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M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Did you know your thoughts and feelings, even the way your muscles move is based on the that same principal as the old third grade law of physics? He who's smart have dealt it. It's true. Stink moves from an area of high concentration to an area of low concentration. Same way your neurone transmitters move from one neurone to the next, or calcium floods over your muscles cells. So stink moves normally towards your nose which is the closest ones, there you go.
Now the AP Biology people aren't going to be asking who passed gas during the test. But they will be asking about cellular transport, so you need to understand this kind of stuff. I'm going to begin with passive transport and go through diffusion, osmosis and facilitate diffusion. Then I'll finish off with active transport, which includes membrane pumps, endocytosis and exocytosis.
Let's start this whole thing off with passive transport. What is passive transport? The idea of passive transport is that, like it suggests, it's passive. That means it doesn't require any cellular energy from the cell. Instead where does that energy come from? It comes from heat. Now if you increase the heat, this speeds up how fast things move in and out of the cell. Alternatively, if you decrease the heat, that slows down the movement of transport of things into and out of the cell, such as nutrients or waste. And this is a good reason why we put our things in fridges. If we don't want bacteria and fungi to eat our food, we put in the fridge. That four degree Celsius keeps everything moving slowly, and so the molecules can't get in into the bacteria and they can't grow very fast. Let's move in to the first kind of passive transport, that's called diffusion.
Diffusion is defined as the random movement of particles from an area of high concentration, to an area of low concentration. Now that gets confusing to people. Let me put it this way. Imagine you had a big box of ping pong balls and you dropped them, what will the ping pong balls do? They fall down, they start bouncing. Do they stay in this nice cube of ping pong balls bouncing up and down in the air? If they do, run away because the universe is about to end.
What the will do however, you know is they'll scatter all over the place. Why? Do the ping pong balls say, "Oh! look over there. There is some place I've not been. Let's go over there." No, it's random movement. Things are random. Look at our election results sometimes. So if we take a look at this, what's going on is that, the ping pong balls are like molecules. They're just randomly going from where there is a lot of then to where there is not a lot of them. Now what we call the difference in the amount of stuff in this area and the amount of stuff over there? That's called the concentration gradient.
In Science, and this is a little tip, if you ever see gradient, that means the difference between two areas. Whether it's a concentration gradient or whether it's a thermal gradient. Thermal, what does that mean? Heat. So that will be a temperature difference between two areas.
In general, things move from a higher concentration to lower concentration. The steeper your gradient, the bigger the difference, the faster diffusion happens.
Like one time I went on a three day train trip, by the time I got done, we didn't have a shower. So I had an incredibly high level of stink. Around me was a very low level of stink. So the bio just spread off me like in waves. Whereas if I was just doing a little bit of exercise, after first I showered, the rate of diffusion will be a little bit lower.
Now here is the trick question that they like to ask on the AP Biology exam. Imagine you had say oxygen on the outside of the cell; high concentration on the outside of the cell, low concentration on the inside of the cell. And you had, say carbon dioxide, on the inside of the cell, not very much on the outside of the cell. Do the concentration gradients of the two different molecules, have any impact on the rate of diffusion, of one of them? No. The reason why, the oxygen molecules are just going from where there is a lot of them, to where there is not so many. They don't know anything, they're not alive. The carbon dioxide are just moving from where there is lots of them, to where there is not so many. Why? They just do by random movement.
So if you keep that in mind, you'll do much better on those kinds of test questions. Let's move on to the second kind of passive transport, that's called osmosis. Now you need to know this definition. If you repeat this definition on any cell transport essay question, you're bound to get a point.
Osmosis is the diffusion of water across a semi-permeable membrane. As you might be able to tell, that means that it's a special form of diffusion. It's specifically of water. People say, "I need to learn something by osmosis." No, you can't. That's movement of knowledge, not water. Now what's this whole semi-permeable thing? That means it's a membrane or barrier that only allows certain things through. It's semi-permeable. Permeable means everything can go through. Semi means some can, some can't.
Now something to remember about your cells. Your cells have a membrane, a living plasma or cell membrane. And that has the ability to change moment by moment what comes in and what doesn't. That makes it a special kind of semi-permeable membrane, that's called selectively permeable.
In the official AP Biology lab on osmosis, they use things called dialysis tubing. So if you need to know what that is, that's a special kind of plastic that has tinny pores that are large enough to allow small molecules. Like say salt ions through, but large molecules like starch can't go through. So let's take a look and see what happens to a cell, when you put it into water.
So if I put a cell, like this red blood cell here, into some water that's roughly the same as it is. Let's suppose the water around it is 80% water, and inside the cell it's 80% water. Everytime a water molecule moves out from here, another water molecule leaves in. That means that there is equal flow in and out. That's called dynamic equilibrium. Now there is this word that 'tonic' it kind of refers to strength or your ability to pull things.
What does 'iso' mean? You've seen that in geometry, in isosceles triangles, it means equal. So equal pull. That means that the water that has the same concentration of stuff, that's not water, is called isotonic. If any of you ever used contact lenses, you know that you use saline solution to soak your contact lenses. Saline solution has added salt to it, to make the water that the contact lenses are soaking in isotonic to your cells. That way, if you put your contact lenses on to your corneas, they don't cause any damage as they suck water out, or dump water in. Let's move on and take a look at some other situation called hypotonic.
You've heard the root what 'hypo' before in things like hypodermic needles or hypothermia. Hypo means below. A hypotonic solution is one that has less pull for water than the cell. And if it has less pull for water than the cell, that means water instead of being poured into the cell, is pushed out. What would be a hypotonic solution? Let's suppose we put some animal cells like these red blood cells into a 100% water, 0% stuff. No salt, no nothing. You know that red blood cells, while they do have water, they do have some stuff; proteins, ions and things like that. So they may only be 80% water.
So as you can see 100, 80 that's a concentration gradient. And the semi-permeable membrane here doesn't allow the proteins or salts to go in and out. So the water moves in and will keep moving in causing that cell to swell. Eventually, it may cause it to lyse which means to burst open. So that's hypotonic. The standard hypotonic solution is pure distilled water.
Now let's take a look at what's called a hypertonic solution. A hypertonic solution, hyper means excess. Like somebody who is hyperactive, they are excessively active, like me. A hypertonic solution has a much stronger pull for water. What makes it have such a high pull for water? Because it's got too much stuff.
So if we had say 40% stuff like salt, that leaves only 60% of water. So if this again was 80% water, 80% to 60%, high to low. Scientists are lazy, they always like to set things up so it's always high to low because that way we can think...what do things do? They fall down from high to low. So the water will move from the high concentration of water, on the inside of the cell, out. Which will cause the cell to shrivel, sometimes they'll call that crenate, just to confuse you. It just means they get smaller.
And this is why if your mum tells you, "Hey you have got a sore throat? Go ahead and gaggle with hot salt water." What are we trying to do? Well your sore throat is caused by your cells inside of your throat getting inflamed, and there is a bunch of bacteria there. To reduce the inflammation, that needs hot salty water to suck some of the excess water out of your cells, so they feel better, plus the bacteria that are attacking it they die. Let's move on.
That's for animal cells, what about plant cells? Because remember, they've got that rigid cell wall. Well you can see that they do the same kind of thing. In a hypertonic, very salty water, water leaves them and they undergo what's called plasmolysis; where the cell inside of the cell wall shrivels. Now the cell itself acts kind of like the inner tube inside of a bike tire. It's inflation helps hold that cell wall rigid. So if you plasmolyze a plant cell, and this happens to the entire plant, then it loses what's called turgor pressure. And it starts to wilt. That's why salting the earth to your enemies was often used in the medieval and pre-medieval warfare. Because if you invade somebody's land and you really want to hurt them, put a bunch of salt in their farmlands because then all their plants will wilt and die.
If you spray some water on a plant cell that is isotonic to the plant cell, the amount of water that moves in or out, is the same.
If you put it into distilled water, it will swell up ,as the water vacuole will start absorbing more and more from that excessively distilled water on the outside. And that makes it turgid. It has high turgor pressure. If you go to the grocery store, my local grocery store, every five to ten minutes and you'll hear this sound of lightening. And pretty soon they are spraying water. Why are they spraying water on their vegetables and fruits? It's not to help them grow, they're already out of the ground, they can't grow any more. What is doing is, it's helping plump them up so that you think they are fresh.
So that is good enough. However, nobody in science really likes to leave things simple. Some guy who's really into Maths said, "Hey how about we try to figure some way to add Mathematics to this?" And that's when they came up with this concept called water potentials, to figure out how much water can go in based on that pressure generated by the cell wall.
Now there is a lot of complicated Math in this, but don't worry too much. Because if they're going to ask an essay question about this, and require that you do some calculations with a more complicated part of it, this negative ICRT stuff, they're going to have to give you that formula. So what is water potential? That is a particular region's ability to donate water to other things. They use the Greek letter Psi, this trident here. Just think in little mermaid, her father came beside and what did he have? The trident because he lived under water, a lot of potential there.
So water potential is are made up of two things; the effects of the amount of stuff or solute in the water, and then the amount of pressure. If you have a lot of stuff inside the cell, it makes it pulling water. But eventually, so much water comes in that the pressure inside the cell builds up, and water starts being squatted out. If I put a million dollars into a back seat of a little Toyota Prius, people start going in really fast. That's like having a lot of solute.
People go in, they're like water, they are trying to get in there. But eventually if you have 20 people all fighting over that million dollars inside the Prius, as one person comes in, somebody gets pushed out by shear pressure. That's the effect of the pressure component.
Now to calculate that solute component you do negative, and that's part of the trick. The more stuff you add, that subtracts the amount of water that area has to donate. 'I' is the ionization constant, you usually don't have to worry about that. So just assume it's one. It would be two for something like sodium chloride, because you know the NaCl breaks apart into sodium and chlorine ions, when you put into water. But in general like I said, assume it's usually one. 'C' is the concentration of solutes, which is just the molarity. I know you're having flashbacks of Chemistry right now. That's the molarity of the stuff that's in the solution, the solute.
R is the gas constant, another flashback to Chemistry. Again, they have to give this to you. It's 0.0821 and then a bunch of units, who cares. T is the temperature and here's the trick. If you ever see the temperature 27 degrees Celsius, don't use 27, use 300. Because what they're doing is, that's Kelvin. This temperature here is in Kelvin, that's that absolute temperature where zero is when everything stops moving. Water freezes at 273 Kelvin. So you're going to need to convert the Celsius into Kelvin, and that's why I say look out for 27 degrees. Because as a teacher, I hate it when students maker mistakes on simple Math. 300 is much easier number to do, because it's times three and then you add two zeros. So that's why they often use 27 degrees.
So you plug this in and you'll get a number. Let me skip this to the end. Always assume it's 0.32, 0.34. And let me show you they're going to use this in the lab.
So usually what you'll see on a lab essay question on this, is you'll see some graph kind of like this, where you take a bunch of potato cores, or zucchini cores, or pieces of a potato or zucchini.
And you plunk it into several different molarities of solution. I may plunk a potato core into zero molar water, that's distilled water. And what I'll see is that it absorbs more and more water as the water moves in. And it increases its mass by say 18%. When I plunk it into 0.2 molar sucrose solution, I'll see that some water moves in. At 0.4 molar solution I see that it lost water. What does that tell me, why is it losing water? Because it has more water than the outside solution. So if I look where they intercept with the zero percent change, it's around 0.3. And that's why as it goes down here, we see more and more water being lost because we're putting in hyper and more hypertonic solutions.
The last form of passive transport is called Facilitated diffusion. Now you remember diffusion is that spreading out of stuff from areas of high concentration, to low concentration. To facilitate means to make something easier. So why do we need to make things easier? Well small molecules, like oxygen gas that don't have a charge, they can go straight through the membrane. Other molecules though, that are charged like sodium ions and chloride ions, they're too charged to get through or large molecules can't fit through the membrane. So we need to facilitate their entry. Let's take a look at the membrane.
You remember that the cell membrane has these phospholipids with a hydrophobic fatty acid tails. Those block the movement of most things through the membrane. So if we want stuff to move in, let's just build a tunnel through it. And that's what you'll see in facilitated diffusion, these protein channels. It's kind of like this is walls of the classroom and these are the doors. Because for some reason, my students can't fit in through the cracks in the wall, they instead need this big portal called "a door" to get into my classroom. Now when I open the door, I'm not running outside and grabbing kids and throw them inside, no. I just open the door and they come in or out.
That's facilitated diffusion. Remember, facilitated diffusion is a form of passive transport, it doesn't require any energy. There are several different ways that you can have your proteins set up. This is a protein channel, and all it is is just a door, a pathway through. These right here are carrier proteins, where they will change shape as things move in, kind of a revolving door at a hotel. Now one last thing that I'll touch on before we go into active transport, is the fact that this a selectively permeable membrane. The dialysis tubing I mentioned before, it's got a membrane but it just has holes. Your cells sometimes do want stuff to move in, other times they don't feel like it.
So what they is that they have little flaps in these channels, that can open or shut just like opening or closing a door, through the doorway. And here is a common example. Glucose is the major fuel of the cell. You usually want it able to come through the cell. How do you get the door for the glucose channels open? You already know the answer. What do diabetics need to inject? You know, it's insulin. What does insulin do? It lands on this protein channel, causes the door to open, glucose goes in. Now what if you don't need any more glucose? Then your insulin levels drops, the door closes and the glucose stops moving in. That's it.
Now we're going to go on to active transport. Now remember, active transport is when the cell has to provide it's own energy. This is the movement of materials that requires the cell to spend some of its own energy, usually in the form of ATP. And this moves things from an area of low concentration to an area where it's already high in concentration. That's moving up a concentration gradient, rather than just letting it tumble down. Just think like a boulder, if you have to move a boulder, it's really easy to move it down a hill from high concentration to low concentration. But to get it to the top of the hill, takes a lot of energy.
Now the first kind of active transport is called membrane pumps. This is very much like facilitated diffusion where you had protein channels, but it's kind of like my door.
I open my classroom door and yes, a few kids will run in and diffuse in out because out of sheer boredom. But when that class bell rings, a lot of the times you have to stand outside the door and say, "Come on in." I'm spending my energy yanking the kids in to class. And that's the thing your cells would do with a lot different chemicals. And so what you do is, you had proteins, much like those protein channels. But instead of just going and allow stuff in or out, then say go and force things in using energy. And there is a number of different ways that they can work. A very common way is to have, if here are some stuff that I want to move in, these orange little hexagons, the protein is like this right now. And it has a shape that allows the special shapes to come in. It's tertiary structure, remember tertiary structure. Mention that active transport thing when you're talking about the membrane pumps, and that will get you another point, when you talk how specific these membrane pumps are. Let's suppose these is say, sodium.
I want sodium to come into the cell. Now I have it come in here. I'm sorry this is within the cell, I want to pump it out. So the sodium comes in and then I spend some energy to change shape and pop that sucker out. Now, a cell is often working really hard to get a high concentration of sodium on the outside of the cell, and a high concentration of potassium on the inside the cell. So if you can remember the sodium potassium pump, this is one of the most common examples of membrane pump used in Biology.
So if you see sodium potassium pump or sometimes they'll just shorten it, because it's kind of an enzyme. They'll say sodium potassium Atpase, because it's using ATP to do that. That's what they're talking about.
So that's part one of active transport. The other two types of active transport are very different. The first of these is endocytosis.
Now in Science, one of the tricks is that, scientists like to use their own little 'lingo'. It's kind of like you teenagers.
You're constantly changing slang so that we adults get confused. My generation, bad meant good, just to confuse the heck out of their parents. Nowadays, things are 'diss' or 'sketch', weird. But when scientists do it, they go to a language called Latin usually, and they reap out from Latin various roots. Endo means into cyt means cell, osis means a process. So this means 'into the cell process'. So what's going on here is that, the cell wants to get some large molecule in, or perhaps even something as large as a bacteria, if you're a white blood cell. So white blood cell that's trying to eat the bacteria, that's much too large to fit threough a membrane pump. So what it does is instead, it will corrupt the bacteria and wrap that bacteria in a bag full of membrane. And now that bacteria is trapped inside the white blood cell trying desperately to get out, but it's too lat. And so that's what we see here.
There's two subdivisions of endocytosis. And one way to keep this straight, is to think about yourself. You wrap large things in your mouth and then swallow them. There is two kinds of things. There is eating, and that's called phagocytosis and then there is drinking, that's liquid. That's pinocytosis. So if a cell is doing phagocytosis of a large solid particle like a bacteria, that's phagocytosis. That's a sub set of endocytosis. If it's trying to take in droplets of oil for example, as a fluid, that'll be pinocytosis. Phage is a root word that means eating. So eating by the cell process. Pino means drinking, drinking by the cell process.
What we see here is, let's suppose we don't want some random thing and we want something specific. And so we see here these little stars shapes are landing on specialised protein receptors. Remember protein receptors, that's another thing that will get you points in AP Biology essay, when you're talking about cell transport. When it lands on that receptor, that causes the membrane here to buckle inwards and pull it in.
Now what if this was something you wanted to dump out. What would you call this process that means out of the cell process? Well guess what the root word for out of is 'exo'. You've seen it's words like exit.
So exocytosis is the export of large materials, like proteins contained in membrane sacks. And what you do is you just run endocytosis in reverse. And here we see a sack filled with something that we want dump outside the cell. You'll very often see this concept intertwined in an essay, when they're talking about protein synthesis and they will make you describe the process of protein synthesis. And then they'll say and how is this exported from the cell? That's when you bring up exocytosis. You mention that and you're good to go. What's involved in here is the golgi apparatus, or the golgi bodies, or golgi complex, whatever they want to call them, golgi.
So what happens is that the proteins are made on the rough endoplasmic reticulum, put in the low membrane sack, sent to the golgi apparatus, which modifies those proteins. And then they send those off to the membrane of the cell, and then it dumps it out. And that's it. That's active transport.
So you've got the membrane pumps, endocytosis and exocytosis.
So there we go. We have now covered the basis of cellular transport, you know that there is active and passive transport. Passive transport requires no energy expenditure from the cell. And it includes diffusion, a special case of diffusion called osmosis, which involves just water. And then facilitated diffusion, which involves protein channels which allow stuff in and out of the cell. That active transport is the stuff that requires the expenditure of ATP, and there is three kinds of that as well. There is the membrane pumps which are spending ATP to move stuff in and out through specialised proteins in the cell membrane. There is endocytosis, which wrap things in membrane as you take them into the cell. And then there is exocytosis which is just endocytosis run in reverse. You take a membrane sack, merge it with another membrane, done that stuff. With this you're good to go on the test.[0:24:00]
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