If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Cell membrane introduction

AP.BIO:
ENE‑2 (EU)
,
ENE‑2.C (LO)
,
ENE‑2.C.1 (EK)
,
ENE‑2.C.2 (EK)
,
ENE‑2.C.3 (EK)
,
ENE‑2.C.4 (EK)
,
ENE‑2.C.5 (EK)
Learn about how phospholipids form the cell membrane, and what types of molecules can passively diffuse thorugh the membrane. By William Tsai. . Created by William Tsai.

Want to join the conversation?

  • aqualine ultimate style avatar for user ㄚ蛤
    Why cell membrane prefer the nonpolar molecules?what does " polar " exactly mean?
    (12 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Sylvain
    How do the cells know how big to get?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Renie Johnston
      It is a question of surface area to mass ratio. The bigger a cell gets the less surface area it has to its mass. The cell must have enough surface area to bring in the raw materials it needs for aliostasis. Thus the size of the cell is limited by the surface area of the cell itself.
      (18 votes)
  • leafers seed style avatar for user Brandon Tom Agulto Gonzales
    wait, if the fatty acid tails are hydrophobic then how can h2o pass through it?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Sam
    There is a mistake at , he meant hydrophilic heads not hydrophobic heads.
    (6 votes)
    Default Khan Academy avatar avatar for user
  • leaf grey style avatar for user patoof
    What are polar and nonpolar molecules?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • duskpin sapling style avatar for user Rana
      A polar molecule is one that has a permanent dipole. This means that the molecule has an unequal distribution of electron density, which can be caused by differing electronegativity of the atoms within the molecule or by the presence of loan pairs on the central atom (such as in water, H2O). An example of a polar molecule is H-F.
      A non-polar molecule is one that does not have a permanent dipole. There is a relatively equal sharing of electrons (for instance, in diatomic molecules such as H2 and O2) or there is a symmetry in the way the electrons are distributed (such as in CO2 - even though oxygen is more electronegative, the lewis structure of the molecule is O=C=O so the equal but opposite pull of electrons cancel each other out, making it a non-polar molecule).
      (6 votes)
  • leaf red style avatar for user Jack McClelland
    If "hydrophilic" means "loves water", does it dissolve in anything?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • leaf blue style avatar for user Peterson
      In the case of our human cells, and their hydrophilic exteriors, then no, they do not dissolve. If you think about it, if they were to dissolve, then our skin would dissolve whenever we bathe or wash our hands, because it (our skin) is made up of cells that have membranes of hydrophilic molecules.
      (5 votes)
  • piceratops seed style avatar for user Nurul  Hidayah
    What is the function of glycocalyx?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Ras_Mekonen
      Glycocalyx is a glycoprotein-polysaccharide coating outside of the plasma membrane of bacterial cells. It can also be found in epithelial cells of animals. This molecule is unique to a specific type of cell. It is like an ID and serves as a recognition of the cell.
      (4 votes)
  • aqualine ultimate style avatar for user milind
    Is glucose absorbed in the same way fructose is absorbed by the intestine ?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • hopper cool style avatar for user Steven Yan
      Technically no. Glucose enters the enterocyte through a Na+-dependent transporter (SGLUT-1). Fructose is not co-transported with sodium, but it enters by another transporter (GLUT5). That is on the apical or luminal side. However, on the basolateral side, both are transported out of the enterocyte through another transporter (GLUT-2) into the bloodstream.

      Glucose and galactose are absorbed in the exact same way through the same transporter.

      That may be more specific than you may have wanted, but I hope that is all clear to you.
      (5 votes)
  • leafers seedling style avatar for user Katy Ching
    why is fatty acid tails hydrophobia?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • male robot johnny style avatar for user frigui.sara
      Fatty acid tails are made of hydrocarbons, which are nonpolar. Since water itself is polar, and fatty acid tails are not (and similar things are attracted each other and vice versa), fatty acid tails are hydrophobic, therefore not attracted to water
      (2 votes)
  • leaf red style avatar for user Jack McClelland
    What if a cell was not permeable at all?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

So when you go swimming or showering, have you ever wondered why don't your cells in your body fill up with water or why don't the substances in your cells leak into the pool? Well, the reason is because we actually have a very important structure that prevents this from happening. This is what we call the cell membrane. The cell membrane is what's on the outside of a cell. So if we have a very basic picture of a cell here with a little nucleus on the inside, this pink outside layer is what we call the cell membrane. The cell membrane can protect our cell from the outside environment, and it can determine what can enter and leave our cell. This is a property that we call semi-permeability. It is somewhat permeable. Somethings can enter, while other things cannot. So since this is such an important part of our cell-- in fact, it's one of the reasons why we can actually survive in the world. So what actually makes up this structure? Well, the main building block of a cell membrane are what we call phospholipids. There are other substances that make up our cell membrane, but the most important building block are phospholipids. And so phospholipids have three major components. The first is a phosphate head group. The second is a glycerol backbone, and the third are two fatty acid tails. So the way we draw this is we give the phosphate head group kind of like a head. It's a circle. And two fatty acid tails hang down from it, kind of like strings on a balloon. So the way I kind of remember this is a phospholipid looks like a balloon, but with two strings. Now, where's our glycerol backbone? Well, our glycerol backbone is actually what it sounds like. It's what holds the fatty acid tails to our phosphate head. It's the backbone of this molecule. So it's usually not drawn in the picture. But just remember that it's there, and it holds our two fatty acid tails to our phosphate head group. So this structure actually has a very interesting property. Up here, this head group is actually hydrophilic or polar. So hydrophilic means that it's water loving. This phosphate head group will do whatever it can to get to water. It loves water. But these fatty acid tails, because they're very, very long carbon chains, this is hydrophobic. I remember hydrophobic because a phobic or phobia is fearing. So hydro is water, so it's water fearing. These two fatty acids will do whatever it can to get away from water. A molecule that has both of these things together is what we call and amphipathic molecule. It means that the molecule has a hydrophobic section and a hydrophilic section. So in water, what would this do? So let's say we put a ton of these molecules in water. Once in water, the hydrophobic heads want to be as close to water as possible. But the tails don't. So what will happen is these phosphate groups are going to cluster together while the tails try to shield themselves away from water. But since this is a substance that's in water, water's going to be down here, too. So this will actually form a really unique structure, because the fatty acid tails are going to start grouping like this. And the phospholipids are going to be kind of upside down so that the phosphate head groups can be close to water, while this inside section can be hydrophobic and away from water. This is what we call a phospholipid bilayer. This is the basic structure of a cell membrane. And like we mentioned, this inside section is going to be hydrophobic. So now we have the structure that looks kind of like this. We call this our phospholipid bilayer, or lipid bilayer for short. But doesn't this section here also interact with water? How can this structure be like this if this section here still touches water? And we know that the fatty acid tails don't want to touch water. Well, in a cell in real life. What actually happens is we end up with the structure that forms a circle like this. Now, this is a fairly crudely drawn picture. In a cell, this wall is actually pretty thin compared to the entire body. So you'll notice that this water here doesn't become a problem anymore, because in our actual cells, water can be on the outside and on the inside. And no matter where this cell membrane touches water, it's always going to be the phosphate head groups that are hydrophilic that are seeking out water. And inside the cell membrane, we actually have a hydrophobic section. So moving on to new picture, we mentioned before that the cell membrane is semi-permeable, and we're going to explore that a little bit more. So I've taken the liberty of pre drawing a very long picture of a cell membrane. So as we mentioned, the cell membrane is actually a sphere that surrounds our cell. For the sake of this lesson, we're going to draw it out in a straight line. And we're going to say that this can be the outside environment or the extracellular, and this can be the inside or the intracellular. So you'll notice that the cell membrane has these phospholipids packed really closely together, so usually small molecules are what can pass through the cell. Another property of the cell membrane that we've discussed is that this inside section right here is really hydrophobic. So generally small, nonpolar molecules can pass through our cell membrane. This is what we call passive diffusion. So what is a good example of a small nonpolar molecule? Well, the most common type of small nonpolar molecule tend to be gasses, things like, O2, for example, or CO2. These are things that surround us every single day. And our cell, in a sense, breathes these molecules in and out of our cell. So gases can very easily pass through our cell membrane, and it's very fast. They are small and they are nonpolar. So what else does our cell interact with every single day? Well, the most common one is water. So water's actually a pretty small molecule, and it's polar. So something else that's similar to water is ethanol. This is like alcohol that we can drink. So how do these interact with our cell membrane? Well, we said that the cell membrane likes small molecules, so these can actually pass through our cell membrane. But our cell membrane prefers nonpolar molecules. So these are actually going to pass through really slowly, and they can pass through because they're so tiny that they kind of sneak by, but pretty slowly, because this very hydrophobic region is still not going to like having the water in there. So if we have small polar molecules, what about something that is a large but nonpolar, like benzene? Benzene can actually pass through our cell membrane. Even though it's large, it's nonpolar. So it's going to get along really well with that hydrophobic region in our cell membrane, but it's going to pass very slowly. Now, as a little bit of a fun fact, benzene used to be used in labs for students and researchers to wash their hands. Scientists actually found out that benzene can pass through our cell membrane and cause harm to ourselves. What about something that is large and polar? Well, a molecule like this would be sugar or glucose. Glucose actually cannot pass through our cell. It's large and it's polar. It's the complete opposite of what the cell membrane allows to pass through the cell. So glucose will have to be absorbed by ourselves through other means, but it cannot pass through the cell membrane. What about charged molecules? These are also all over the place. What's an example of a charged molecule? Well, something like a chloride ion, a sodium ion, or any sort of ion. Another pretty common charged molecule are actually amino acids. And since these are charged, they're so incredibly polar or charged that they also cannot pass through. So, in summary, our cell membrane protects our cells and determines what enters and leaves, a property that we call semi-permeability. And this cell membrane is made up of a whole bunch of phospholipids put together. Since our cell membrane has a very large hydrophobic region, it prefers nonpolar molecules. And since these phospholipids are packed so closely together, our cell membrane also prefer small molecules to pass through. So our cell membrane is semi-permeable, allowing, generally, small and nonpolar molecules to pass through the cell membrane.