Cell membrane: 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 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 semipermeability. It is somewhat permeable. Some things 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 fury. 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 an 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 is 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 this 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 a new picture, we mentioned before that the cell membrane is semipermeable, 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 gases, things like o two, 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 is 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 can kind of sneak by, but pretty slowly because this very hydrophobic region is still not going to like having water in there. So if we have small polar molecules, what about something that is 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 our cells. 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 our cells 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 semipermeability. 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 prefers small molecules to pass through. So our cell membrane is semipermeable, allowing generally small and nonpolar molecules to pass through the cell membrane.
The cell membrane is also called the plasma membrane or plasma lemma. It makes the outer protective wall of the cell. It maintains the shape and size of the cell, and obviously any exchange between the inside and outside of the cell happens through the cell membrane. Only you. Now let’s talk about its structure. It’s very thin and pliable. Its thickness is only about 10.
Main constituents of the cell membrane are lipids, proteins and carbohydrates. Lipids make up about 40%, proteins about 55%, and carbohydrates about 5% of the membrane. Now let’s talk about each of these components one by one. First, the lipids. The lipids are arranged in two layers, like a sandwich. Thus they make a lipid bilayer. It mainly has two types of lipids, phospholipids and cholesterol. Along with them, there are other types of lipids as well.
Phospholipids are by far the most abundant among them. This is a phospholipid molecule. It has a phosphate head, which is hydrophilic, means it likes to be with water, and a lipid tail, which is hydrophobic. This means it likes to stay away from water. So the phospholipid molecules arrange themselves in this type of bilayer form, where phosphate groups are on the surface in contact with water and lipid groups are on the inside away from water.
This is called hydrophobic interaction. An interesting thing about this is that in this arrangement, the phospholipid molecules do not make any strong bonds with each other. These molecules are actually freely moving along the plane of the layer. Thus the membrane is actually in a liquid state, not the solid one. This is called the fluid mosaic model of the membrane.
Now let’s continue with the next lipid cholesterol. Cholesterol molecules are dissolved in the lipid bilayer. It controls the fluidity of the membrane. At a modest concentration, it decreases the fluidity, or in simple words, it makes the membrane more rigid and at high concentrations, it increases the fluidity of the membrane. So these were the lipids talking about the permeability of the lipid bilayer.
It’s semipermeable, as it’s made up of lipids. Other lipids and lipid soluble substances can dissolve in it and thus pass through it. Water and water soluble substances, as well as large molecules, cannot pass through it. The lipid bilayer makes the backbone of the membrane to which the proteins and carbohydrates are attached. And that brings us to the protein component of the cell membrane proteins are mostly in the form of glycoproteins.
Broadly, they are categorized into two types, integral membrane proteins and peripheral membrane proteins. Integral membrane proteins are tightly integrated with the lipid bilayer and peripheral membrane proteins are loosely attached to lipids or integral membrane proteins.
The membrane proteins serve as transport proteins to allow passage of water and water soluble substance receptors to receive signals as second messengers in intracellular signaling, as enzymes, as adhesion molecules to attach the cell to extracellular metrics or make cell to cell contact form a submembrane cytoskeleton that provides strength and resilience to the membrane. And finally, they may be expressed as antigens.
So these were proteins. Now let’s see the last component of the cell membrane. Carbohydrates. Most of the carbohydrates on the cell membrane are in combination with proteins and lipids in the form of glycoproteins and glycolipids. The carbohydrate portion in these molecules is usually dangling on the outer side of the cell. Along with these are also other carbohydrates attached loosely to the outer surface of the cell.
All these carbohydrates cover almost the entire outer surface of the cell. This carbohydrate coating of the cell is called glycocalyx. Functions of the carbohydrates are as follows. Most carbohydrates have a negative charge, so they make the outer surface of the cell electronegative, which repels other negatively charged objects. Second neighbor cells may get attached to one another through their glycocalyx.
They may serve receptor functions and finally they may be involved in immune reactions. So this was all about the cell membrane. Now let’s have a quick summary. The cell membrane makes the outer wall of the cell. The central lipid bilayer is made up mainly of phospholipids and cholesterol. It’s fluidic in nature and permeable to only lipid and lipid soluble substances.
Attached to the lipid bilayer are proteins that serve as transport proteins, receptors, second messengers, enzymes, adhesion molecules, submembrane, cytoskeleton and antigen. And we also have carbohydrates, making the outer coat called glycocalyx. The carbohydrates give the cell surface a negative charge, help one cell to attach to the other, serve as receptors, and participate in immune actions.
how fluid and solutes move within our body. So let’s get started. The human body likes to maintain a homeostatic environment to make sure that our fluids and solutes are equally balanced. And to do this, it has different types of transport processes that allow this to be achieved.
So in this review, I’m going to be talking about the two types of diffusion, known as simple diffusion, and facilitate diffusion, along with osmosis and active transport, and hydrostatic pressure and oncotic pressure, also known as colloidal osmotic pressure. First, let’s talk about the processes that move substances within the cell, specifically that cell membrane.
Here you’re going to see a phospholipid bilayer that is found within the cell membrane. And this phospholipid bilayer acts as this medium to really allow substances to flow in and out of the cell. So here you see the circular, yellow little balls. Those are known as the hydrophilic heads.
And then coming off those heads are the hydrophobic tails, and they really just come together to help form this barrier that separates the extracellular part, the outside of the cell, from the intracellular part, the inside of the cell. And then scattered within this membrane are these channel carrier proteins. Now, one thing I want you to remember about this bilayer is that it is very particular.
It only allows certain substances to go in through certain processes. So really, based on the size and if this solu is charged, will depend on how it’s going to enter or exit the cell. So first, let’s talk about simple diffusion. Simple diffusion is just as its name says, it’s a very simple process because it requires no energy from the cell, and it’s a passive form of transport.
So what happens with this process is that molecules, hence solutes, are going to move from a high concentration to a low concentration. So here we see outside of our cell a lot of solutes. There’s a high concentration of them, and according to simple diffusion, they are just going to easily diffuse, hence move through that phospholipid bilayer to the inside of the cell, where there’s not a lot of solutes until homeostasis has been achieved.
So this mass movement will continue until we have equilibrium, where we have a balance of these solutes. Now, an important thing to remember about simple diffusion is that only tiny, non charged molecules are going to be able to go straight through this phospholipid. Bilayer. So we’re talking about things like oxygen, carbon dioxide and so forth.
Now, if bigger charged polar molecules want to move in and out of the cell, they need to do it through a different process known as facilitated diffusion. And facilitated diffusion is very similar to simple diffusion, but with facilitated diffusion, it’s going to use these special helper proteins that are found within that phospholipid membrane to move these solutes, hence molecules, to and from the cell.
So again, these molecules, hence solutes, are going to go from a high concentration to a low concentration. It’s going to go down that concentration gradient. It’s a passive form of transport, requires no energy, but it’s going to allow big molecules that are charged and polar to move to and from the cell. They just can’t go straight through that phospholipid bilayer.
So we can move glucose and ions to and from the cell. Now we have a different type of transport that’s sort of going to do the opposite of what diffusion did. Because with diffusion, we went from high to low concentration. We were going down the concentration gradient. It was a very simple process. We didn’t have to go against it. But sometimes our body wants to move against this concentration gradient and wants to go from a low concentration to a high concentration.
And this is where active transport comes into play. So with active transport, there’s going to be the movement of molecule tense solutes from a low concentration to a high concentration through these special proteins found within that phospholipid bilayer. But it’s going to use energy in the form of ATP. So here we have our phospholipid bilayer.
Notice, on the inside of the cell, we don’t have a lot of solutes, but on the outside of the cell we have a lot of them. So with this transport process, we want to go from low to high. So we’re going against that concentration gradient rather than just down the concentration gradient. So when we go against it, it requires effort, a lot of energy. So this is where we utilize ATP.
So in order to move this molecule from the inside of the cell to the outside of the cell, it’s going to flow through this special protein channel. But ATP is going to help energize this process, and we’re going to be able to flow to that higher concentration. Now, let’s take a look at osmosis. So with osmosis, we’re talking about the movement of water.
Water is going to move through a semipermeable membrane that is only permeable to water and nothing else. And it’s going to do this through a passive process. It’s a passive form of transport, requires no energy. And the whole goal of osmosis is to achieve homeostasis in the terms of water. Water wants to shift around until we’ve equaled out the concentration of water inside and outside the cell, and we’ve equaled out that solute concentration.
So whenever you’re trying to understand osmosis, you can look at it one of two ways. One way is that you can remember that water will move from a high water concentration to a low water concentration. Or water is going to move from a low solute concentration. So the solutes, hence the dissolved substances that are in that water are low.
And that water wants to move to a fluid, hence water situation where the solutes are high, it has a high osmolarity. There’s a lot of solutes in it. So water is attracted to solutes, hence water is attracted to sodium. You got a lot of sodium on board. It’s going to draw a lot of water in. So those are two ways you can look at osmosis.
And when we’re talking about a cell, we’re talking about water moving in and out of the cell. And it all really depends on that solute concentration, whether it’s high inside of the cell or outside of the cell. So let’s look at this illustration here. On the outside of the cell or extracellular part of our cell, there is a lot of water, but there’s not a lot of solute.
So it has a low osmolarity on that extracellular fluid. But on our inside of the cell, notice there’s a lot of solutes, but not a lot of water. So it has a high osmolarity in there. So according to osmosis, effortlessly, no energy needed. That water wants to achieve some homeostasis. It’s really salty or high osmolarity inside of that cell.
So that water is going to be drawn through a semipermeable membrane, and it’s going to go and enter into that cell until it’s tried to equal out osmolarity inside and outside of the cell. And whenever that’s achieved, osmosis will cease.
Now, some problems that can arise whenever fluid does move in this direction with osmosis, because we have a cell that has such a high osmolarity, is that too much water can go inside that cell and it can cause it to swell and rupture and the flip side can happen. Let’s say that inside of the cell had a low osmolarity, but the outside of the cell had a high osmolarity, had a lot of solutes, and it didn’t have a lot of water.
Well, too much water can leave that cell and go in the opposite direction to extracellular fluid, and we could dehydrate that cell and shrink it. Now, the neat thing about this osmosis process is that in healthcare, we can actually use this to benefit our patient, because sometimes patients come in with fluid volume deficit or fluid volume overload, and they need certain fluids to help correct that imbalance.
And we can manipulate this osmosis process with those fluids based on the solute concentration of these fluids to help either rehydrate that cell or dehydrate that cell, depending on what’s going on with that patient. So if you’d like to watch more in depth review over iv fluids and this whole osmosis process, you can check out these videos up here.
So, we just reviewed how certain transport processes move fluid and substances to and from that cell through that cell membrane. Now let’s look at some processes that move fluid from the capillaries to the interstitium, also known as the tissue space, by talking about hydrostatic and oncotic pressure. Hydrostatic pressure and oncotic pressure are two pressures that literally work the opposite of each other, but they work beautifully together to help maintain fluid going across our capillary wall into our inner stitchum, our tissue space.
With oncotic pressure, it’s going to pull water across that capillary wall, and hydrostatic pressure is going to push water across that capillary wall. So first, let’s talk about oncotic pressure. So, oncotic pressure, you may hear also referred to as colloidal osmotic pressure. So if you also hear that term as well, that’s what it’s talking about, because sometimes that can get a little confusing. And this is that pulling force on water created by proteins, specifically the protein albumin, which is known as a colloid.
And a cool thing about albumin is that a lot of it hangs out in our blood plasma, and it is way, way too big to pass through that capillary wall, so it just hangs out there in that blood plasma, in that intravascular space in high concentrations. And whenever it does this, by hanging out in high concentration, it creates an osmotic pressure, which pulls water through a process known as osmosis.
And we just talked about osmosis, and we know that osmosis occurs because water loves to be where there’s a high concentration of something, hence solutes. And in this case, we’re talking about albumin. So there’s a lot of albumin hanging out in this capillary wall, which is going to result in water being pulled in. So water is going to stay inside that capillary, which is what we usually want. So, again, just to drive home that point, let’s look at this illustration.
We have this example of a capillary, and in white, you see all these colloids, hence albumin, the proteins hanging out within this vessel, and it’s highly concentrated. So what it’s going to do is it’s going to pull water from that surrounding area, that interstitial area with the fluid in there, and it’s going to cause water to stay inside that vessel. And the reason it’s doing that is because there’s a high concentration of that albumin inside that vessel.
It causes osmotic pressure to occur, which is going to pull water in there, and water is going to stay inside that vessel, hence our capillary. Now, sometimes problems arise in some patients where they don’t have enough of this albumin in their blood plasma, and they’re experiencing a condition known as hypoalbuminemia. And this can happen in cases of liver or kidney failure, because your liver makes albumin. So you just don’t have enough in your blood.
Or the patients, let’s say, had severe burn, so we’ve dropped those levels. So what do you think is going to happen if you don’t have enough albumin in your blood plasma? Well, your oncotic pressure is really going to be affected. You’re not going to have a lot of it, because there’s not enough of it hanging out in the blood to create that pressure.
So instead, water is going to leave that blood plasma, go into that interstitial space, and we’re going to experience swelling. Now, let’s talk about hydrostatic pressure. So this is the opposite of oncotic pressure, because it creates a pushing effect on water across that capillary wall. And in other words, really what hydrostatic pressure is, is it’s the pressure or force of a fluid inside a restricted space. So in our body, when we’re trying to think of that, the restricted space is going to be our blood vessels, hence our capillaries, and that fluid is going to be our blood.
So what happens is that this pressure is created somewhere and it’s created by our heart. So our heart contractions create hydrostatic pressure. And hydrostatic pressure varies throughout your circulatory system. It’s really high in the arteries, and we need it to be high in the arteries because your arteries take that fresh, oxygenated, nutrient rich blood and needs to push it out throughout your body. So we need hydrocytic pressure to be high.
But as we get closer to the venous system, it gets lower, because the venous system’s job is to take that used blood back to the heart so we can make it better again, give it more nutrients. So whenever you’re looking at the capillary and you’re trying to figure out where these pressures are highest on the end of the arterial part of the capillary is where the hydrostatic pressure is the highest versus where it’s the lowest, which is the venous end of the capillary.
And the whole goal of hydrostatic pressure is that it needs to create a process known as filtration, because we need to get this water and solutes out of the capillary into their inner stitchal fluid so it can go and do its thing and then come back to us. So what hydrostatic pressure does is it’s that pressure that pushes that water and solutes out of the capillary into the interstitial fluid, which is again known as filtration.
So, as you can see with these two processes, oncotic pressure and hydrostatic pressure, how our body needs them to work, we have one that’s going to push out the water and the nutrients, which is hydrostatic pressure, and then we have the other oncotic pressure, which is going to pull it and keep it inside the vessels. Go to Home
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