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.
Cell transport is movement of materials across cell membranes. Cell transport includes passive and active transport. Passive transport does not require energy whereas active transport requires energy to proceed. Passive transport proceeds through diffusion, facilitated diffusion and osmosis.
Cellular transport is a basic process of the cells and it's essentially how does the cell get stuff into the cell or out of the cell? Now you can divide it down to 2 basic categories passive versus active transport. Passive transport is like you suggest it's passive it means just like you being a passive observer of TV doesn't require any energy from your point of view. And the cells typically the energy is being provided in the form of ATP.
Whereas active transport does require that the cells spend some energy in order to get material into the cell or toss it out of the cell. And again that is usually in the form of ATP, now those of you who have taken Physics know to move something generally it does take some energy. So where is the energy coming from? It's usually coming from the environment in the form of heat energy. Now within passive transport there's 3 sub divisions. There's diffusion, facilitate diffusion and osmosis, now the most important one to understand is diffusion because that's what facilitated diffusion and osmosis are based on. So what is diffusion?
Diffusion is the movement of group of particles from an area where there's lots of them to an area where there's not so much of them. Now how did they do that? Why do they do this? Well you should know that all things are made of atoms and they're not sitting still, there're actually moving, even material like me that's a solid my arms right now are kind of jiggling a little bit they're not moving very far but there're still jiggling. And a gas or a liquid like inside of your cells, your cells are made mostly of liquid and so the molecules are actually able to flow and flow around and flow and move around.
Now there's something about the universe that just despises order, its called entropion. Entropion our universe is always increasing and it's why when I drop these you're going to see them scatter because of this was a nice wonderfully universe they would stay in this nice cube form but instead did they know that there was any skittles over there? No they did not, is just that they move in random directions and there's a lot more skittles here than there was over there. So they just spread we can could that diffusion of skittles, that was simple now what if I'd had this happen inside of a, well if I dropped skittles now they bounce a fair amount. If I drop them so they don't bounce you'll see that they're not moving out. Why? Because they can't fit through the glass, in order to allow them to fit through to facilitate their movement I would have to punch holes and that's what cells do.
If a molecule or particle is too large to fit through the cell membrane or perhaps it has a charge and therefore it can't get through. The cell membrane can have protein pores or channels which allow those particles to go through the cell membrane easily but again there're just moving from where there's lots of them to where there's not so much. The last kind of passive transport is called osmosis, osmosis is the diffusion of water through a semi permeable membrane and it's a special case of diffusion where it's focused solely on the water movement nothing else.
Alright onto active transport and again this is when the cell is having to spend its energy in order to get stuff to come in or out. Now the most simplest to understand of this are the pumps that are in the membrane made of proteins, so these proteins are like regular pumps they pump stuff and just like if you're on a boat and you have a hole in the bottom if the boat, you ram a hole through you see water moves in passively. Why? Because there's a lot of water on the outside not so much on the inside, that's like facilitated diffusion. Well how would you get that water to move from inside the boat where there's not so much back into the ocean? You'd have to pump it out either with a bucket or with a belch pump or some other kind of pump. That's what facilitated diffusion is but again it's spending energy to move stuff against a concentration grade or difference between concentrations.
The next one is something called endocytosis endo is a root word that means into cyto means the cell. Alright so how does that work? That's how you get a large chunk maybe even something that is of size of another cell say bacterial cell being eaten by the white blood cell. If it's too big to fit through any pore in the membrane how do you get it through? Let's suppose this is a bacteria and this is a white blood cell, this wants to get that inside so it can kill it. So what this white blood cell starts doing, is it starts extending its membrane and extends it more and finally mergers these 2 pieces of membrane. And now this bacteria here has moved inside of the cell and it's trapped inside of this little sack. That's endocytosis, the reverse this is called exocytosis out of the cell process.
How does that work? Well let's suppose we have a cell and I just made some large molecule or group of molecules that it needs to dump out. Now to keep things organized it'll put them inside of sacks, so let's suppose these are a bunch of these molecules that are too large to go through the membrane by themselves, so we put them in a sack. What do we do, we allow this small sack called a vesicle which means just a small vessel or sack. We allow it to move to the membrane, once it's at the membrane it starts to merge with it and dumps its materials out. That's exocytosis, so endocytosis sucking in, exocytosis dropping out.
Now just like when you ingest or take in particles through your mouth that's enter your body, you talk abut I ingested a big Mac and I ingested a coke. Well we usually don't use the verb ingest so instead we talk about we eat the big Mac, we drink the coke. Just like you break it down to solids and liquids they also break it down for endocytosis. There's phagocytosis which means eating by the cell process and pinocytosis which means drinking by the cell process. And what do you think the difference is, this is taking in solids by, this is taking in liquids by this process of and that's active and passive transport.