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Lecture Comments (4)

1 answer

Last reply by: Bryan Cardella
Tue Sep 30, 2014 10:41 AM

Post by King Calculus on September 29, 2014

You speak of the neuron and the electrical energy that takes place along the axon. Is this similar to what is going on in Parkinson's disease where there is too much electrical activity in the neuron, or is it too little neural activity in the in the basal ganglia or too much in the substantia nigra? By the way, you're a great teacher, thanks for doing what you do, you're a hero.

1 answer

Last reply by: Bryan Cardella
Sun Jul 13, 2014 11:47 AM

Post by Brady Dill on July 12, 2014

Osmosis: What is the reason water is pulled to areas of lower water concentration? What force pulls it? Some sort of pressure?

Cellular Transport

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Passive Transport 0:05
    • Movement of Substances in Nature Without the Input of Energy
    • High Concentration to Low Concentration
    • Opposite of Active Transport
    • No Net Movement
  • Diffusion 3:55
    • Definition of Diffusion
    • Examples
  • Facilitated Diffusion 7:32
    • Definition of Facilitated Diffusion
  • Osmosis 9:34
    • Definition of Osmosis
    • Examples
  • Concentration Gradient 15:55
    • Definition of Concentration Gradient
  • Relative Concentrations 17:32
    • Hypertonic Solution
    • Hypotonic Solution
    • Isotonic Solution
  • Active Transport 22:49
    • Movement of Molecules Across a Membrane with the Use Energy
    • Example
  • Endocytosis 25:53
    • Wrapping Around of Part of the Plasma
    • Examples
    • Phagocytosis
    • Pinocytosis
  • Exocytosis 29:40
    • Releasing Material From Inside of a Cell
    • Opposite of Endocytosis

Transcription: Cellular Transport

Hi, welcome back to, this is the lesson on cellular transport.0000

When we talk about cellular transport, there are two main categories, passive or active.0006

We are going to start with passive transport, 0011

which is the transport that happens across the membranes of cells without the use of energy, it is very natural thing.0013

So this is the natural movement of surfaces in nature, without the input of energy. 0020

You do not need ATP, you do not need to force molecules against where it is going, it just happens naturally.0024

It is about particles bumping into each other and kind of spreading out over time.0030

It is a movement from a high concentration to a low concentration, whether it is solute or solvent.0036

Solute this is particles dissolved in water.0043

We will talk about diffusion in a bit.0057

Diffusion is passive transport of particles in water or air.0059

The term solute is usually used to describe something dissolved in water, salt, sugar, food coloring, a lot of different things.0065

And then, solvent is the H₂O, that is nature solvent.0073

In other science, you will talk a lot about other solvents, other liquid substances.0080

But water, that is the classic solvent for biology.0089

Sometimes, we talk about diffusion of the solvent which is actually osmosis.0093

We will talk more about that on later slide.0098

Passive transport is the opposite of active transport.0102

Like I said, active transport will be the use of energy, forcing molecules against what is called the concentration gradient.0105

Let me give you an example of passive transport, a practical example, one you can do at home.0112

This is a container with water in it.0119

Here is a bunch of solvent, we are going to put solute in there.0123

This is a bunch of red food coloring, dye.0131

And then, on this side, on the right hand side, I am going to put a bunch of blue food coloring and just let it sit.0135

We are not going to mess with it, we are not going stir it because that would be applying energy,0147

adding a force in there to expedite the process.0152

But if we just let it stay, you can probably guess what is going to happen.0156

The red spreads out, the blue spreads out, and we get a purple solution.0163

If you know enough about paints and the color wheel, you have heard of this, red and blue makes purple.0174

I want you to think that once you know the blue spreads out, once the red spreads out, they just stay still.0183

There are still going to be movement of the food coloring molecules around but there is no net movement.0191

The term no net movement means that, if you measure everything as it is in a whole system, 0198

you do not have a significant change in where the things are.0211

Across a membrane will make more sense, let me give you some examples of that later on this lesson.0214

You can still have red food coloring and blue food coloring moving around, they are not static.0219

But there is no net movement all in all in the whole solution, to reach what is called dynamic equilibrium.0225

I will bring up that term a little bit later.0232

Diffusion, this is a kind of passive transport involving the movement of particles in air or water,0237

from a high concentration to a low concentration.0243

That food coloring example in the previous slide, a good example of diffusion of that food coloring substance.0246

Here this could be sugar or salt.0253

you drop a sugar cube in water, it is very highly concentrated in that sugar cube.0256

Overtime, gradually those pieces dissolved and eventually spread out throughout the fluid, throughout the solid.0262

Obviously, if you do not put a spoon in there like poke it,0271

you are going to accelerate the process but then that would not be entirely passive, right?0273

Here you have a case where it is reached dynamic equilibrium.0278

Equilibrium meaning like kind of equal, in terms of being balanced, and dynamic meaning not static.0293

Static is just still an unchanging, but it is dynamic because you still get these little red dots,0299

whatever molecule this is, it could be sugar, kind of moving around through the fluid.0305

But all in all, the concentration here and here remains about equal.0311

It has reached equilibrium, they cannot spread out any more than they are, but it is not static.0319

In terms of membrane, I will bring it up in a little bit.0326

Air freshener definitely diffuses through the air, same with cologne or perfume.0329

I did an example in class with students where I spray cologne and as they smell it, as they can actually smell it, they stand up.0334

And then, standing up in sequence as the cologne molecules spread out 0344

or defused through the classroom is a good visual depiction of like how these molecules are actually spreading out.0350

In terms of like air freshener, spraying that in a room comes out highly concentrated from that container.0357

And then, it diffuses, it spreads out through the room and it reaches its dynamic equilibrium.0365

But, it does not stop there and eventually goes away because there is the hole under the door,0370

there is the vent, and there is the window.0376

It will eventually leave, that is why air fresheners does not last forever, same with cologne or perfume.0379

Eventually, it comes off your body, it is not going to stay there forever.0384

Oxygen in your blood stream, definitely is a practical diffusion example for organisms.0390

Every time you inhale oxygen out of the air, it goes from your lungs into your bloodstream.0395

The oxygen gets highly concentrated in your bloodstream, compared to what it once was.0401

You needed oxygen, that is why you inhale it.0405

Now, it is highly concentrated in your blood because of your heart beating, gets pumped through out of your body.0407

As the blood comes up to the neighboring cells that it delivers oxygen to, 0413

you do not have to use energy to get oxygen into the cells, it just naturally diffuses in there.0417

It goes from a high concentration in your bloodstream to a lower concentration, 0422

as your cells needed it, it was lacking in oxygen, into the cells.0427

That is how it spreads out in your cells through this passive process known as diffusion.0431

CO₂, carbon dioxide goes the opposite direction also passively.0436

As it builds up in your cells because of some waste product of metabolism,0441

it ends up in your bloodstream and goes the opposite direction back to your lungs that you can exhale it.0445

Facilitated diffusion, if you facilitate a process in real life with the social circumstance, you are helping it along,0454

you are helping make it happen and kind of leading it, right.0465

Facilitated diffusion is still diffusion, in terms of cellular movement, moving something in and out of a cell.0468

But here, we have an assistance, something that is helping it along.0476

But no energy is used, this is still passive.0480

It is a type of diffusion which membrane proteins assist with the movement of particles from high to low concentration.0484

Still diffusion but membrane proteins assisting it along.0490

Here is the extra cellular fluid meaning the fluid, the aqueous environment outside of the cell.0494

Here is the intra cellular fluid also known as cytoplasm, with your cytosol.0500

Here are membrane proteins imbedded in the phospholipid bilayer, you can see here.0506

Here is the molecule we are talking about, there is definitely a higher concentration outside than inside, you can tell it visually.0512

Diffusion would move it gradually inside but it is not just simple diffusion here, you have facilitation of that.0521

These protein and these, are helping move these molecules.0528

It could be an amino acid, it could be a hormone, it could be sugar molecules.0534

Regardless what it is, it is helping to move it in without the use of energy.0543

Think of it as like a saloon door, you do not have to really push very hard on a saloon door.0548

You know the one that go like this to get inside or to get outside.0553

You are just going to barely bump the door then it will move.0557

It is the same idea here, proteins that do this, you do not need to attach an energy molecule0561

to manipulate them, in order to do this.0566

They will naturally do it to help diffusion along across the membrane.0569

Osmosis, this is talking about diffusion of water.0575

Now, we are focusing on the solvent rather than solute.0579

This is the natural movement of water from a high concentration to low concentration of water.0582

We are focusing on water here, not salt or sugar.0591

Another way to think of it is this, you can also think of it as water moves to where there is more solute, which is less water. 0594

It is by concentration.0606

I will give you some examples below so that this will make sense, 0615

in terms of why we are focusing on concentration rather than volume.0619

You can think of it as more solute is where it is going because on one side of the membrane, 0624

if you have more solute, like more salt, well then there is less water.0628

If you consider all that solution, if there is 100% of it there, well if it is 5% salt, it is 95% water.0633

On the inside of the cell, if it is 1% salt, it is 99% water, in terms of what is in the solution.0642

Let us give some examples here.0649

I am now drawing a U Tube not the web site, this is the original U tube.0654

See, it is a U tube, it is literally a U tube.0665

We have water in this U shaped tube, another important aspect is this.0669

This is a membrane that is permeable only to water.0675

It is a semi permeable membrane, it is only letting water back and forth, nothing else will go across.0691

Salt would not, sugar would not, amino acids would not, food coloring would not, just water, H₂O.0696

That is important for this osmosis distinction0704

because we are talking about the movement of the water, not movement of particles, in this case.0706

Here is the water level, I want to say it starts out being about equal on both sides.0713

Water here, water here, equal volumes on both sides,0719

but we are going to have a difference in the amount of particles on both sides.0722

On this side, there is that concentration of solute.0728

On this side, we have a lot more of, let us say it is salt.0737

Clearly, I made a lot of dots here.0748

Visually, you could tell there is a higher concentration of solute here and slightly less concentration of water compared to the other side.0751

Since water goes to where there is less concentration of water, goes to a lower concentration of water, 0758

or to where there is more solute, where is it going to go?0765

It is going to go to the left.0770

Water will go to the left to try to equalize the concentration of solute on both sides, until it reaches dynamic equilibrium.0774

And if it cannot, it will keep going until there is no water here and it is overflowing at the top on the other side.0782

That is the movement that osmosis makes happen.0788

Let me give you a different example, just so that we can make sure the distinction is correct, in terms of the understanding.0793

Semi permeable membrane only permeable to water, but now a slightly different case.0804

We are going to say that we are starting out with very different levels of water on both sides of the membrane.0813

Here is still our membrane.0823

We are starting out with less than half of the volume on this side and this side.0826

Let us say it is a little bit different.0832

You got just a little bit of solute there, but on this side now, we have got a lot of solute, sugar, salt, whatever it is.0834

Okay, that is good enough.0851

Now, the question is, what direction it is going to go? Is it going to the left? Is it going to go to the right?0853

Well, since water goes to where there is less concentration of water, not to where there is less in volume. 0859

The water this time goes to the right.0867

This is an important distinction because if you are thinking that the definition is wrong, 0870

if you think that water is going towards where there is less water by volume, 0873

meaning the ml of water, you think it is going to go here but it does not.0876

Even there is less amount of water here, there is a higher concentration here of water.0881

Let us say, it is 99% water and 1% salt, over here could be 97% water and about 3% salt,0887

which is close to the ocean level of salt concentration.0895

This really does suck water out of the left, water goes from left to right.0899

It will continue to do that until there is an equalization of the concentration of solute on both sides,0904

and concentration of water on both sides.0910

There is osmosis.0912

This is why drinking seawater, if you are stranded in the middle of ocean on a raft, will be a problem.0915

Try to catch a fish and eat the meat inside of there and will be enough water to let you survive for a while.0923

But, drinking seawater will actually hasten how quickly you will reach your demise because this exact scenario will occur.0930

You will be drinking this stuff that is really highly concentrated with salt.0941

As it goes down your digestive track, in your bloodstream, it will suck water because of osmosis, 0945

out of your cells and will dehydrate you even faster.0950

Concentration gradient, whether we are talking about diffusion or osmosis,0956

concentration gradients are gradual changes in solute or solvent concentrations over a distance, in a particular solution.0961

Here actually, we are talking about concentration gradient of solute, of stuff dissolved in liquid.0968

Here, we have got a little arrow that shows the concentration gradient from high to low.0974

It is going in this direction for Na⁺, sodium ions.0980

These little hexagons represent all the sodium ions.0985

Outside of the cell, there is more of them and you can see the arrow is pointing in here because they are gradually going this way.0988

The interesting thing is, when you look at amino acids, the arrow is going this way, as if they have been pulling amino acids in.0997

If that is the case, amino acids this would be actually active transport, 1006

if they are still getting pulled in against the concentration gradient.1012

Maybe this particular membrane would not let this particular Na⁺ diffuse naturally.1017

But, if these are being put out like this one is, they are being pushed out into the extracellular space, 1024

this will actually be active transport of these amino acids.1030

If I were to erase all of these arrows, we would be saying that,1035

actually the concentration gradient would move all of these little amino acids out of the cell.1039

Because, the concentration gradient for the amino acids, in this case would be the opposite direction as the sodium ions.1046

Relative concentrations, when we look at cells in relation to the amount of solute inside and outside the cell,1054

some things will happen in terms of movement, in terms of movement of water, specially.1062

Let us pretend we got a red blood cell here, they are really, really tiny but we are zoomed in.1068

Here is a red blood cell and RBC, let us say you put the red blood cells in what is called a hyper tonic solution.1075

Hyper, that does not mean just like you are hyper.1082

In terms of a prefix, hyper like if you say hyper speed, that is really fast excessive speed.1087

Hyper here can mean too much excess, like hyperglycemic means the sugar level in your blood stream is too high,1094

or hypertonic means too much solute or excess solute.1102

Tonic here is referring to solute, think about tonic water.1107

It is like club soda but with a little bit extra tonicity, a little bit extra solute in there like quinines,1113

is an ingredient in there that gives little extra flavoring.1120

Tonic here, we are talking about hypertonic excess solute in the solution outside of the cell.1123

This red blood cell has been put in an environment where there is a lot more solute outside of it than inside of it.1132

Let us assume that this membrane is only permeable to water, just like with that U tube example from a couple of slides ago.1144

The membrane of a red blood cell only will let water through.1151

Here, we are not going to talk about diffusion, we are going to talk about osmosis.1155

What direction will osmosis move the water?1159

Water will leave the cell and it will shrivel up like a resin.1164

It will probably die especially if the relative tonicity, the hypertonicity outside the cell is way more than on the inside.1174

You know if there is a tiny bit more cellular on the outside, maybe there is just a little bit of water will move out,1183

and then there will be no net movement because it is balanced out the concentration and it has reached a dynamic equilibrium.1190

Oftentimes, putting a red blood cell in the solution like ocean water or salt water, it is going to shrivel out.1197

Next scenario, here is red blood cells again, hypotonic solutions.1206

Hypo means the opposite of hyper, like hypoglycemic too little sugar content in the bloodstream.1210

If you are going to be weak, you need to eat something, if you are hypoglycemic.1219

Hypodermis means the layer of tissue beneath your dermis like deep in your skin, that is below or under.1223

This means like too little, under, below, same with tonic, this is the opposite.1232

You have got hardly on the outside of the cell compared to the insides.1241

The inside, definitely you got a hypotonic solution.1246

I can even erase these and make it pure H₂O, that would be the most hypotonic you can get.1252

As you may guess, it is going to be the opposite here.1259

If the membrane is only permeable to water, water will rush in and it will swell, potentially pop.1264

Especially, if you have a huge difference in the amount of water concentration on either side, 1273

it might swell so much that just pops.1277

A red blood cell will burst and rupture, typically in that case.1281

Isotonic, I think you can catch where this is going.1285

Iso means equal, here we got a case where the solution is just fine and dandy.1291

I’m going to visually try to indicate that it is the same concentration on the inside and outside.1298

Now, that does not mean that you have literally the same amount of solute particles1306

on the outside, as you do on the inside.1309

Because, on the inside there is less fluid than what it is in, it is about percentages.1311

Isotonic would be like, if the outside is 1% salt, the inside is 1% salt.1316

Keep in mind also, this does not mean there is no movement of water at all back and forth.1322

There is no net movement meaning for the amount of water that is going in,1327

the same amount is going out, it is equalized.1340

This is a dynamic equilibrium situation.1343

Thankfully, your red blood cells in your bloodstream are in isotonic environment.1345

The plasma, the fluid of your blood, that is not going to make your red blood cells shrivel and pop, that would be terrible.1351

You will be losing a lot of red blood cells all the time.1358

Your red blood cells get damage for other reasons but thankfully, your bloodstream is not isotonic scenario for your blood cells.1361

Active transport, this is the movement of the molecules across a membrane with the use of energy,1371

and usually we are talking about ATP.1378

There are other energy molecules in a cell but usually ATP is used to pump particles against the concentration gradient.1382

That is the big deal, against the concentration gradient.1390

If it is going with the concentration gradient, just let it happen.1392

It is going to happen naturally, the stuff will gradually come in or gradually go out.1397

But, if you have a lot more than out, and you have some tiny bit on the inside 1401

but you want to force them out, you got to use energy.1405

An example is the Na⁺ K⁺ pump, sodium potassium pump, I am going to give you a little drawing here of a neuron.1409

This relates to physiology, here is a neuron, this is the cell body, there is the nucleus.1430

Here are these things called dendrites that allow it to receive a signal.1436

What happens is, you have what is called an action potential that gets the electric flow along what is called the axons.1443

This is the axon of the neuron.1449

Thanks to active transport, your neurons function will actually get an electric signal across the axon to the next nerve cell,1453

or the next neuron, or to a gland or to a muscle, or to whatever needs to be stimulated.1463

This is the receiving end, the neuron receives a signal.1470

What has to happen is you need what is called an action potential to exist here, 1473

which is kind of an electrical stimulation that stimulates the next region.1477

It is a domino effect of electric flow along this, how does that happen?1482

This first part, you get sodium coming in and potassium moving out.1486

Na⁺ comes in, potassium goes out, and then that triggers the next part.1507

What actually happens here, that triggers the next part to do the same thing.1515

You get this influx of positive ions and efflux, the moving out of positive ions with potassium.1519

You will get this like up and down kind of electrical graph that stimulates next regions, and it happens in fractions of a second.1526

That has to happen through active transport, it is a pumping in of sodium and it is a pumping out of potassium.1535

Thanks to using ATP in pumping those charged ions, you actually can get electrical stimulation.1542

It happens all the time, every second of your life, all throughout your body.1548

Endocytosis, the next two examples that I am going to give you are regarding cells, in the sense that,1554

if you have particles that are too big to move through membrane proteins, 1561

channels like those sodium potassium pumps or facilitated diffusion, how do you get it inside of the cell?1567

Well, endocytosis is the wrapping around of part of the plasma membrane to consume large molecules, 1574

macromolecules, that otherwise would not fit through membrane proteins.1580

Amoeba is a classic example of doing endocytosis.1584

Here is an amoeba, you have got your nucleus here, ribosomes, particles dissolved, little vacuoles.1588

And what this amoeba will do is, let us say here is our amoeba,1595

and there is a little protist or little bacteria that want to swallow it up.1603

That little cell is not going to fit through their little membrane proteins but here is what the amoeba can do.1609

It can actually use energy to push and pull on its plasma membrane to, 1616

here is what it really does, it uses energy to push and pull on the cytoskeleton.1628

Remember, the cytoskeleton from the cell lesson, 1634

it is like microtubules and microfilaments that are touching the inside of plasma membrane.1636

Using energy, you can pull uncertain parts of it, let loose the other parts,1641

and you can make the plasma membrane do something where it actually looks like a mouth.1645

It looks like it is going to wrap around this bacterium or this little unicellular protist,1651

and that is exactly what is going to happen.1658

It forms what is called pseudopodia.1660

Pseudopodia means fake feet, it looks like it is kind of crawling or swimming through the water.1663

It uses these pseudopodia, I will label that for you.1672

This is a pseudopod and plural would be pseudopodia. 1679

This little cell, it is lunch.1688

The next step, the last thing that will happen is this just pinches off.1690

The beauty of endocytosis is now, this is inside the cell contained its own little vesicle.1694

If you remember lysosomes from the cell lesson, lysosomes will fuse with this.1702

And that little enzymes will break down the cell into little bits of food particles for the amoeba to eat, 1710

and let us not forget the nucleus.1718

And that is how amoebas typically consume food is endocytosis,1720

pulling in those large cells or large molecules that cannot fit through their little membrane of proteins.1724

Two types of endocytosis, phagocytosis is cell eating, that is just what you saw here, cell eating is bringing in solid stuff.1733

Pinocytosis replace what ended up in that little vesicle with what looks just like fluid, cell drinking.1743

Now, it is not that they are just drinking.1751

If we would to talk about this as being a vesicle that they took in and it is mainly water,1755

oftentimes, they do because of what is in the water and it might be ions.1762

It might be tiny dissolved particles, but they still call it cell drinking because 1767

it looks like it did not take in a solid structure like a cell or macromolecule, like a large sugar, that is endocytosis.1772

Exocytosis is the releasing of material from inside of the cell, 1781

by fusing a membranous vesicle with a plasma membrane in dumping that stuff out.1785

In a sense, it is the opposite of endocytosis.1789

Think about that sequence with the amoeba taking in that molecule, just reverse it.1793

Imagine, the amoeba having little vesicle and just fusing and dumping it out.1797

This happens all the time in your body, even if there is not an amoeba moving around through you,1804

Here is the end of the neuron, this is called the axon terminal.1809

This is the receiving end, part of the cell body of what is called the postsynaptic neuron.1814

It is because this region right here is called a synapse.1821

It is a little tiny space between two neurons.1827

This neuron is about to communicate with this one.1831

These little red arrows is about sending signals and starting the action potentials down this neuron.1833

Here is how it works, number one there is a mitochondrion.1841

Let us focus on number 2, this is what is called a synaptic vesicle.1845

A synaptic vesicle is just a little container of neurotransmitters at the end of a neuron, at the end of an axon, specifically.1850

Neurotransmitters are little signal molecules that get dumped out into the synapse 1863

to bind with the receiving end of this neuron to stimulate.1868

It was just kind of like a little messenger molecule that connects them.1874

To get them out there in the synapse, a synaptic vesicle which is made up of that phospholipid bilayer material,1878

that always is the same as what is lining here.1884

It just fuses with it and that little sac, once it fuses with it, opens up and through exocytosis dumps these across.1888

They move across by passive transport, they just drift across and they bind to these little protein receptors here,1896

to start the process of an action potential on this neuron.1905

This has to happen to allow neurotransmitters to get dumped into a synapse,1908

and allow your brain to communicate with itself and communication from your body into your body.1913

Thank you for watching