Transport Across the Cell Membrane (Nursing)

00:00 So let's switch gears and talk about the fluidity of the membrane.

00:04 So there are several structures as we said, embedded throughout the membrane and a lot of these structures need to be mobile, they need to be able to move around the cell.

00:15 And so the dynamic flow of the membrane allows for this to happen.

00:21 As well, the membrane proteins and the lipids within it usually stay in their half of the bilayer.

00:28 So if you think about the inner half of the bilayer versus the outer half of the bilayer the membrane proteins are mostly moving lateral although some flipping can occur.

00:39 As well, emebedded within our membranes, we have cholesterol.

00:44 Cholesterol is actually there to stabilize the membrane and make it so that it is not too fluid.

00:51 So while we do want our membranes to be dynamic, we also need it to have some type of structural integrity and the cholesterol acts as kind of like the support beams in our membranes so that there is something there that makes it have a little bit of rigidity.

01:10 Now, the purpose of the plasma membrane is of course to separate our external and internal environment but because of the structure of the plasma membrane where you have the phosphate heads and the fatty acid tails, it is selectively permeable.

01:28 Why? It's selectively permeable because everything cannot pass through the fatty acid tails, they almost act as like a gatekeeper.

01:38 So, it is permeable to small, non-polar, uncharged molecules because those things can pass through the gate.

01:48 They can pass through fatty acid tails because they do not have a charge and they're not polarized in any manner.

01:55 But if you're trying to get something across the membrane whether it's getting it in or getting it out, it usually requires some type of transmembrane protein that can act as a channel or transporter in order to allow these substances to move across this gate or fatty acid tails that block everything.

02:16 And as well, sometimes things are so big that the only way they can actually get in is through vesicular transport where it is brought in through a vesicle or it is released from the cell through a vesicle, and we'll talk about that a little later.

02:33 So when it comes to movement across the plasma membrane, it usually involves a concentration gradient.

02:41 So concentration gradient is the difference in concentration of a chemical between one side of the plasma membrane and the other side of the plasma membrane.

02:50 So think about it from the standpoint of: one side might be really crowded while the other side is less crowded and things are gonna try to move away from being really crowded to less crowded.

03:02 The same way you would do, let's say in any situation where you're in a really crowded room and there's an opportunity to move away from the crowd.

03:11 As well as concentration gradients, we also have electrical gradients.

03:15 This is especially important in cells and the nervous system and in the muscular system which we will talk about later.

03:22 And if you put those two together, you can also have a gradient known as an electrochemical gradient which becomes important when we talk about cellular respiration.

03:32 So in order to move substances across the cell, there's usually two ways that this is done.

03:38 It can be a passive process where things just move down their concentration gradient.

03:44 Examples of passive processes include simple diffusion, where non-polar, uncharged substances can move passively through the plasma membrane down their concentration gradient without any help from anything else.

03:59 However, if you're trying to move things like water or ions, you might need a little bit of help.

04:05 In this we use facilitated diffusion.

04:08 Facilitated diffusion involves the use of channels and transport proteins in order to move things down the concentration gradient.

04:17 In both of these processes, it's passive because there's no energy required.

04:23 The same way, if you put a ball at the top of the hill and let it go without actually pushing it, it will roll down the hill on its own.

04:32 These processes are the same way.

04:35 A third passive process is osmosis.

04:38 Osmosis is specific for the movement of liquid across the plasma membrane.

04:44 And in this, it's also a passive process and involves movement of liquid from a higher liquid concentration to a lower liquid concentration.

04:55 Active processes involve energy input in order to move things from one side of the membrane to the other.

05:03 There are multiple types of active processes including primary and secondary transport as well as vesicular transport.

05:11 In both of these, we are usually moving something against their concentration gradient.