# crossing the membrane

**Channel:** M A
**Source:** https://www.youtube.com/watch?v=rGX1sTlFltQ
**Transcript page:** https://www.withtranscript.ai/video/rGX1sTlFltQ

## Chapters

- 0:03 — Plasma Membrane and Selective Barriers
- 6:43 — Passive Diffusion and Transport Proteins
- 14:40 — Active Transport and Sodium Pump
- 22:07 — Membrane Gradients Drive ATP Synthesis
- 26:29 — Vesicle Transport Across Membranes
- 32:33 — Endocytosis, Phagocytosis, and Immunity
- 43:31 — Enveloped Viruses and Cell Entry

## Transcript

**[0:03] Speaker A:** Transport of small molecules.

**[0:10] Speaker A:** As we all experienced in the previous chapter, our cells live in a sea of chaos, noise and entropy.

**[0:28] Speaker A:** The plasma membrane is a physical boundary between the highly organized inside of the cell and the chaotic external environment.

**[0:44] Speaker A:** However, despite the importance of separating the inside of living cells from non living outside of the cell, the plasma membrane must also allow for a dynamic interplay and exchange between the cell and its surroundings.

**[1:12] Speaker A:** So the plasma membrane is like the cell wall around the jail.

**[1:21] Speaker A:** It forms a barrier that blocks the free exchange of molecules between the cytoplasm and the external environment of the cell.

**[1:36] Speaker A:** However, this analogy is going a bit farther. Not everything is stacked in the jail. The guards can come and leave if they wish to do so, while the prisoners must get very, very special permission in order to leave.

**[2:04] Speaker A:** And their visitors must get some sort of permission to get is applied to cells as well. Some things can enter while others cannot.

**[2:23] Speaker A:** The internal composition of the cell is maintained because the plasma membrane is selective permeable to small molecules.

**[2:43] Speaker A:** Specific transport proteins, carrier protein channel proteins mediate the selective passage of small molecules across the membrane, allowing the cell to control the composition of its cytoplasm.

**[3:06] Speaker A:** So, as we discussed earlier, the plasma membrane forms a stable barrier between two polar watery compartments.

**[3:22] Speaker A:** And because of its hydrophobic interior, the lipid bilayer of cell membrane restricts the passage of most polar molecules.

**[3:39] Speaker A:** This barrier allow the cell to maintain concentrations of chemicals in its cytosol that is different from those that are in the extracellular fluids and each of the intracellular membrane enclosed compartments.

**[4:10] Speaker A:** So the idea here is that molecules that are hydrophilic a lot of time, in most of the cases they are going to be repelled by the hydrophobic membrane. They are not going to be able to enter the cell. While hydrophobic molecules such as steroids can get in without any help from channels, they simply can cross the plasma membrane as is.

**[5:01] Speaker A:** So generally, hydrophobic molecules can cross the plasma membrane by diffusion.

**[5:14] Speaker B:** In a cell membrane, phospholipids are arranged in a double layer or bilayer, with the polar hydrophilic heads immersed in water and the nonpolar hydrophobic hydrocarbon tails in the anterior of the membrane away from water, the membrane functions as a selective barrier because most molecules have trouble moving through both polar and non polar regions.

**[5:40] Speaker A:** Let's go back to water. Although water can slowly diffuse across the synthetic lipid bilayer, cells use dedicated water channels that greatly increase the permeability of their membrane to water.

**[6:05] Speaker A:** So actually, without the proper control of the water to get inside the cell, the cell will burst and actually we want to control this level.

**[6:24] Speaker A:** So that's the reason that we need those type of channels that will be able to have some control over the amount of water that is getting in the cell, getting out.

**[6:43] Speaker A:** Diffusion. The term diffusion describes the spontaneous net of movement of particles down their concentration gradient. So let's start with passive diffusion.

**[7:02] Speaker A:** Passive diffusion is the simplest mechanism by which molecules can cross the plasma membrane. The net flow of molecules is always down their concentration gradient from compartment with a high concentration to one with a lower concentration of that molecule.

**[7:35] Speaker A:** So during simple passive diffusion, a molecule moving throughout the phospholipids bilayer towards the watery solution at the other side of the membrane.

**[7:56] Speaker A:** The direction of the transport is determined simply by the relative concentration of the molecule inside and outside of the cell.

**[8:14] Speaker A:** And no energy is needed here. It's like an analogy to this couch potato. You don't need to use any energy in such a situation simply because the power is coming from the concentration gradient.

**[8:40] Speaker A:** So simple diffusion is the basic way by which molecules can move from one place to another.

**[8:56] Speaker A:** Facilitated diffusion. Actually here it requires some level of permission. Channel proteins, which are similar in some ways to the cathode versus a dog door that you have in order to let your cat or your dog in or out. If you are using cat door, probably large dogs will not be able to cross this type of permission to get in and out. If you have a cat dog, larger animals will not be able to get in.

**[9:45] Speaker A:** The raccoon is likely to stay outside of your home. But if you have a large dog and you want this dog to get in and out through a, through a special dog, well, I mean here the raccoons will be able to get there as well. So channel proteins are mainly determine about the size of the molecule, while transporter carrier or carriers actually require a bit more strict permission in order to get in or out.

**[10:31] Speaker A:** Wh Freeman have a lot of very good animations that I like to use and actually they are quite helpful in teaching I would say.

**[10:48] Speaker A:** So let's take this one that is describing the passive transport. So let's start with the very basic simple diffusion. So here you can see that what we have is high concentration of a certain molecule on the outside of the cell, low concentration inside the cell. And since this molecule can cross the membrane, so the molecules are going to cross the membrane where the net direction of movement will be according to their concentration gradient. And eventually you're going to have an equal concentration of this molecule in both sides of the the membrane.

**[11:55] Speaker A:** So that's the simple diffusion, facilitated diffusion, which are by channel proteins are again according to the size of the molecule in most of the cases.

**[12:20] Speaker A:** So here for charged substances can cause the membrane. And yes, you need a certain inducer to open up the channel. And so that's the first step, some modification of the channel that need to take place. But now when the channel is open, molecules can get in and out according to their concentration gradients. Again, they don't need to be recognized by the channel proteins.

**[13:10] Speaker A:** It's simply again like the doggy door or cat door that can allow some animals to get in and out according to their size.

**[13:22] Speaker A:** And this activity is not required energy.

**[13:31] Speaker A:** What about carrier proteins or transporter here actually we need more intimate contact between the molecule that we want to get in or out and the carrier. The carrier actually is quite specific to one molecule or another. Like glucose transporters can let glucose in the cell in or out, but in general it's going in where it's going to be metabolized. So eventually we are going to get the same concentration of glucose from one side and the other side of the membrane. The differences between the channels and the carrier transporters is the channel need to recognize the molecule to be entered inside the cell.

**[14:40] Speaker A:** Active transport.

**[14:45] Speaker A:** So active transport is mediated by transporters coupled to an energy. So and the classical example here is the sodium potassium pump.

**[15:08] Speaker A:** Normally the concentration of the sodium is significantly higher outside than inside of the cell.

**[15:22] Speaker A:** So the concentration of the potassium on the other hand is higher inside the cell as compared to the outside of the cell.

**[15:40] Speaker A:** And those ion gradients are maintained by the sodium potassium pump.

**[15:52] Speaker A:** The sodium potassium pump uses energy derived from ATP hydrolysis in order to transport the sodium and potassium against their concentration gradient.

**[16:14] Speaker A:** So what is the process by which the sodium potassium pump is working?

**[16:21] Speaker A:** So first the sodium ions bind to high affinity sites inside the cell.

**[16:36] Speaker A:** So they are going to bind to three sites which have high affinity to sodium ions. And this binding stimulates the hydrolysis of ATP and phosphorylation of the pump. So this pump have an ability to hydrolyze ATP and to become a substrate to the phosphate, in other words, to become phosphorylated.

**[17:26] Speaker A:** And the phosphorylation of the pump is inducing a conformational change that expose the sodium binding sites to the outside of the cell and reduce their affinity to sodium. So the pump is upon phosphorylation is going to change conformation. And this change of conformation actually flipped the pump in a way that the sodium now is exposed to the outside of the cell and Also, this process is reducing the affinity of the sodium to the pump.

**[18:30] Speaker A:** So because of this, the bound sodium is released into the extracellular fluid.

**[18:46] Speaker A:** At the same time, high affinity potassium binding sites are exposed on the cell surface.

**[19:01] Speaker A:** So the binding of extracellular potassium to those sites are going to take place, which is stimulating the hydrolysis of the phosphate group bound to the pump.

**[19:25] Speaker A:** So this induces a second conformational change, exposing the potassium binding sites to the cytosol and lowering their affinity, their binding affinity, so that the potassium is released inside the cell. Please note that the pump has three binding sites for sodium and two binding sites for potassium. So each cycle three sodium ions outside of the cell and two potassium inside to the cell, an activity which caused the cell 1 molecule of ATP.

**[20:34] Speaker A:** Have quite a lot of nice animation to explain different things in biology. You can find them on YouTube as well.

**[20:45] Speaker C:** The sodium potassium pump is an active transport mechanism that is driven by the breakdown of ATP and works through a series of conformational changes. In a transmembrane protein, 3 sodium ions bind to the cytoplasmic side of the protein, causing the protein to change its conformation. In its new conformation, the molecule becomes phosphorylated at the expense of a molecule of ATP.

**[21:13] Speaker C:** The phosphorylation induces a second conformational change that translocates the three sodium ions across the membrane. In this new conformation, the protein has a low affinity for sodium ions and the three bound sodium ions dissociate from the protein and and diffuse into the extracellular fluid. The new conformation has a high affinity for potassium ions, two of which bind to the extracellular side of the protein. The bound phosphate now dissociates and the protein reverts to its original conformation, exposing the two potassium ions to the cytoplasm on the inside of the cell. This conformation has a low affinity for potassium ions.

**[21:57] Speaker C:** So the two bound potassium ions dissociate from the protein and diffus interior of the cell.

**[22:07] Speaker A:** So I want to take you to another system where you are going to see a link between the passive transport and the active transport.

**[22:24] Speaker A:** So what we are talking here is the process in glycolysis and this is a process that take place in the mitochondria.

**[22:40] Speaker A:** Mitochondria have two membranes, so one of the membrane actually is going to facilitate both activities, which is active transport followed by passive transport. And that's going to be quite beautiful. I want to tell you now because in Howard University a few years ago, a structural biologist decided to open an Animation stream in his lab and he put a beautiful, beautiful animations on the web for people to use.

**[23:31] Speaker A:** And he developed a company called Biovision as well. So I think that it's still affiliated with Howard University and we are going to hear from them quite a lot. So let's start so glycolysis, the citric acid cycle.

**[23:59] Speaker A:** So what you are going to see here, and I'm going to stop this from time to time, is that here below you have a very high concentration hydrogens. These yellow dots are hydrogens. And this is a process that is mediating by active transport of hydrogen to this part of the cell. So first we are going to have concentration, high concentration of hydrogen. And this is mediating by active transport.

**[24:48] Speaker A:** And then the high concentration of hydrogen is going to drive a motor. And this is by passive transport. The passive transportation hydrogen are leading to this motto to go around and to make ATP.

**[25:20] Speaker A:** So this is the ATP synthesis which is drived by the passive transport of hydrogen that was initially made by the active transport by certain pumps that put it in the other side of the membrane. So the active transport put the hydrogen at very high concentration on one side of the membrane. And from here it is activating the ATP synthesis that is making ATP for us.

**[26:07] Speaker A:** So in general the active transport costs ATP, but the passive transport is not just that. It's not cost energy, but actually we are making energy ATP by this process.

**[26:29] Speaker A:** Okay, now let's actually switch gear towards another direction about another way that actually we discussed earlier. I want to say the vesicles, we start to talk about them earlier when we talked about the way that membranes can be fused together. And actually this is a great way to take certain things from one side of the membrane, no matter what they are, and to put them on the other side of the membrane. And here we don't have the same limitations that we add when we are using channels or carriers or even pumps. Here it's different situation.

**[27:21] Speaker A:** So again, just repeat what we did earlier. What are vesicles? Vesicles are pocket made of membrane that is separated from the cytoplasm of the cell. And here you see those blue dots that. Well, I mean no matter what they are, they can cross the membrane simply because the ability of the vesicle to be fused with the large membrane.

**[27:54] Speaker A:** And that makes vesicles a carrier of molecules in and out the cell without crossing the membrane directly. In other words, without the need of the direct cross of this molecule when they are using carriers on all the other things that we discussed up until now, and just to remind you once again that we can use those type of vesicles in an artificial way for delivery of drugs to the cell. We can even make some modifications related to the kind of reception for lipids that we want to use. So the interaction is going to probably be with more specific cells or if you really want to make this activity to be really specific, we can integrate into this vesicle some membrane proteins that can recognize the cell of the target cell. So that's another way to deal with this.

**[29:07] Speaker A:** So think that what you have, it's here, it's not water, but a drug. So the drug is going to be delivered inside the cell and that's actually going to work much better than drug that you need to take orally and then it need to cross the stomach and to be absorbed by the gastrointestinal system and eventually to reach to the entire body and hopefully also to find the cell that we need to take care of. So that's actually quite promising direction. But mother nature is using vesicles all the time. When we talk about the endoplasmic reticulum, Golgi apparatus and all this, all the movement of proteins are by vesicles.

**[30:04] Speaker A:** So here we are, Golgi apparatus. A vesicle is leaving the Golgi, moving towards the plasma membrane. That's the plasma membrane, that's the vesicle. And the interaction is take place first by interaction with the heads, mean hydrophilic interactions, and then it's going to follow by hydrophobic interactions. And whatever was on one side of the membrane is on the other side.

**[30:43] Speaker A:** And it can also go the other way around. We can actually pick up bacterial cells, macrophages for example, they can put them into a phagosome which is going to move inside the cell and it's going to be treated in the phagolysosome.

**[31:07] Speaker A:** So here we are.

**[31:13] Speaker A:** Both exocytosis and endocytosis can be mediated by vesicles.

**[31:24] Speaker A:** So the exoticytosis, the vesicle is made inside the cell. The fusion to take place with the plasma membrane and what it is inside the vesicle is going to be outside of the cell and the endocytosis is the other way around. Here we have something that we want to put inside the cell. So the vesicle is going to be made towards the inside of the cell. It's going to be pinched inside the cell where this cargo means the things that was earlier outside the cell, now they're going to be inside the cell.

**[32:23] Speaker A:** And from here they can find their way to other locations or for other activities.

**[32:33] Speaker A:** So let's go again to McGrew Hill Animation to give you the general idea of endocytosis.

**[32:47] Speaker C:** The substances taken in by single celled organisms are often particles or large polar molecules that cannot cross the hydrophobic plasma membrane. The substances taken in by single celled organisms are often particles or large polar molecules that cannot cross the hydrophobic plasma membrane. Many single celled eukaryotes employ endocytosis to ingest such food particles.

**[33:12] Speaker C:** In this process, the plasma membrane extends outward and surrounds the food particle. Cells use three major types of phagocytosis, pinocytosis and receptor mediated endocytosis. If the material the cell takes in is particulate, such as a bacterium or a fragment of organic matter, the process is called phagocytosis.

**[33:39] Speaker C:** If the material the cell takes in is liquid, it is called pinocytosis. Specific molecules, such as low density lipoproteins are often transported into eukaryotic cells through receptor mediated endocytosis. Molecules to be transported first bind to specific receptors on the plasma membrane. The interior portion of the receptor protein is embedded in the membrane. The protein clathrin coats the inside of the membrane in the area of the pit.

**[34:10] Speaker C:** When an appropriate collection of molecules gathers in the coated pit, the pit deepens and seals off to form a coated vesicle which carries the molecules into the cell.

**[34:23] Speaker C:** Exocytosis is the reverse of endocytosis. This process results in the discharge of material from vesicles at the cell surface to the outside of the cell.

**[34:36] Speaker A:** There is one more thing that I want to say here is that vesicles, when they are moving towards the outside of the cell and also the inside of the cell, actually they are moving by using the highways of the cell, which are microtubules, and they're also using motor proteins in order to go directly to where they need. So again, biovisions, which add a very, very nice work at this level, are going to help us again to show you this. So here it is. The vesicle is carried by motor proteins that look like that very much. They are moving the cargo from one place to another and are taking them directly to their destination, which is probably here, outside of the cell.

**[35:46] Speaker A:** So the activity of the endocytosis can be quite specific. It's usually quite specific because here we need some interaction, specific interaction between the material that we want in inside the cell and the membrane. So that can be small molecules or fluids, but it can be also larger organisms. And another things that this process is Going to be required is by certain proteins called caldrin, which are going to bend the membrane in such a way that it's going to turn into a vesicle. And then they are also helping this vesicle to be pinched in the cell means to be separated entirely from this plasma membrane.

**[37:06] Speaker A:** So receptors on the surface of the cell bind to specific molecules and in general we call this receptor mediated endocytosis.

**[37:21] Speaker A:** So let's put everything together when it comes to endocytosis. Endocytosis is a method enters into the cell without crossing the plasma membrane directly.

**[37:40] Speaker A:** So this is so pinocytosis when external fluid is engulfed. And receptor mediated endocytosis is the material to be transferred bind to certain specific molecules on the plasma membrane. So endocytosis is a method of entering into the cell without crossing the plasma membrane.

**[38:13] Speaker A:** Can be a molecule that can be a virus. Viruses are using this system a lot. Viruses in many ways are actually some sort of a vesicle here. I'm talking about envelope viruses. They are actually using the system quite cleverly in order to enter the cell.

**[38:37] Speaker A:** In a second we will discuss this. Bacteria, all of them from the outside of the cell, can actually bind to the membrane of the cell and can induce the cell to form a vesicle.

**[38:59] Speaker A:** A phagocytosis of an entire cell, like bacteria take place a lot in macrophages, neutrophils, and here it's going to be very effective and specific, even if you have antibodies that can help the cell to recognize the bacteria in a more efficient way. And this is a process that we called opsonization, receptomediated phagocytosis. So the way that it look like is more or less like this. Well, bacterial cells can be engulfed by macrophages, by neutrophils, but they're going to be recognized much better if they are covered by antibodies. Antibodies are made by the B cells of the immune system and those are proteins that know how to bind specifically to different type of things, I mean to their antigens. And antigen can be pretty much everything, including bacteria. So when you have antibodies which are specific to this specific bacteria, they're going to bind to its surface.

**[40:36] Speaker A:** And actually antibodies have a unique shape and it's like a Y shape in a way. So this region which is marked by red, that's called the FC portion of the antibody.

**[40:55] Speaker A:** Now we actually have FC receptors on neutrophils and macrophages. So neutrophil and macrophages are going to bind to this bacteria in a Very specific way, if you have antibodies against the bacteria. So the interaction between the neutrophil or macrophage with the bacteria, indirect, but it's specific, it's strong, it's going to make the neutrophil macrophage to make a vesicle around the bacteria. So think about this. What we have is actually other receptors that can bind to other molecules. So eventually the membrane of the neutrophilian macrophage is going to go around the bacteria. They are going to trap the bacteria in a vesicle. Right now this vesicle is not really pinched inside the cell, but you can probably get the idea what it's really going to take after this. And actually I have also animation for you to give you the entire picture. But that's actually the beginning of the receptor mediated endocytosis, where the receptor is FC receptor in such a situation.

**[42:28] Speaker A:** And this is the way that the immune system is working specifically and strongly against bacterial cells. If we have antibodies against them, if we're able to develop antibodies against them.

**[42:44] Speaker D:** The uptake of bacteria by phagocytes is an active process which requires the triggering of specific receptors on the phagocyte. FC receptors, which bind antibody coated bacteria, are one of the receptors capable of triggering phagocytosis. Binding of the aggregated antibody molecules to FC receptors on the phagocyte causes the cell to engulf the bacteria.

**[43:10] Speaker D:** The phagocyte first produces pseudopods, or ruffles that surround the bacterium and then fuse, trapping the bacterium within what is now an intracellular vesicle, the phagosome. Within the phagocyte lysosomes fuse with the phagosome, delivering their enzymatic contents to degrade the engulfed bacterium.

**[43:31] Speaker A:** Let's go back to viruses. Viruses in many ways can be looked like vesicles. And here we are talking about enveloped viruses.

**[43:43] Speaker A:** Viruses in general have a genetic material, a capsid protein capsid to cover them. But many viruses, as we discussed the first lecture, actually are coming with a membrane that they were able to pick up from the previously infected cell. So they actually look like vesicle and they act like vesicles. First, they want to recognize a receptor on the surface of the cell because they want to use the most effective way to enter the cell, which is receptor mediated endocytosis. So viruses like HIV have certain proteins on their surface, gp120, for example, and they can recognize two different receptors on helper T cells.

**[44:46] Speaker A:** And now when they can recognize the cell, the membrane of the virus can be fused with the plasma membrane and bingo, the virus is inside the cell. So that's actually the tricks that viruses are using in order to enter the cell. Again, we are talking about envelope viruses. Not all viruses are enveloped, by the way. There are viruses that need other type of tricks in order to get in.

**[45:19] Speaker A:** But envelop viruses simply need to have fusion with the plasma membrane. They need this interaction with the plasma membrane to be specific. That's the reason that viruses are specific to certain cells. And then fusion of the envelope of the membrane of this virus put the virus inside the cell. Bingo. HIV is a good example for this. But there are a lot of viruses that are using the very, very same trick. So here you can see it again. I mean, that's virus which is enveloped. This is the envelope of the virus. The virus needs to recognize receptors on the surface of the cell. The virus must have attachment proteins that are the proteins that can recognize the receptor. And that's actually determine the specificity of the virus to infect certain cells. And when this virus is bound to its target cell, the membrane can be fused with the plasma membrane, and bingo, the viruses inside.