 00:02:41.351
So they're gonna move along

00:02:41.351 --> 00:02:43.557
with their concentration gradient.

00:02:43.557 --> 00:02:46.459
So carbon dioxide can actually diffuse quite well

00:02:46.459 --> 00:02:48.806
across cellular membranes.

00:02:48.806 --> 00:02:52.682
Another molecule that can is molecular oxygen.

00:02:52.682 --> 00:02:55.933
Molecular oxygen can also diffuse quite well

00:02:55.933 --> 00:02:57.767
across cellular membranes.

00:02:57.767 --> 00:02:59.300
So if I have a higher concentration

00:02:59.300 --> 00:03:03.990
of oxygen on the outside than I have on the inside,

00:03:03.990 --> 00:03:06.754
because it's small and it's non-charged,

00:03:06.754 --> 00:03:08.423
it's not gonna have problems.

00:03:08.423 --> 00:03:09.703
It's not gonna be particularly attracted

00:03:09.703 --> 00:03:11.467
to the hydrophilic heads,

00:03:11.467 --> 00:03:12.996
but they're small and they're gonna be able

00:03:12.996 --> 00:03:14.228
to pass right between them,

00:03:14.228 --> 00:03:15.597
it's going to be indifferent to them,

00:03:15.597 --> 00:03:16.984
and then it's gonna be able to pass

00:03:16.984 --> 00:03:18.980
through all of these hydrophobic tails,

00:03:18.980 --> 00:03:21.163
and since you have a higher concentration on the outside

00:03:21.163 --> 00:03:23.019
than the inside, you're just going to have more

00:03:23.019 --> 00:03:24.024
in a given amount of time,

00:03:24.024 --> 00:03:26.596
more random interactions of the ones going in that direction

00:03:26.596 --> 00:03:27.967
than the ones going in that direction,

00:03:27.967 --> 00:03:30.706
so you would have a net inflow into the cells.

00:03:30.706 --> 00:03:33.471
So these things are going to be able to diffuse fairly...

00:03:34.491 --> 00:03:35.862
whoops, these things are going to be able

00:03:35.862 --> 00:03:39.586
to diffuse fairly naturally.

00:03:39.586 --> 00:03:41.375
And of course, they are going to be obstructed

00:03:41.375 --> 00:03:44.509
by just the structure, by all of these molecules here

00:03:44.509 --> 00:03:46.296
that make up the actual cellular membrane,

00:03:46.296 --> 00:03:48.409
but they're going to be able to get through.

00:03:48.409 --> 00:03:50.476
Now, what about things that would have a lot

00:03:50.476 --> 00:03:52.148
of trouble getting through?

00:03:52.148 --> 00:03:53.982
So things that would have a lot of trouble getting through,

00:03:53.982 --> 00:03:58.347
would be things like a sodium ion.

00:03:58.347 --> 00:04:00.292
A sodium ion.

00:04:00.292 --> 00:04:04.714
Or a potassium ion.

00:04:05.314 --> 00:04:07.009
Why would they have trouble getting through?

00:04:07.009 --> 00:04:08.006
Well, let's just imagine.

00:04:08.006 --> 00:04:09.516
Let's say I have a higher concentration

00:04:09.516 --> 00:04:12.518
of sodium on the outside...

00:04:14.298 --> 00:04:17.782
than I have on the inside.

00:04:17.782 --> 00:04:20.407
Well, they might be attracted to the hydrophilic heads here

00:04:20.407 --> 00:04:22.193
that have some charge,

00:04:22.193 --> 00:04:24.167
but there's no reason why they would then want

00:04:24.167 --> 00:04:25.537
to go any further.

00:04:25.537 --> 00:04:28.316
They're going to be attracted to the hydrophilic heads

00:04:28.316 --> 00:04:29.659
that have charge...

00:04:31.489 --> 00:04:34.423
and the hydrophobic tails have nothing interesting for them.

00:04:34.423 --> 00:04:37.476
They're gonna wanna maybe clump around the...

00:04:38.556 --> 00:04:39.439
phosphate heads,

00:04:39.439 --> 00:04:42.619
but not be able to migrate all the way through.

00:04:42.619 --> 00:04:45.614
So things that have outright charge are gonna have trouble

00:04:45.614 --> 00:04:47.567
just passively diffusing.

00:04:47.567 --> 00:04:49.865
We'll see in future videos that there's other ways

00:04:49.865 --> 00:04:51.419
for them to get through.

00:04:51.419 --> 00:04:53.464
You have things like channel proteins,

00:04:53.464 --> 00:04:54.793
which essentially give them tunnels,

00:04:54.793 --> 00:04:56.187
and we'll talk more about that.

00:04:56.187 --> 00:05:00.437
But just naturally, natural diffusion is going to be hard

00:05:00.437 --> 00:05:01.992
for things like this.

00:05:01.992 --> 00:05:04.478
Now, what about things that are in between?

00:05:04.478 --> 00:05:06.590
What about things like water molecules?

00:05:06.590 --> 00:05:08.169
And water is incredibly important,

00:05:08.169 --> 00:05:11.280
because cells are living in an aqueous environment.

00:05:11.280 --> 00:05:13.069
They're surrounded by water on the inside of the cell

00:05:13.069 --> 00:05:14.741
and the outside of the cell.

00:05:14.741 --> 00:05:16.411
And water is in between,

00:05:16.411 --> 00:05:18.874
because it doesn't have an outright charge,

00:05:18.874 --> 00:05:20.894
but it has a partially.

00:05:20.894 --> 00:05:24.167
Water molecules, oxygen, two hydrogens.

00:05:24.167 --> 00:05:26.048
Oxygen likes to hog the electrons.

00:05:26.048 --> 00:05:28.208
It has a partial negative charge on that end.

00:05:28.208 --> 00:05:30.182
The hydrogens have their electrons charged,

00:05:30.182 --> 00:05:32.226
have a partial positive charge end...

00:05:33.496 --> 00:05:35.726
a partial positive charge on that end.

00:05:35.726 --> 00:05:38.470
And we call these phosphate heads hydrophilic,

00:05:38.470 --> 00:05:39.631
because they're attracted to water,

00:05:39.631 --> 00:05:41.211
and water is attracted to it.

00:05:41.211 --> 00:05:44.810
So the water molecules, for sure,

00:05:44.810 --> 00:05:47.150
are going to be attracted,

00:05:47.150 --> 00:05:50.840
are going to be attracted to the hydrophilic heads.

00:05:50.840 --> 00:05:54.980
but their charge isn't so strong that they can't,

00:05:54.980 --> 00:05:56.373
if you have enough interactions,

00:05:56.373 --> 00:05:57.951
a lot of them will be attracted,

00:05:57.951 --> 00:05:59.809
but some of them will actually make it through.

00:05:59.809 --> 00:06:01.365
The water molecule is small enough,

00:06:01.365 --> 00:06:04.476
and its charge is not strong enough, I guess you could say,

00:06:04.476 --> 00:06:05.637
It has some polarity,

00:06:05.637 --> 00:06:07.286
but it's going to be able to make it through,

00:06:07.286 --> 00:06:10.537
not as easily as carbon dioxide or the molecular oxygen,

00:06:10.537 --> 00:06:13.972
but it will be able to slowly diffuse through.

00:06:13.972 --> 00:06:15.667
And as we'll see, there's other ways

00:06:15.667 --> 00:06:17.246
that this can be facilitated,

00:06:17.246 --> 00:06:19.291
where the water can go through once again.

00:06:19.861 --> 00:06:22.215
We'll see in future videos things like aquaporins,

00:06:22.215 --> 00:06:25.513
tunnels through the membranes,

00:06:25.513 --> 00:06:26.762
so it doesn't have to deal with all

00:06:26.762 --> 00:06:28.857
of this business right over here.

00:06:28.857 --> 00:06:30.923
And of course, if you have really large molecules,

00:06:30.923 --> 00:06:35.218
if you had a big, honking protein right over here,

00:06:35.218 --> 00:06:36.518
this would have trouble.

00:06:36.518 --> 00:06:37.355
This would have trouble.

00:06:37.355 --> 00:06:38.911
It would have trouble even physically getting

00:06:38.911 --> 00:06:41.117
through the gaps, not to mention whether parts

00:06:41.117 --> 00:06:43.647
of it are hydrophobic or hydrophilic.

00:06:43.647 --> 00:06:46.085
So hopefully this gets you a sense of the types of things

00:06:46.085 --> 00:06:49.614
that can diffuse through a cellular membrane.

00:06:49.614 --> 00:06:52.212
This is a form of passive transport,

00:06:52.212 --> 00:06:55.052
and in the next video,

00:06:55.052 --> 00:06:58.321
we'll talk about facilitated passive transport.


Hypotonic, isotonic, and hypertonic solutions (tonicity)

WEBVTT
Kind: captions
Language: en

00:00:00.539 --> 00:00:02.611
- [Voiceover] I have three different scenarios here

00:00:02.611 --> 00:00:05.397
of a cell being immersed in a solution,

00:00:05.397 --> 00:00:07.593
and the cell is this magenta circle,

00:00:07.593 --> 00:00:09.444
that's the cellular membrane.

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I have the water molecules depicted

00:00:11.087 --> 00:00:14.072
by these blue circles, and then, I have the solute

00:00:14.072 --> 00:00:17.295
inside of the solution, inside of the water solution

00:00:17.295 --> 00:00:18.891
that we depict with these yellow circles.

00:00:18.891 --> 00:00:21.422
I've clearly exaggerated the size of the water molecules

00:00:21.422 --> 00:00:25.384
and the solute particles relative to the size of the cell,

00:00:25.384 --> 00:00:26.836
but I did that so that we can visualize

00:00:26.836 --> 00:00:28.584
what's actually going on.

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We're going to assume that the cellular membrane,

00:00:30.595 --> 00:00:34.405
this phospholipid bilayer, is semipermeable,

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that it will allow water molecules to pass in and out,

00:00:37.391 --> 00:00:39.453
so a water molecule could go from the inside

00:00:39.453 --> 00:00:42.330
to the outside, or from the outside to the inside,

00:00:42.330 --> 00:00:44.025
but we're gonna assume that it does not allow

00:00:44.025 --> 00:00:46.099
the passage of the solute particles,

00:00:46.099 --> 00:00:47.795
so that's why it's semipermeable.

00:00:47.795 --> 00:00:50.244
It's permeable to certain things,

00:00:50.244 --> 00:00:52.488
or we could say, selectively permeable.

00:00:52.488 --> 00:00:55.049
Now, what do we think is going to happen?

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Well, the first thing that you might observe is

00:00:57.212 --> 00:01:00.666
we have a lower concentration of solute on the outside

00:01:00.666 --> 00:01:02.730
than we have on the inside,

00:01:02.730 --> 00:01:05.095
so at any given moment of time, you will have

00:01:05.095 --> 00:01:07.319
some water molecules moving in just the right direction

00:01:07.319 --> 00:01:11.894
to go from the outside to the inside, and you will also have

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some water molecules that might be in just the right place

00:01:13.923 --> 00:01:16.452
to go from the inside to the outside,

00:01:16.452 --> 00:01:18.058
but what's more likely to happen,

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and what's going to happen more

00:01:19.464 --> 00:01:21.546
over a certain period of time?

00:01:21.546 --> 00:01:23.113
The water molecules that are on the outside,

00:01:23.113 --> 00:01:25.287
and we talk about this in the osmosis video,

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they're going to be less obstructed by solute particles.

00:01:28.662 --> 00:01:31.373
If this one happens to be moving in that direction,

00:01:31.373 --> 00:01:33.872
well, it's gonna make its way to the membrane,

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and then, maybe get through the membrane,

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while something, maybe, if this water molecule

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was moving in this direction, well, gee,

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it's gonna be obstructed now, maybe this is bouncing back,

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and it's gonna ricochet off of it,

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so the water molecules on the inside are more obstructed.

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They're less likely to be able to fully interact

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with the membrane or move in the right direction.

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They're being obstructed by these solute particles.

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Even though you're going to have

00:01:57.013 --> 00:01:58.902
water molecules going back and forth,

00:01:59.874 --> 00:02:01.920
in a given period of time, you have a higher probability

00:02:01.920 --> 00:02:04.196
of more going in, than going out,

00:02:04.196 --> 00:02:06.523
so you're going to have a net inflow.

00:02:06.523 --> 00:02:08.376
Net inflow

00:02:10.217 --> 00:02:14.307
of H2O, of water molecules.

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Now, a situation like this, where we're talking about a cell

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and it's in a solution that has a

00:02:19.122 --> 00:02:20.799
lower concentration of solute,

00:02:20.799 --> 00:02:23.870
it's important that we're talking about a solute

00:02:23.870 --> 00:02:25.704
that is not allowed to go to the membrane,

00:02:25.704 --> 00:02:27.868
the membrane is not permeable to that solute.

00:02:27.868 --> 00:02:30.644
We call this type of situation, this type of solution

00:02:30.644 --> 00:02:32.226
that the cell is immersed in,

00:02:32.226 --> 00:02:34.867
we call this a hypotonic solution.

00:02:38.193 --> 00:02:39.980
Hypotonic solution.

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Anytime we're talking about hypotonic,

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or as we'll see, isotonic and hypertonic,

00:02:46.112 --> 00:02:49.040
we're talking about relative concentrations of solute

00:02:50.945 --> 00:02:53.628
that cannot get through some type of a membrane.

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The word hypo, you might've seen it in other things.

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It's a prefix that means less of something, so in this case,

00:02:58.840 --> 00:03:02.875
we have a lower concentration of solute in the solution

00:03:02.875 --> 00:03:05.241
than we have inside of the cell, and because of that,

00:03:05.241 --> 00:03:06.914
you're going to have osmosis,

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you're gonna have water molecules going from the outside,

00:03:12.088 --> 00:03:14.591
I should say, to the inside.

00:03:14.591 --> 00:03:17.364
That's actually going to put pressure on the cell.

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The cell itself might expand, or it could even,

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if there's enough pressure,

00:03:22.729 --> 00:03:24.873
it might even explode.

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Now, let's go to the next scenario.

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In this scenario, we have roughly

00:03:29.869 --> 00:03:32.830
equal concentrations of solute on the outside

00:03:32.830 --> 00:03:35.923
and on the inside, at least, I tried to draw them that way.

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In this situation, the probability of a water molecule,

00:03:38.530 --> 00:03:39.966
in a given period of time,

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going from the outside to the inside,

00:03:41.886 --> 00:03:45.086
or from the inside to the outside, is going to be the same,

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so you're not going to have any net inflow or net outflow.

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You're always gonna have water molecules

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going back and forth, but there's not gonna be

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any net inflow or outflow.

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Let's see, let me write no net,

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no net flow.

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In this type of solution, where you have

00:04:01.269 --> 00:04:04.113
the same concentration of solute in the solution,

00:04:04.113 --> 00:04:08.787
as you do inside the cell, we would call this an isotonic.

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This is an isotonic solution.

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Isotonic solution.

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The prefix, iso, refers to things that are the same.

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It has the same concentration of solute,

00:04:21.633 --> 00:04:22.872
and so you have no net inflow.

00:04:22.872 --> 00:04:26.185
Hypotonic solution, you have water molecules

00:04:26.185 --> 00:04:28.346
going into the cell, the cell expanding,

00:04:28.346 --> 00:04:29.791
kind of like a filling balloon.

00:04:29.791 --> 00:04:33.030
Isotonic solution, no net flow.

00:04:33.030 --> 00:04:35.813
Of course, you could imagine in this last scenario,

00:04:35.813 --> 00:04:38.863
I have a higher concentration of solute on the outside

00:04:38.863 --> 00:04:40.527
than I have on the inside.

00:04:40.527 --> 00:04:42.022
We can guess what's going to happen.

00:04:42.022 --> 00:04:43.719
First, what would I call this?

00:04:43.719 --> 00:04:46.501
Well, I have more of something in the solution,

00:04:46.501 --> 00:04:48.788
so I would use the prefix hyper.

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I have more of it, more, hypertonic.

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This is a hypertonic solution.

00:04:56.522 --> 00:05:00.287
Once again, the solute can't go across the membrane,

00:05:00.287 --> 00:05:01.943
but the water molecules can,

00:05:01.943 --> 00:05:03.635
and you're gonna have water molecules

00:05:05.822 --> 00:05:07.491
going from the outside to the inside,

00:05:07.491 --> 00:05:09.501
and from the inside to the outside,

00:05:09.501 --> 00:05:13.138
but the probability that the ones on the inside

00:05:13.138 --> 00:05:15.790
are gonna be less obstructed to go out,

00:05:15.790 --> 00:05:18.055
than the ones on the outside to go in,

00:05:18.055 --> 00:05:19.965
so you're going to have a net outflow.

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You have a higher probability of things

00:05:21.133 --> 00:05:24.069
going from the inside to the outside,

00:05:24.069 --> 00:05:27.188
than you do from things going from the outside to the inside

00:05:27.188 --> 00:05:29.392
because they're gonna be more obstructed,

00:05:29.392 --> 00:05:32.143
so they're gonna be held back, I guess, in different ways.

00:05:32.143 --> 00:05:34.593
In this situation, you're gonna have the water

00:05:34.593 --> 00:05:38.213
escape the cell, and the cell actually might shrivel up.

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Since it's gonna lose that pressure from the water,

00:05:41.331 --> 00:05:44.398
the cell itself might shrivel up in some way.

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You could actually see this in actual living systems.

00:05:46.795 --> 00:05:48.583
If you were to put a red blood cell

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into a hypotonic solution,

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the water's gonna rush into it,

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and it's gonna blow up.

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It's going to expand, so it's gonna look like a

00:05:58.152 --> 00:06:01.160
overinflated red blood cell, and an isotonic solution

00:06:01.160 --> 00:06:02.542
is gonna look the way that we're used

00:06:02.542 --> 00:06:04.206
to seeing a red blood cell,

00:06:04.206 --> 00:06:06.879
actually, having kind of that little divot

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in the middle area, while over here,

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it's all going to expand.

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Then, in the hypertonic solution,

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the water's going to escape the red blood cell,

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then you would actually see it kind of shrivel up,

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shrivel up a little bit like this because

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we have a net outflow of water molecules.