AP Biology Cell Membranes and Transport

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We are going to continue our discussion of cell structure and function by focusing on cell membranes and transport across cell membranes.0002

We will begin by talking about the structure of cell membranes.0012

Cell membranes are composed of a phospholipid bilayer embedded with proteins.0015

Right here is the chemical structure of a phospholipid, and phospholipids have a particular property of being amphipathic.0021

Amphipathic refers to the fact that they have one region that is hydrophobic and the other that is hydrophilic.0031

The fatty acid tails right here are hydrophobic.0044

They are attached to a hydrophilic head region composed of glycerol bonded to a phosphate group, which may in turn be bonded to another group.0060

Right here, the other group is choline, although it could be serine or another group.0080

This region is hydrophobic.0088

The phosphate in its attachment groups are hydrophilic. Therefore, this is an amphipathic molecule.0091

Because of this, phospholipids congregate into bilayers, and this allows them to sequester the fatty acid tail, which are hydrophobic from the water on the outside of the cell.0098

This is a typical phospholipid bilayer structure with a hydrophobic region on the inside and the hydrophilic heads pointing outwards exposed to either the cytoplasm or the extracellular space.0115

This particular phospholipid is called phosphatidylcholine, and that is named after the type of bonds that form it as well as this choline group out here.0142

You could also have another phosphoglycerol. It has a phosphate group in the glycerol, but again, it might be phosphatidylserine or another group.0152

As I mentioned, the phospholipid bilayer contains proteins, and actually the cell membrane can be up to half protein and then, the other half phospholipids.0166

This description of the cell membrane is known as the fluid mosaic model, and let's look first at the word mosaic.0177

Mosaic is referring to the fact that it is composed of a phospholipid bilayer embedded with proteins.0185

Here, this shows, as I mentioned, the hydrophilic head regions pointing out and the hydrophobic tail regions sequestered within the cell membrane.0190

There are two major types of plasma proteins, and those are peripheral proteins.0200

Peripheral proteins are loosely associated with either the outside of the cell...they are loosely associated with the membrane0205

either at the extracellular space - let's say that this is the extracellular side - and that this is the intracellular or the cytoplasmic side.0213

Peripheral proteins are associated with one of these sides, whereas, integral proteins are embedded within the cell membrane.0226

These proteins are also amphipathic. They also have a hydrophobic region and one or more hydrophilic regions.0238

As expected, the hydrophobic region of a protein is here, embedded in the area where the lipids, which are hydrophobic are also found.0245

And the hydrophilic regions can extend outside the cell either into the extracellular space or in the cytoplasmic space.0256

These proteins do not always traverse the entire membrane. They may actually just be partially embedded.0263

You could have a protein like this, which is sticking out into the cytoplasmic side, but it does not go all the way through.0269

Those proteins that do go all the way through from one side to the other are called transmembrane proteins.0274

Another important feature of cell membranes is that cholesterol is embedded among the lipid bilayer.0289

There will be cholesterol molecules in here, and they help the cell membrane to maintain its fluidity.0297

The cell membrane is not immobile. Everything does not just stay packed in there.0302

There is actually movement, but the movement is lateral.0307

Phospholipids can actually switch places.0310

They can move back and forth, and they can actually...this one might switch places with the one adjacent. The same thing can happen over here.0313

Rarely, a phospholipid from one layer will actually flip with one with the opposite layer. Mostly, the movement is lateral.0320

Movement within the cell membrane is mostly lateral, and this is also applies to proteins.0329

Some proteins have the ability to actually move around laterally within the lipid bilayer.0335

Although, others are actually fixed in place by the cytoskeleton, by elements of the cytoskeleton.0340

Again, this fluidity refers to the fact that these are not just stuck in place. Things can move around.0350

Now, how does cholesterol help to maintain this fluidity? Well, a couple of ways.0357

First of all, because these cholesterol molecules are wedged in here, they prevent the lipid, the fatty acid tails from packing together.0362

Instead of being packed together where they cannot move, there is a little bit of space in some of these regions, and that allows for this lateral movement.0374

In addition, cholesterol stabilizes the membrane by preventing it from solidifying at low temperatures.0381

And the overall consistency of the cell membrane is similar to oil, like a type of oil that you would cook with.0390

These membrane proteins play very important functions within the cell, and we are going to talk in detail about some of these functions.0400

But to give you an overview for right now, one function of membrane proteins is that they may be enzymes.0406

Since they extend into the cytoplasmic space or at least some of them do, they can actually catalyze reactions within the cell, so there may be enzymatic activity in this region.0418

That is one function. They can function as enzymes.0428

Second is to allow for adhesion to neighboring cells, and there are various structures such as tight junctions and gap junctions and desmosomes0431

that we are going to talk about that allow for communication between neighboring cells, and they hold cells together.0441

Some proteins, which extend into the extracellular space, actually allow the cell to adhere to neighboring cells.0449

Third function is transport.0457

The cell membrane is only selectively permeable. It only allows certain substances to pass to the phospholipid bilayer.0461

In order for other substance to pass through, they need a transport protein.0468

There is a couple ways in which the proteins can transport it. It can be a channel, or it can undergo a conformational change.0473

Ether way, some of these proteins act as transfer proteins, and that is why if you look at this one it is traversing the entire cell membrane, so it could function as a transport protein.0480

Fourth function: proteins in the cell membrane maybe receptors.0492

We are going to talk in the next lecture about cell signalling, but one major way that cell signalling occurs is through receptors.0498

Molecules from another cell, for example, can come along and bind to a protein and signal the cell to grow, change in metabolism. They can be invaders that are being engulfed.0507

Again, intermembrane proteins can act as receptors.0523

And finally: recognition.0527

Cell membranes have particular types and amounts of protein depending on the type of cell, so a cell in the GI tract will have different membrane proteins than a neuron.0534

And these proteins allow other cells to recognize the cell type, and actually, what is very important in recognition are glycoproteins.0544

You can tell from the name that this is talking about proteins with...it is actually carbohydrate chains usually that are bonded to the protein.0556

You will have proteins on the cell membrane that are extruding out into the surface of the cell, and they will have this particular carbohydrate side chains that label the cell.0565

Actually, membrane lipids can also be able to help out with the labelling. They are also glycolipids.0576

Carbohydrate chains on the lipids, in the lipid bilayer can also help to label the cell.0584

OK, now, we have looked at the structure of the cell membrane. Our next step is to discuss how substances are transported across the cell membrane.0594

Cell membranes are selectively permeable to substances, as I mentioned. What this means is that some substances can cross more easily than others.0603

Some substances cannot cross at all on their own. This allows the cell to control what enters and what leaves.0611

Permeability is dependent on several factors, in particular, size, polarity and charge.0620

First, looking at size: small molecules are more permeable to the cell membrane.0628

Very small molecules can much more easily get through the cell membrane than large molecules, so the cell is less permeable to large molecules.0633

Polarity: thinking about the structure of the cell membrane, because of that hydrophobic internal region, a molecule that can cross easily is going to be a hydrophobic molecule.0643

It is going to be a non-polar molecule.0654

Polar molecules have a much harder time crossing. Although, if they are small enough, like water - water is polar, but it is small - a very small polar molecule can cross.0656

But in general, the ones that cross most easily would be both small and non-polar.0667

Charge: again, because of that hydrophobic central region, an uncharged molecule is going to have an easier time crossing than a charged molecule such as ions.0672

Ions cannot cross to the cell membrane on their own. They need the help of a transport protein.0682

Glucose is a larger molecule. It is polar.0687

It also is not going to cross the cell membrane very easily.0689

Looking at examples of substances that do cross the cell membrane more easily, those are things such as oxygen which is small, hydrocarbons, hydrophobic.0693

I talked about water being small, and yet it is polar, CO₂ small and non-polar and urea.0704

These all can cross relatively easily, whereas, larger polar molecules ions like glucose, sodium, hydrogen and calcium0711

all are going to have a difficult time crossing, if they can even cross at all without help.0721

There are two general types of transport across the cell membrane. The first is passive, and the second is active.0730

The major difference is that active transport requires energy, and in the cell, the main form of energy is ATP.0739

Active transport requires the input of energy in the form of ATP. Passive transport does not require the input of energy.0747

In this slide, we are looking just at passive transport, and there are two types of passive transport.0756

The first is simple diffusion, and the second is facilitated diffusion.0761

Starting out with simple diffusion, simple diffusion means that the substance that is crossing the cell membrane does not require a transport protein.0766

First of all, neither type requires energy, and you have to consider why that is.0777

It is actually because with diffusion of either type, a molecule is moving down its concentration gradient.0782

With diffusion, a molecule is moving down its concentration gradient.0790

Substance moves down its concentration gradient. What do we mean by down its concentration gradient?0798

Well, an area of higher concentration to an area of lower concentration- higher to lower concentration.0808

For example, imagine that on one side of the cell, I have a lot of oxygen.0820

We will have oxygen represented as just these red dots, and let's say I have a lot of oxygen molecules over here; and as you know, these can cross the cell membrane on their own.0830

They do not need the help of a transport protein.0849

Over here on the other side, let's say I have a lower concentration of oxygen.0852

Oxygen is going to want to move from the region of higher to lower oxygen concentration, so it is going to want to go in this direction.0856

Input of energy will not be required.0863

Now, if I wanted to move oxygen from this side to the other side, it would require the input of energy, and the molecule is only moving down its own concentration gradient.0866

There could be, let's say, CO₂. There could be tons of CO₂ - putting that in blue - on this side.0879

And that is not going to prevent the oxygen from moving this direction, even though there is a lot of solute on this side because it is only considering its own concentration gradient.0887

The CO₂, if it is moving down its concentration gradient, is actually going to move in the opposite direction.0895

Alright, that is simple diffusion. In simple diffusion, we are considering substances like oxygen and molecules like CO₂ that do not need the help of a transport protein.0906

They can actually cross the cell membrane.0916

Facilitated diffusion refers to substances that are still moving down their concentration gradient, but they require the help of a transport protein, for example glucose.0921

Glucose is not able to cross the cell membrane on its own generally, and I may have a lot of glucose on this side, so yellow is going to be glucose.0933

Glucose naturally wants to go down its concentration gradient, but it cannot because it cannot cross through the cell membrane.0949

However, it can use a transport protein, and this could be in a form of a channel or a protein that actually just change confirmation0957

and will allow the protein to go to the other side or...excuse me I did not mean transported to go through to the other side.0967

It is still moving down its concentration gradient. That is the important thing.0980

The fundamental difference: facilitated diffusion requires a transport protein, simple diffusion does not.0983

Water, as I mentioned, can actually cross the cell membrane on its own, but it does so relatively slowly because its polar.0991

There are particular channels called aquaporins, and they transport water across the cell membrane via facilitated diffusion.0999

And these are necessary in order to transport large quantities of water more quickly.1018

Cells like the kidney that need to move a lot of water around contain a lot of aquaporins.1023

Again, water can get across, but it can get across a lot faster through an aquaporin.1028

Osmosis is a special type of diffusion. It is diffusion of water down its concentration gradient.1039

Let's just review some terminology before we go on to discuss osmosis.1046

First of all, recall that if you have a solution, there are two parts to the solution: the liquid part, which is the solvent, and the solute, which is the substance dissolved in this liquid.1053

OK, this is a solution, and within this solution, we have a liquid, which is a solvent, and a solute, which is a substance dissolved in the liquid.1084

And as I mentioned before with diffusion, diffusion is the tendency of a substance to move down its concentration gradient.1093

If I had a container and within this container I had some water and I just dropped some sugar in the water, the sugar initially, say in a clump, is eventually going to disperse throughout.1101

It is going to move from areas of low - let's say its glucose concentration - from areas of high glucose concentration, where I dropped it, to areas of lower glucose concentration.1118

And eventually, it is going to end up being evenly dispersed throughout.1131

Now, these glucose molecules might still move around, but they are not going to all clump in one place. Overall, they are going to tend to disperse throughout.1140

Diffusion, again, is the movement of a substance from a region of higher concentration to region of lower concentration.1152

Now, let's apply that to water. Water can do the same thing.1160

Osmosis is the diffusion of water down its concentration gradient.1164

Water is going to have a tendency to move from an area of low water concentration - excuse me - high water concentration to an area of low water concentration.1167

It is going to move down its concentration gradient from a region of higher concentration to region of lower concentration.1179

Looking at these two, which is U shape, imagine that I had a membrane across here, and it was a semipermeable membrane.1186

Water can cross this membrane, but the solute that I put in cannot.1201

Imagine that I fill this tube up with water and it reaches a certain level, and on this side, I put in some sort of solute.1206

It could be salt. It could be sugar, something dissolved in here, and this substance cannot cross the membrane.1217

Water can cross the membrane.1227

What is going to happen is the water is going to move from the area of high water concentration, which is over here on the right1229

because on the left, it is lower water concentration because some of the space is being taking up by the solute.1236

The flow of water is going to be from this side to the other side.1242

What is going to end up happening, since the solute cannot cross, is I am going to end up with the level1250

of the water rising on the left side, as water moves to that side, and ending up lower on the right.1261

And this is going to occur until the point at which the concentration of water is...until it equalizes.1268

Let's say I have some solute over here but a little smaller amount, OK?1277

Then, this is going to move the water from this side to the left side until the concentration of water ends up equal on both sides.1286

That is osmosis, and osmosis can also occur in a cell because water can diffuse across the cell membrane.1298

A few terms that you should be familiar with for the test that relate to osmosis: hypertonic, hypotonic and isotonic.1308

And when you talk about hyper or hypotonic, you are talking about one solution relative to another solution.1319

Let's say I have two solutions, and the one on the left has a lot of solute in it. This other one, solution two, it has a much lower solute concentration.1326

If I compare the two, I would say that number one is hypertonic, so it has a relatively high solute concentration relative to number two.1346

Hypertonic solution has the higher solute concentration.1363

Hypotonic is the opposite. I would say that two is hypotonic relative to number one.1375

Hypotonic indicates a lower solute concentration.1386

Looking over here, this side, number one is hypertonic. Side two was relatively hypotonic.1395

Now, once the water is done diffusing across, and it has equalized the solute concentration on both sides, what we are going to end up is a situation where the two sides are isotonic.1406

This means they have the same solute concentration.1418

At this point, there is going to be no net movement of water. Water may still move across, but there is going to be no net movement.1424

It is going to be overall unchanged.1431

The solute concentration is not going to change overall, although, water may move back and forth.1434

We often talk about hyper, hypotonic, isotonic in biology relative to the cell. It is relative to the tenacity of the cell.1442

You may hear about experiments, and we will talk about situations where a cell is placed in a solution; and the solution could be, like in this case, hypotonic.1454

This solution out here is hypotonic. It is less concentrated, less solute concentration relative to the cell.1468

We could place a cell in a solution that is actually hypertonic, so here is a hypertonic solution relative to the concentration of substances inside the cell.1478

Think about what is going to happen in that situation.1498

If you put a cell in a hypotonic solution, water, as usual, is going to go from a region of higher water concentration to a region of lower water concentration.1501

With this hypotonic solution, the higher water concentration is outside the cell.1515

That means that water is going to enter the cell, and if the solution is very hypotonic, so much water is going to rush in that eventually the cell will burst or lyse.1520

The cell may lyse. The cell may actually burst because water is rushing in.1535

The volume increases and increases and eventually cell just bursts open.1540

Let's look at the situation for a hypertonic solution. In this case, again, water is going to move down its concentration gradient.1543

It is going to move from a region of higher water concentration to lower water concentration, which, in this case, is going to move outward.1552

Water is going to move from inside the cell to outside of the cell.1565

The end result will be that this cell will shrink.1570

If you place a cell in a solution that is isotonic, it has the same concentration of solutes or we would mostly say the same osmolarity.1575

Osmolarity refers to solute concentration.1587

If these two solutions have the same osmolarity, inside the cell and outside, and it is in isotonic situation, water may leave the cell.1594

Water may enter the cell, but there is not going to be any net change.1608

The amount of water that leaves the cell, the volume, is going to be equal to the amount that enters the cell, and the cell size or cell volume will not change.1612

Again, three situations: hypotonic water is going to rush into the cell the cell may lyse even. It will get bigger, possibly will lyse.1625

Hypertonic solution, outside the cell, water will leave the cell. The cell will shrink.1633

And isotonic solution, no net movement of water. The cell size will remain unchanged.1640

Recall that plant cells have cell walls, and one of the functions of cell wall is to prevent lysis in a hypotonic environment like this.1645

And in fact, in a slightly hypotonic environment, plants can maintain their rigidity.1654

They have turgor from all the water inside the central vacuole pushing against the cell wall of the plants1660

and allows the plants to maintain some rigidity so that their stems and leaves maintain their shape.1667

That is one function of the cell wall.1674

OK, this is osmosis.1676

We talked already about passive transport. Passive transport does not require the input of energy.1679

Diffusion, simple diffusion and facilitated diffusion are both forms of passive transport.1686

Osmosis is a passive transport of water. It is a form of diffusion.1693

Active transport requires the input of energy in order to move a substance against its concentration gradient.1697

Recall in diffusion a substance is moving down its concentration gradient. It is moving from higher to lower concentrations just like it wants to.1705

First, looking at regular active transport, not yet a cotransport, consider a substance such as sodium, and there is a relatively high concentration of sodium outside the cell.1715

There is a relatively low concentration of sodium inside the cell.1745

Potassium is the opposite situation. There is a relatively high concentration of potassium within the cell and a relatively low concentration outside the cell.1754

Looking at which direction things are going to want to move in, sodium is going to want to go into the cell.1771

Potassium is going to want to leave the cell. It is going to want to go down its concentration gradient and so is the sodium.1779

However, in order to maintain the correct concentrations of these ions in the cell, the cell does not just allow them to move in the directions they want to go.1785

Instead, there is what is called a sodium potassium pump that maintains the correct balance of sodium and potassium, and this is done through active transport.1797

Recall first that, since sodium and potassium are ions, they cannot just cross the cell membrane on their own because of their charge.1810

Also because I want to move them against their concentration gradient energy is going to be required, so ATP will be required.1818

The sodium potassium pump pumps sodium out of the cell.1826

The sodium is going to be pumped out of the cell at the same time that potassium is being pumped against its concentration gradient, and it is being pumped into the cell.1838

And for each ATP that is used, three sodium ions are pumped out, for every two potassium ions that are pumped in, so three sodium out for every two potassium in.1853

Again, this is going to require ATP, so ATP is going to have to be hydrolyzed to ADP to provide the energy for this process of pumping sodium out of the cell1875

against its concentration gradient and pumping potassium into the cell against its concentration gradient.1888

There is another consequence of this.1900

If three sodium are being pumped out for every two that are being pumped in, there is a net loss. This means that there is a loss of one positive charge for each cycle.1902

For each ATP used or each turn of this pump, there is a loss of one positive charge because three positive charges, three sodiums are going out. Only two are coming in, net loss of 1+ out.1921

The result is going to be that the cell has a relatively negative charge compared to the outside.1936

That charge difference is called membrane potential, and we will talk about this in more detail when we talk about the neurological system, the nervous system.1943

but different cell types have different membrane potentials.1954

For example a typical neuron has a membrane potential of approximately -70mV, and that is maintained by the sodium potassium pumped in animal cells.1958

In plant cells, fungi, bacteria, the membrane potential is actually maintained by a hydrogen pump.1972

The hydrogen pump maintains the membrane potential, this chemical gradient - excuse me - electrical chemical gradient1982

because there is a chemical gradient of sodium, potassium or hydrogen as well as an electrical gradient, this difference in potential.1989

The hydrogen pump does the work in plants, animals and fungi, whereas, the sodium pump that mainly maintains that gradient in animal cells.1997

OK, this is called primary active transport because in this case, the ATP is being used directly by the transport protein.2006

There is a second type of active transport called cotransport.2016

Let's look over here on the right at this purple protein that is going to be the cotransport protein.2021

Cotransport also uses energy, but it uses it in a slightly different way.2032

Cotransport uses the energy of a substance moving down its concentration gradient to move another substance that is moving against its concentration gradient.2037

What it does is it harnesses the energy from one substance that is going the direction that it wants to go, and it uses that to push a substance going against its concentration gradient.2049

A typical example would be glucose. Glucose is involved in cotransport, is often cotransported.2062

The sodium glucose cotransport occurs in the kidney.2071

Let's say that I have got over, on this side, this high concentration of sodium, and sodium really wants to go into the cell, where there is a low concentration of sodium.2079

This cotransport protein, this is going to be the sodium glucose cotransport protein.2091

The sodium wants to go down its concentration gradient, so as it is moving down, it is releasing energy that is harnessed to move glucose.2104

Let's say that the glucose has a high concentration in the cell, for example, and a lower concentration outside of the cell; then, glucose does not want to enter the cell.2114

It has to move via active transport.2131

If you look at this, the sodium and the glucose are moving in the same direction, but the sodium is moving down its concentration gradient,2136

whereas, the glucose is moving against its concentration gradient.2144

The glucose requires energy to move in that direction, whereas, the sodium does not.2148

And actually, if there is a high concentration of glucose out here and the glucose is needed to move in the other direction,2153

the sodium still could move down its concentration gradient and provide energy to push the glucose the other way.2159

Glucose and sodium or whatever, two things are being transported. In some systems, they are physically being transported in the same direction; in others, they are not.2167

All that matters is that one is going down its concentration gradient, and the other is going against its concentration gradient.2176

I said that active transport requires energy, and here, I am talking about the release of energy.2184

Where is the energy used? Where does this energy come from?2190

Well, it comes from this earlier step.2193

In order to maintain the gradient of the sodium in this case, if I just let this go, eventually, more and more sodium would come inside2196

and equal concentration of sodium inside and out of the cell, and there would be no net movement of sodium to harness, in order to transport the glucose.2204

For this pump to continue to work or this cotransport to continue to work, the cell has to maintain the sodium gradient.2213

The energy for cotransport is used in maintaining the gradient of the substance moving down its concentration gradient, so it is in an earlier step.2219

There is two types of active transport: primary active transport, in which the energy is used directly by the transport protein and secondary active transport,2230

in which the energy is used at an earlier step, which is to maintain a concentration gradient for a substance that will eventually move down its concentration gradient.2239

OK, these are two types of active transport, and we also talked about a couple of types of passive transport.2251

Active and passive transport are methods through which substances, molecules, can move into the cell or out of the cell.2260

Now, If a cell needs to uptake larger substances such as proteins or release larger substances such as proteins, it usually uses endocytosis and exocytosis.2272

These both require energy. They require energy usually in a form of ATP.2286

First, looking at endocytosis, which is shown here, here is the inside of the cell.2295

This is a cytoplasm.2301

Out here is the extracellular space, and we see this particle; and it is moving towards the cell.2304

The cell membrane invaginates and actually pinches off, as shown here, to form a vesicle, and then, that vesicle ends up in the intracellular space.2312

This is called endocytosis, and the opposite process can occur.2324

If I started out, if I actually went from right to left, if I started out with a substance in the cell and...so, if I had a substance in the cell, and it was packaged in the vesicle,2331

that vesicle could actually fuse with the cell membrane and then, be extruded and released outside the cell; and that is exocytosis.2349

For example, exocytosis could be used to release a protein that was made on the rough endoplasmic reticulum.2368

That protein would be, then, processed in the Golgi apparatus, packaged in the vesicle, and then, it could be released from the cell.2375

Or sometimes, this vesicle will fuse with the cell, and the protein will just end up on the cell membrane.2382

This is how cell membrane, proteins end up integrated into the membrane, or you could have a substance that has actually been released completely.2388

Neurotransmitters are released, so example, neurotransmitters are released through exocytosis also cell membrane proteins.2395

Endocytosis can be used to uptake substances such as invaders.2412

Cells of the immune systems, leukocytes, white blood cells, may ingest invaders, pathogens such as bacteria.2419

And then, the vesicle will fuse with the lysosome, which contains enzymes that can digest this invader.2429

There is a couple types of endocytosis that you should be familiar with. One is pinocytosis.2440

This is sometimes called cellular drinking because it is the process through which the cell ingests liquids.2447

If the cell pinches of a vesicle like this, and it contains a liquid, that is called pinocytosis.2455

Phagocytosis is sometimes called cellular eating. This is when solid particles are ingested by the cell through endocytosis.2462

And what I just described here with a cell-ingesting bacteria, a leukocyte, a white blood cell-ingesting bacteria, that would be a type of phagocytosis.2476

There is another type of endocytosis know as receptor-mediated endocytosis, and what this allows is for the cell to take up specific molecules.2488

The receptor shown here have specificity the same way that an enzyme has specificity for its substrate.2499

Here, we have a substance that is going to bind, and this substance that binds is going to be, once it bound, called the ligand.2508

You have a receptor and a ligand, and these two are specific.2516

They are shaped and the have a specificity to one another, so that instead of just ingesting non-specific molecules,2521

this allows certain molecules such as, a good example is LDL, cholesterol, to be ingested.2532

Here, you see the receptor binding to a ligand, and let's use LDL as an example; and these are LDL receptors and LDL.2540

The cell, again...the plasma membrane invaginates.2550

It is going to pinch off and form a vesicle in the last step, and this type of vesicle has the name coated vesicle because it is coated with its receptors bound to its ligand.2552

And then, these can be released within the cell, and then, this vesicle can actually go back; and the receptors can be recycled.2574

Or they can make their way back out to the cell membrane in a vesicle, be reused, ingest more ligand.2583

Again, LDL is a form of cholesterol found in the body that it is stands for low-density lipoprotein.2592

And there is actually a disease called familial hypercholesterolemia, which is caused by a defect in the LDL receptor.2603

Some people have a genetic defect in their LDL receptors, and the result is that they do not have enough receptors or enough functional receptors; so they do not uptake LDL.2612

If there is too much cholesterol left floating around because the cells are not taking the LDL up, the result can be heart disease, early onset of heart disease due to high cholesterol.2624

And that is a result of a defect in these receptors.2635

Let's look at some examples to sum up what we have discussed today.2641

Example one: list three characteristics of substances that are permeable to the cell membrane.2645

What type of substance will be most permeable to the cell membrane? Well, first of all, something that is small.2651

Small substances are most easily ingested, so size.2658

Second would be polarity. Non-polar molecules such as hydrocarbons are much more able to cross the cell membrane because of the hydrophobic interior of the phospholipid bilayer.2665

The third characteristic is charge. Actually an uncharged molecule is going to cross much more easily than an ion.2680

Again, this is because of the hydrophobic interior of the phospholipid membrane.2692

Molecules that are permeable to the cell membrane are likely to be small. They are going to be non-polar; and they are going to be uncharged.2699

Oxygen, CO₂, water can cross because even though it is polar, it is quite small.2707

Hydrocarbons, these are all more readily permeable to the cell membrane.2714

Example two: an animal cell is placed in pure water, what would be the outcome and why?2721

Let's sketch this out, so pure water2728

Now, I put an animal cell on it, and the animal cell is going to have all kinds of solutes, substances, in the cytoplasm, whereas, the pure water is not.2734

Therefore, what you have done is to place the cell in a hypotonic environment.2745

This is hypotonic relative to the solute concentration- osmolarity within the cell.2752

Remember that water wants to go from high concentration of water to a relatively low area of low concentration of water.2757

Out here, there is a very high concentration of water because there are relatively few solutes, whereas inside, there is a lot of solutes.2769

Water is going to rush into the cell. The cell is going to become larger and larger, and it is eventually going to burst or lyse.2777

Cell takes up water. Water is going to cross the cell membrane.2786

Via osmosis, water is going to cross. Via diffusion, via osmosis, cell will take up water and eventually lyse or burst.2793

Now, let's say we repeat this procedure in a plant cell. What would possibly happen, and why would this outcome be different?2806

Again, I have a hypotonic environment, but now, I have a plant cell; and recall that an important difference between a plant and animal cell is the presence of the cell wall.2817

Therefore, water is going to rush in, and the cell will increase in size but it will not lyse. Plant cell will not lyse due to the cell wall.2828

The cell wall provides the strength and the structure so that this cell will not just keep on absorbing water to the point that it actually lyses.2846

Example three: why is an energy required for passive transport?2857

Recall that in passive transport, substances are moving down their concentration gradient. This is the direction that a substance wants to move in.2862

If there is for example a high concentration of sodium, sodium is going to want to move to an area of low concentration of sodium. Sodium will want to move in this direction.2885

That is not going to require the input of energy because it wants to move in that direction, whereas, if the sodium were to move in the opposite direction, energy input would be required.2902

What is the source of energy used to drive a molecule against its concentration gradient in cotransport?2914

How are molecules, substances, moved against their concentration gradients in cotransport?2922

Well, energy from another substance moving down its concentration gradient is harnessed to move a second substance against its concentration gradient.2928

Remember that it requires energy to maintain the concentration gradient of the first substance, otherwise, that concentration gradient would seize to exist.2973

But the immediate source of the energy during cotransport is the release of energy from a substance moving down its concentration gradient.2984

For example, sodium moving down its concentration gradient provides the energy to move glucose against its concentration gradient in sodium-glucose cotransport.2992

What is the difference between simple diffusion and facilitated diffusion?3003

Facilitated diffusion requires a transport protein. Facilitated diffusion is used to transport substances that cannot cross the cell membrane on their own.3007

Maybe they are too large, or they are polar; or they are charged.3028

How are they the same? Well, both do not require energy.3032

In both facilitated and simple diffusion, substances are moving down their concentration gradients, and they do not require the input of energy.3042

The difference is that facilitated diffusion requires a transport protein.3049

Example four: match each of the following terms with its definition, and we have exocytosis, pinocytosis, receptor-mediated endocytosis, phagocytosis and regular endocytosis.3056

Looking at the possible definitions, molecules bind to specific receptors on the cell membrane and then, enter the cell via phagocytosis.3070

If the molecules are binding to specific receptors, and endocytosis is occurring, that would be receptor-mediated endocytosis.3083

The example I gave was with LDL. They bind to LDL receptors, and are ingested into the cell via endocytosis.3092

B: molecules contained in vesicles leave the cell by fusion of the vesicle with the cell membrane.3101

If a molecule is leaving or substance is leaving the cell in a vesicle, that is considered exocytosis.3108

So, it exiting- exo. If it is entering the cell, it is endocytosis.3119

Therefore, exocytosis, number one up here, is B, the uptake of fluids by the cell through a form of endocytosis.3123

Recall that there are a couple of types of endocytosis.3133

One is considered the cell-drinking sometimes cell-ingesting fluid by endocytosis, and that is pinocytosis.3136

D: the uptake of particles of the cell through the endocytosis, again, a particular form of endocytosis.3148

But this time, instead of fluids, we are talking about particles. It is sometimes called cellular eating.3155

And that is phagocytosis, such as when a macrophage or leukocyte ingest bacteria through the formation of a vesicle from the cell membrane.3160

Finally, molecules enter the cell by formation of vesicles derived from the cell membrane, so just the general term is endocytosis.3172

OK, we have all those taken care of, and that concludes this lesson on Educator.com.3182

Thanks for visiting.3189