0:00:01.417,0:00:03.500 In this video, we're going[br]to explore a little bit 0:00:03.500,0:00:05.680 about the cell membrane. 0:00:05.680,0:00:07.710 So just as a little[br]refresher, let's say 0:00:07.710,0:00:11.620 this is a picture of our cell[br]with a little tiny nucleus 0:00:11.620,0:00:12.840 in the middle. 0:00:12.840,0:00:16.580 Our cell membrane is what's[br]on the outside of our cell, 0:00:16.580,0:00:18.840 so our cell membrane is[br]what protects our cell 0:00:18.840,0:00:22.640 from a really harsh[br]outside environment. 0:00:22.640,0:00:24.280 If it weren't for[br]the cell membrane, 0:00:24.280,0:00:26.205 we wouldn't be alive[br]today, because there 0:00:26.205,0:00:29.290 would be nothing to protect[br]us from the outside world. 0:00:29.290,0:00:32.549 So we're going to talk[br]about the main three things 0:00:32.549,0:00:35.860 that make up the cell membrane--[br]the first, phospholipids, 0:00:35.860,0:00:39.157 the second, cholesterol,[br]and the third, proteins. 0:00:39.157,0:00:40.990 So the first one we're[br]going to talk about-- 0:00:40.990,0:00:43.400 and this makes up the[br]majority of what's 0:00:43.400,0:00:46.402 in our cell membrane--[br]are phospholipids. 0:00:46.402,0:00:47.860 And just for the[br]sake of time, I've 0:00:47.860,0:00:51.830 predrawn a picture of[br]the cell membrane here. 0:00:51.830,0:00:56.100 And you'll notice that all[br]of these individual pieces 0:00:56.100,0:00:59.480 are phospholipids, and a[br]phospholipid looks like this. 0:00:59.480,0:01:04.140 It has that polar head group,[br]that polar phosphate group. 0:01:04.140,0:01:07.285 And it has two fatty acid tails. 0:01:07.285,0:01:08.910 And so this is the[br]way that we normally 0:01:08.910,0:01:11.275 represent what a[br]phospholipid looks like. 0:01:11.275,0:01:12.650 And in the cell[br]membrane, you can 0:01:12.650,0:01:15.490 see that these phospholipids are[br]packed pretty closely, pretty 0:01:15.490,0:01:18.792 tightly together, all[br]throughout the entire membrane. 0:01:18.792,0:01:20.250 And we're looking[br]at this membrane. 0:01:20.250,0:01:22.210 This is kind of like[br]a cross-section. 0:01:22.210,0:01:25.670 You can imagine that we[br]cut the membrane in half. 0:01:25.670,0:01:27.250 So what we have here[br]is actually what 0:01:27.250,0:01:30.350 we call our[br]phospholipid bilayer, 0:01:30.350,0:01:35.189 and sometimes it's also[br]called the lipid bilayer. 0:01:35.189,0:01:37.230 The second thing that we[br]can find in our membrane 0:01:37.230,0:01:39.280 is cholesterol. 0:01:39.280,0:01:43.240 Now, we often hear cholesterol[br]in foods and cholesterol 0:01:43.240,0:01:45.474 in our blood, and we[br]think it's a bad thing. 0:01:45.474,0:01:47.140 But in this case[br]cholesterol is actually 0:01:47.140,0:01:49.960 very important for[br]our cell membrane. 0:01:49.960,0:01:51.420 And cholesterol looks like this. 0:01:51.420,0:01:52.961 And again, just for[br]the sake of time, 0:01:52.961,0:01:56.240 I've predrawn what[br]cholesterol looks like. 0:01:56.240,0:02:00.040 And you'll notice that[br]cholesterol has a lot of rings, 0:02:00.040,0:02:03.660 and this gives cholesterol[br]a pretty stable structure. 0:02:03.660,0:02:05.580 And what cholesterol[br]does is cholesterol kind 0:02:05.580,0:02:08.870 of inserts itself[br]between phospholipids, 0:02:08.870,0:02:10.209 kind of like that. 0:02:10.209,0:02:12.000 And the way I think[br]about it is cholesterol 0:02:12.000,0:02:13.950 is kind of like a buffer. 0:02:13.950,0:02:17.680 It maintains the fluidity[br]of our cell membranes. 0:02:17.680,0:02:19.910 So as temperatures[br]become lower, cholesterol 0:02:19.910,0:02:22.070 will help increase the fluidity. 0:02:22.070,0:02:23.900 And as temperatures[br]become higher, 0:02:23.900,0:02:27.350 cholesterol will help reduce the[br]fluidity of the cell membrane. 0:02:27.350,0:02:29.400 So cholesterol keeps[br]our cell membrane 0:02:29.400,0:02:33.215 in kind of a happy middle[br]ground of fluidity. 0:02:33.215,0:02:35.340 And the third thing that[br]makes up our cell membrane 0:02:35.340,0:02:38.600 are proteins, and proteins[br]are actually a big one. 0:02:38.600,0:02:40.475 And depending on[br]the cell, some cells 0:02:40.475,0:02:42.100 will actually have[br]a significant amount 0:02:42.100,0:02:44.280 of protein in the membrane. 0:02:44.280,0:02:47.370 And so proteins can[br]take two major forms. 0:02:47.370,0:02:49.460 The first is you can[br]have a protein that 0:02:49.460,0:02:51.110 crosses the entire membrane. 0:02:51.110,0:02:53.420 We call this an[br]integral protein. 0:02:53.420,0:02:57.740 We also can call this a[br]transmembrane protein. 0:02:57.740,0:03:00.930 And this can occur throughout[br]different areas of the cell, 0:03:00.930,0:03:02.770 like that. 0:03:02.770,0:03:06.090 And some proteins actually kind[br]of sit on top of the membrane, 0:03:06.090,0:03:07.560 like this. 0:03:07.560,0:03:10.550 Or they might sit on[br]another protein, like that. 0:03:10.550,0:03:13.340 And these are what we[br]call peripheral proteins. 0:03:13.340,0:03:16.120 There are some[br]very rare proteins 0:03:16.120,0:03:19.540 that actually can go halfway[br]through the membrane. 0:03:19.540,0:03:22.380 And even rarer, there[br]are occasionally 0:03:22.380,0:03:26.330 a few proteins that actually[br]can be found inside the cell 0:03:26.330,0:03:29.620 membrane, like this,[br]between the two 0:03:29.620,0:03:32.330 phospholipids[br]inside our bilayer. 0:03:32.330,0:03:34.680 Now, proteins are[br]a very big player 0:03:34.680,0:03:36.370 in the function[br]of cell membranes. 0:03:36.370,0:03:38.580 They actually carry[br]out nearly all 0:03:38.580,0:03:41.750 of the membrane processes[br]that we can think of. 0:03:41.750,0:03:45.950 And the two biggest things[br]that proteins do is, the first, 0:03:45.950,0:03:48.240 they can actually[br]act as receptors. 0:03:48.240,0:03:49.690 So the proteins[br]can actually tell 0:03:49.690,0:03:51.980 the cell what's going[br]on in the outside world. 0:03:51.980,0:03:54.340 They act as communication. 0:03:54.340,0:03:56.160 And the second thing[br]that proteins can do, 0:03:56.160,0:03:59.220 which generally occur in[br]transmembrane proteins, 0:03:59.220,0:04:01.820 is that proteins can actually[br]help transport molecules 0:04:01.820,0:04:05.335 in and out of the cell. 0:04:05.335,0:04:07.210 So now that we know the[br]function of proteins, 0:04:07.210,0:04:11.570 why do you think proteins[br]that are lipid-bound or bound 0:04:11.570,0:04:15.679 within our lipid bilayer, like[br]this one here, is so rare? 0:04:15.679,0:04:17.720 Well, it's because if[br]the role of proteins 0:04:17.720,0:04:20.730 is primarily to act as[br]receptors-- to communicate 0:04:20.730,0:04:24.760 with our outside world-- or[br]to act as transport-- to allow 0:04:24.760,0:04:26.690 things to go from the[br]inside to the outside 0:04:26.690,0:04:29.310 or the outside to the inside--[br]the proteins that are kind 0:04:29.310,0:04:32.330 of stuck in between don't really[br]have a big role in our cell 0:04:32.330,0:04:33.640 membrane. 0:04:33.640,0:04:37.470 And lastly, there's one very[br]important type of molecule 0:04:37.470,0:04:41.810 that actually binds to our[br]lipids or our proteins, 0:04:41.810,0:04:43.760 and these are carbohydrates. 0:04:43.760,0:04:46.740 And we call these[br]glyco for short. 0:04:46.740,0:04:52.160 So they would be glycoproteins,[br]or they might be glycolipids. 0:04:52.160,0:04:55.870 And what these do is they play[br]a big role in communication. 0:04:55.870,0:04:58.710 So for example, it allows[br]a cell to recognize 0:04:58.710,0:05:00.400 another cell in our body. 0:05:00.400,0:05:02.160 If they play a role[br]in communication, 0:05:02.160,0:05:04.270 in cells recognizing[br]other cells, 0:05:04.270,0:05:07.100 where do you think[br]these sugars would go? 0:05:07.100,0:05:08.970 Well, these sugars[br]would mainly occur 0:05:08.970,0:05:11.390 on the outside of our membrane. 0:05:11.390,0:05:14.680 So they would kind of[br]stick out on proteins-- 0:05:14.680,0:05:16.910 these would be[br]glycoproteins-- and they 0:05:16.910,0:05:20.510 can be on peripheral[br]or integral proteins. 0:05:20.510,0:05:23.570 Or they might stick out[br]on lipids, like this. 0:05:23.570,0:05:26.900 And these would be glycolipids. 0:05:26.900,0:05:28.910 Now, this a little[br]confusing to look at it. 0:05:28.910,0:05:30.920 What we've just drawn[br]is a cross-section 0:05:30.920,0:05:32.300 of our cell membrane. 0:05:32.300,0:05:34.320 But what if we were looking[br]at the cell membrane 0:05:34.320,0:05:37.990 from the outside, kind[br]of like a top view? 0:05:37.990,0:05:39.815 What would that look like? 0:05:39.815,0:05:41.190 Well, again for[br]the sake of time, 0:05:41.190,0:05:43.570 I've predrawn our phospholipids. 0:05:43.570,0:05:45.320 So if we were looking[br]at the cell membrane 0:05:45.320,0:05:49.960 from the outside-- looking onto[br]the top of the cell membrane-- 0:05:49.960,0:05:54.880 all we would see are these head[br]groups of our phospholipids. 0:05:54.880,0:05:57.530 We might see some cholesterol[br]in between our cell membranes, 0:05:57.530,0:05:59.530 like this. 0:05:59.530,0:06:01.580 And we might see[br]some larger proteins 0:06:01.580,0:06:03.130 that are on top of[br]our cell membrane, 0:06:03.130,0:06:07.420 like this, scattered[br]throughout our cell. 0:06:10.130,0:06:13.590 And lastly, we might actually[br]see some glycoproteins 0:06:13.590,0:06:16.256 and glycolipids on the outside. 0:06:16.256,0:06:17.630 And these would[br]attach themselves 0:06:17.630,0:06:27.812 to our proteins and our[br]phospholipids, like that. 0:06:27.812,0:06:29.770 So from the top, this is[br]what our cell membrane 0:06:29.770,0:06:32.390 would look like. 0:06:32.390,0:06:36.210 And you know something really[br]special about this-- this kind 0:06:36.210,0:06:38.600 of looks like a piece of art. 0:06:38.600,0:06:40.800 So if we think back to[br]elementary school, where 0:06:40.800,0:06:43.320 we had the project where[br]we would put a lot of beans 0:06:43.320,0:06:46.490 or different macaroni together[br]to create a piece of art, 0:06:46.490,0:06:48.790 this kind of reminds me of that. 0:06:48.790,0:06:51.100 So this is actually[br]what we call a mosaic. 0:06:51.100,0:06:54.740 So scientists kind of[br]thought the same thing. 0:06:54.740,0:06:57.220 So scientists actually[br]named this model 0:06:57.220,0:07:00.980 of the cell the[br]fluid mosaic model, 0:07:00.980,0:07:06.590 and so the mosaic portion of[br]our cell can be described here. 0:07:06.590,0:07:09.150 Again, you can see that there[br]are a lot of different pieces-- 0:07:09.150,0:07:11.680 different colorful[br]types of pieces-- 0:07:11.680,0:07:15.180 put together to create this[br]beautiful cell membrane. 0:07:15.180,0:07:18.150 But why did we call it fluid? 0:07:18.150,0:07:21.590 Well, the reason we call[br]the cell membrane fluid 0:07:21.590,0:07:24.060 is because these pieces[br]in our cell membrane 0:07:24.060,0:07:25.440 can actually move around. 0:07:25.440,0:07:27.720 They're not set in stone. 0:07:27.720,0:07:29.500 So the proteins[br]and phospholipids 0:07:29.500,0:07:32.400 in our cell membrane can[br]move around, like that. 0:07:32.400,0:07:34.356 This is why we call it fluid. 0:07:34.356,0:07:36.730 What would that look like if[br]we look at the cell membrane 0:07:36.730,0:07:38.640 from the top? 0:07:38.640,0:07:40.060 Well, the movement[br]is actually not 0:07:40.060,0:07:43.810 two-dimensional-- just up and[br]down, or just left and right. 0:07:43.810,0:07:47.220 It can actually go in a lot[br]of different directions. 0:07:47.220,0:07:49.857 So our proteins can move all[br]around the cell membrane, 0:07:49.857,0:07:51.065 and so can our phospholipids. 0:07:55.461,0:07:57.710 So again, this is what we[br]call the fluid mosaic model. 0:07:57.710,0:08:00.570 And just as a little[br]bit of a fun fact, 0:08:00.570,0:08:05.530 this was only really[br]discovered in 1972. 0:08:05.530,0:08:08.502 So it was only 40 years ago[br]that we really figured out 0:08:08.502,0:08:09.960 that our cell[br]membrane was actually 0:08:09.960,0:08:13.289 the fluid mosaic model. 0:08:13.289,0:08:15.080 So in summary, our cell[br]membrane is made up 0:08:15.080,0:08:16.950 of three major things. 0:08:16.950,0:08:18.670 The first are phospholipids. 0:08:18.670,0:08:21.100 These make up the most[br]of the cell membrane, 0:08:21.100,0:08:23.850 and they're kind of like a basic[br]building block for our cell 0:08:23.850,0:08:25.630 membrane to exist. 0:08:25.630,0:08:27.950 The second are cholesterol. 0:08:27.950,0:08:29.766 Cholesterol is[br]scattered randomly 0:08:29.766,0:08:31.140 through our cell[br]membrane, and it 0:08:31.140,0:08:34.240 helps maintain the fluidity[br]of the cell membrane. 0:08:34.240,0:08:37.230 And the third are protein, and[br]proteins carry out nearly all 0:08:37.230,0:08:40.520 of the essential cell[br]membrane functions. 0:08:40.520,0:08:43.150 And together we call this[br]our fluid mosaic model, 0:08:43.150,0:08:44.650 because our cell[br]membrane is made up 0:08:44.650,0:08:47.890 of so many different things,[br]and all of these things 0:08:47.890,0:08:50.810 are always moving[br]around like a fluid.