Understanding Cell Transport Mechanisms

BTEC Applied Science Unit 5. This is the last of the biology videos and this is cell transport mechanisms. Now what is cell transport? Basically it's how stuff gets in and out of cells and it does that by going through the plasma membrane. So what is the plasma membrane? It's a lipid bilayer. And it is semi-permeable. It's made of these molecules, as we'll see, called phospholipids, a bilayer, as in there are two layers next to each other, and semi-permeable, as in some stuff can get through and other stuff can't get through. So a lipid bilayer that is semi-permeable and it regulates, it controls what goes into and out of the cell, which is cell transport. We call it the fluid mosaic model because it looks a bit like a mosaic, as in it's flat, a flat sheet with bits in it. A flexible elastic layer embedded with proteins. So it's not rigid at all. It can actually move an awful lot and flow almost. The molecules can actually change positions. It's a flexible elastic layer embedded with proteins. which we shall see are very important. They do different jobs. There are proteins on the surface and proteins which are actually part of the layer. They are integral to the layer. And this is the fluid mosaic model. So a phospholipid, if you look at this molecule, we have a hydrophilic head. That means it's attracted to water. it loves water and we have a hydrophobic tail and that means it doesn't like water and basically what happens is that the water loving head is attracted to the there's watery fluid outside the cell and inside the cell inside the cell it's cytosol it's the the liquid inside the cell outside the cell in between cells we call that interstitial fluid and so the water loving head is attracted to that the tail is repelled by it and so we end up with this structure where the water loving heads are on the outside of the cell and the inside of the cell and then all these tails in the middle and we end up with a a flat sheet which forms a continuous layer surrounding the cell and as i said before this structure is not rigid there's quite a lot of movement possible You'll see inside, in between these tails, there is molecules. They are cholesterol. And these cholesterol models do a few jobs, which you should remember. They make the layer less fluid and a bit more rigid. They make the layer less permeable to some small molecules. You don't want too much stuff going in and out. And they separate the tails so that they don't crystallize, so that they don't form a solid. So it actually makes it more rigid, but it stops it turning into a solid. So those are cholesterol molecules in the layer. There are two types of transport, passive transport and active transport. Now, the difference between them, passive transport doesn't require any energy input. Okay, it doesn't need any fuel to make it happen. And passive transport, there's a couple of types. First of all, just simple diffusion. If we're talking small substances and non-polar substances, you should remember what a polar molecule is from your Unit 1 chemistry. Small and non-polar substances, for example, oxygen and carbon dioxide, small fatty acids. then they can just travel through the layer. The movement of a substance from a high concentration to a low concentration, that's what diffusion is, again, unit one. And subsubstances can move through the plasma membrane. Others move through pores in the membrane. And this process requires no energy input. No energy is required. It's just a part of the natural. mixing process of diffusion. Okay, osmosis and this is to do with the movement of water. If you like it's the diffusion of water. If there's a difference in the concentration of stuff either side of the membrane, if you look at this diagram, so on one side of the membrane there's lots and lots of salts dissolved in the water, on the other side of the membrane there isn't as much. So what will happen is that water molecules will diffuse through the membrane or through pores, yeah, these proteins called pores, and they will go to the side where it's more concentrated to balance out the concentration. So diffusion of water through the membrane is osmosis, and water molecules can slip through the membrane, or there are special channels, I believe they're called aquapores. that the water molecules can pass through and that's osmosis small charged particles ions or polar molecules cannot pass through the membrane easily they can't go through the layer so they need to go through protein channels which is basically a pore p-o-r-e a hole in the membrane which is through the middle of a large protein complex. So these are protein channels. These channels may be open all the time, and they're called leak channels, or they may be gated. So they may open when something happens, and that's called a gated channel, some kind of stimulus. Lots of Unit 1 biology is coming back into my mind now. And these channels are usually specific. to a particular type of ion so this is one type of facilitated diffusion again it is passive it doesn't require any energy now small polar molecules amino acids they can get through by another mechanism another type of facilitated diffusion and this is another type of uh channel, but they are formed by these things called carrier proteins. Now what happens is that these carrier proteins, the stuff comes down and it binds to one end of the channel, and then the protein changes shape and the stuff travels through the protein and comes out of the other side. Again, this is still a passive process. It's still from high to low concentration. It doesn't require any energy input, but these are carrier proteins. They actually carry the material through the membrane by changing shape. And so we get on to active transport. And active transport, this needs energy. And the energy comes in the form of ATP. You should remember that ATP is produced in the mitochondria. ATP is the chemical energy, which is the petrol that all of our cells use to do things. And ATP is adenosine triphosphate. Triphosphate, it's got three phosphate bits to it, triphosphate. And there are two types of active transport we need to know, pumps and vesicles. So first of all, let's talk about pumps. Now, the thing is here that what we're actually doing is moving the ions against the concentration gradient. So we're taking these in this case, looking at the diagram, we're taking these sodium ions from a place where there's a low concentration to somewhere where there's a higher concentration. And that's going to need energy to do that. OK. They do this by creating an electrochemical gradient across the membrane. Basically, what they do is they create charges. So, you know, positive and negative charges attract each other. Positive charges repel each other. They create charges which create a force on the ions to push them in the direction that they want. These pumps are specific to certain substances, sodium and potassium pumps. You remember very important when we were talking about action potential in Unit 1. Hydrogen ion pumps for adjusting the pH. on either side of the membrane as well. So they are specific to certain substances and they need energy to do this and the energy comes in the form of ATP. What actually happens is that the ATP, there's a chemical reaction takes place, energy is produced to move the ion. The ATP becomes ADP. It loses one of its phosphate groups and becomes adenosine diphosphate. and that provides the energy needed for the pump to work. So these are pumps. The other type of active transport is using vesicles. From Unit 1 you should remember that vesicles are like little taxis and they carry things around through the cell, in this case to the membrane, and then through the membrane and they release them. When stuff is leaving the cell, This is called exocytosis. So when vesicles are taking stuff out of the cell, on this diagram here, that's exo. Exo means outside. So exocytosis. Large molecules. So they need to be taken in a taxi out of the cell. So in the Golgi apparatus, they are packaged, they are put into a taxi, into a vesicle. and then that vesicle travels to the membrane and it merges with the membrane it's basically made of the same stuff as the membrane and then it empties its contents outside the cell and its active transport energy is needed and that is exocytosis where stuff goes out of the cell any guesses what endocytosis is yeah endocytosis Substances are brought into the cell. So basically, this fantastic little animation here shows a white blood cell. And what it's doing is it's gobbling up a pathogen, some kind of perhaps a bacteria. And so this bacteria is entering. It's going inside the white blood cell. Substances are brought into the cell and it basically surrounds it and forms a vesicle. So the actual membrane forms a vesicle around the substance and then that is brought inside the cell and then perhaps in the case of in this case lysosomes will then digest this stuff and break it down. This is an active process energy is needed and a good example is this phagocytosis. when white blood cells or phagocytes engulf pathogens. So exo, stuff going out of the cell, endo, stuff coming into the cell. Here's a summary then of cell transport. This would be a good little flashcard. So cell transport can be passive or active. Passive doesn't require energy. Active does require energy in the form of ATP. Three types of passive, the simple diffusion, osmosis, which is to do with water, and then facilitated diffusion. And that could be from carrier proteins, which are actually change shape, or protein channels, which are basically just pores, P-O-R-E-S, pores. Active can be using pumps. or vesicles and then the vesicles could be exocytosis stuff going out or endocytosis stuff coming in one last thing to talk about in this section a little bit of maths for you to do here are two little cubes there's a one centimeter cubed and a two centimeter cubed work out the surface area of each one work out the volume of each one, and then work out the surface area divided by the volume. That's called the surface area to volume ratio. So if you want to have a go doing that, and if you do it properly, you should see that the big cube obviously has a bigger surface area and it has a bigger volume, but the surface area to volume ratio is less. it's actually half as much so that means for its size the small cube has a bigger surface area And this is why cells are small. Basically stuff needs to diffuse into and out of the cell. For example, oxygen needs to diffuse into the cell. And the rate of diffusion depends on the surface area to volume ratio. If the surface area to volume ratio is big enough, then plenty of stuff will come into the cell. And this is why cells are small. Okay, because they need this oxygen and other stuff coming in. If cells were too big, then the middle of the cell wouldn't get enough stuff. It wouldn't get enough oxygen. It wouldn't get enough nutrients. It wouldn't be able to get rid of the stuff it needs to get rid of. You should remember from Unit 3, do you remember the experiment with jelly and phenolphthalein? Look it up if you don't.

And it is semi-permeable. It's made of these molecules, as we'll see, called phospholipids, a bilayer, as in there are two layers next to each other, and semi-permeable, as in some stuff can get through and other stuff can't get through. So a lipid bilayer that is semi-permeable and it regulates, it controls what goes into and out of the cell, which is cell transport.

We call it the fluid mosaic model because it looks a bit like a mosaic, as in it's flat, a flat sheet with bits in it. A flexible elastic layer embedded with proteins. So it's not rigid at all. It can actually move an awful lot and flow almost. The molecules can actually change positions.

It's a flexible elastic layer embedded with proteins. which we shall see are very important. They do different jobs.

There are proteins on the surface and proteins which are actually part of the layer. They are integral to the layer. And this is the fluid mosaic model. So a phospholipid, if you look at this molecule, we have a hydrophilic head. That means it's attracted to water.

it loves water and we have a hydrophobic tail and that means it doesn't like water and basically what happens is that the water loving head is attracted to the there's watery fluid outside the cell and inside the cell inside the cell it's cytosol it's the the liquid inside the cell outside the cell in between cells we call that interstitial fluid and so the water loving head is attracted to that the tail is repelled by it and so we end up with this structure where the water loving heads are on the outside of the cell and the inside of the cell and then all these tails in the middle and we end up with a a flat sheet which forms a continuous layer surrounding the cell and as i said before this structure is not rigid there's quite a lot of movement possible You'll see inside, in between these tails, there is molecules. They are cholesterol. And these cholesterol models do a few jobs, which you should remember.

They make the layer less fluid and a bit more rigid. They make the layer less permeable to some small molecules. You don't want too much stuff going in and out.

And they separate the tails so that they don't crystallize, so that they don't form a solid. So it actually makes it more rigid, but it stops it turning into a solid. So those are cholesterol molecules in the layer.

There are two types of transport, passive transport and active transport. Now, the difference between them, passive transport doesn't require any energy input. Okay, it doesn't need any fuel to make it happen.

And passive transport, there's a couple of types. First of all, just simple diffusion. If we're talking small substances and non-polar substances, you should remember what a polar molecule is from your Unit 1 chemistry.

Small and non-polar substances, for example, oxygen and carbon dioxide, small fatty acids. then they can just travel through the layer. The movement of a substance from a high concentration to a low concentration, that's what diffusion is, again, unit one. And subsubstances can move through the plasma membrane. Others move through pores in the membrane.

And this process requires no energy input. No energy is required. It's just a part of the natural.

mixing process of diffusion. Okay, osmosis and this is to do with the movement of water. If you like it's the diffusion of water. If there's a difference in the concentration of stuff either side of the membrane, if you look at this diagram, so on one side of the membrane there's lots and lots of salts dissolved in the water, on the other side of the membrane there isn't as much. So what will happen is that water molecules will diffuse through the membrane or through pores, yeah, these proteins called pores, and they will go to the side where it's more concentrated to balance out the concentration.

So diffusion of water through the membrane is osmosis, and water molecules can slip through the membrane, or there are special channels, I believe they're called aquapores. that the water molecules can pass through and that's osmosis small charged particles ions or polar molecules cannot pass through the membrane easily they can't go through the layer so they need to go through protein channels which is basically a pore p-o-r-e a hole in the membrane which is through the middle of a large protein complex. So these are protein channels.

These channels may be open all the time, and they're called leak channels, or they may be gated. So they may open when something happens, and that's called a gated channel, some kind of stimulus. Lots of Unit 1 biology is coming back into my mind now.

And these channels are usually specific. to a particular type of ion so this is one type of facilitated diffusion again it is passive it doesn't require any energy now small polar molecules amino acids they can get through by another mechanism another type of facilitated diffusion and this is another type of uh channel, but they are formed by these things called carrier proteins. Now what happens is that these carrier proteins, the stuff comes down and it binds to one end of the channel, and then the protein changes shape and the stuff travels through the protein and comes out of the other side. Again, this is still a passive process. It's still from high to low concentration.

It doesn't require any energy input, but these are carrier proteins. They actually carry the material through the membrane by changing shape. And so we get on to active transport.

And active transport, this needs energy. And the energy comes in the form of ATP. You should remember that ATP is produced in the mitochondria.

ATP is the chemical energy, which is the petrol that all of our cells use to do things. And ATP is adenosine triphosphate. Triphosphate, it's got three phosphate bits to it, triphosphate.

And there are two types of active transport we need to know, pumps and vesicles. So first of all, let's talk about pumps. Now, the thing is here that what we're actually doing is moving the ions against the concentration gradient. So we're taking these in this case, looking at the diagram, we're taking these sodium ions from a place where there's a low concentration to somewhere where there's a higher concentration. And that's going to need energy to do that.

OK. They do this by creating an electrochemical gradient across the membrane. Basically, what they do is they create charges. So, you know, positive and negative charges attract each other.

Positive charges repel each other. They create charges which create a force on the ions to push them in the direction that they want. These pumps are specific to certain substances, sodium and potassium pumps. You remember very important when we were talking about action potential in Unit 1. Hydrogen ion pumps for adjusting the pH. on either side of the membrane as well.

So they are specific to certain substances and they need energy to do this and the energy comes in the form of ATP. What actually happens is that the ATP, there's a chemical reaction takes place, energy is produced to move the ion. The ATP becomes ADP.

It loses one of its phosphate groups and becomes adenosine diphosphate. and that provides the energy needed for the pump to work. So these are pumps. The other type of active transport is using vesicles. From Unit 1 you should remember that vesicles are like little taxis and they carry things around through the cell, in this case to the membrane, and then through the membrane and they release them.

When stuff is leaving the cell, This is called exocytosis. So when vesicles are taking stuff out of the cell, on this diagram here, that's exo. Exo means outside.

So exocytosis. Large molecules. So they need to be taken in a taxi out of the cell. So in the Golgi apparatus, they are packaged, they are put into a taxi, into a vesicle. and then that vesicle travels to the membrane and it merges with the membrane it's basically made of the same stuff as the membrane and then it empties its contents outside the cell and its active transport energy is needed and that is exocytosis where stuff goes out of the cell any guesses what endocytosis is yeah endocytosis Substances are brought into the cell.

So basically, this fantastic little animation here shows a white blood cell. And what it's doing is it's gobbling up a pathogen, some kind of perhaps a bacteria. And so this bacteria is entering.

It's going inside the white blood cell. Substances are brought into the cell and it basically surrounds it and forms a vesicle. So the actual membrane forms a vesicle around the substance and then that is brought inside the cell and then perhaps in the case of in this case lysosomes will then digest this stuff and break it down.

This is an active process energy is needed and a good example is this phagocytosis. when white blood cells or phagocytes engulf pathogens. So exo, stuff going out of the cell, endo, stuff coming into the cell. Here's a summary then of cell transport.

This would be a good little flashcard. So cell transport can be passive or active. Passive doesn't require energy. Active does require energy in the form of ATP. Three types of passive, the simple diffusion, osmosis, which is to do with water, and then facilitated diffusion.

And that could be from carrier proteins, which are actually change shape, or protein channels, which are basically just pores, P-O-R-E-S, pores. Active can be using pumps. or vesicles and then the vesicles could be exocytosis stuff going out or endocytosis stuff coming in one last thing to talk about in this section a little bit of maths for you to do here are two little cubes there's a one centimeter cubed and a two centimeter cubed work out the surface area of each one work out the volume of each one, and then work out the surface area divided by the volume.

That's called the surface area to volume ratio. So if you want to have a go doing that, and if you do it properly, you should see that the big cube obviously has a bigger surface area and it has a bigger volume, but the surface area to volume ratio is less. it's actually half as much so that means for its size the small cube has a bigger surface area And this is why cells are small.

Basically stuff needs to diffuse into and out of the cell. For example, oxygen needs to diffuse into the cell. And the rate of diffusion depends on the surface area to volume ratio. If the surface area to volume ratio is big enough, then plenty of stuff will come into the cell.

And this is why cells are small. Okay, because they need this oxygen and other stuff coming in. If cells were too big, then the middle of the cell wouldn't get enough stuff.

It wouldn't get enough oxygen. It wouldn't get enough nutrients. It wouldn't be able to get rid of the stuff it needs to get rid of. You should remember from Unit 3, do you remember the experiment with jelly and phenolphthalein? Look it up if you don't.

Transcript for:Understanding Cell Transport Mechanisms

Transcript for:
Understanding Cell Transport Mechanisms