Cell Transport

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Have you ever wondered what it must be like to be inside a cell? Imagine the genetic material, the cytoplasm, the ribosomes---you will find that in almost ALL cells----prokaryotes and eukaryotes. Eukaryote cells in addition have membrane bound organelles.

All of those organelles have different functions. But cells are not isolated little worlds. They have a lot going on inside them, but they also interact with their environment.

It makes sense that to keep a stable environment inside them---otherwise known as homeostasis---they must have some control on what goes in and out of them. A very important structure for this that ALL cells contain is the cell membrane. By controlling what goes in and out, the cell membrane helps regulate homeostasis.

Let’s take a look at the cell membrane. You could have a course on the cell membrane itself---it has amazing structure and signaling abilities. But to stick to very basics, it is made of a phospholipid bilayer.

Bilayer means two layers, so you have these two layers of lipids. Part of them---the head is polar. The tail part is nonpolar.

Some molecules have no problem going through the cell membrane and directly go through the phospholipid bilayer. Very small non-polar molecules fit in this category and are a great example. Like some gases.

Oxygen and carbon dioxide gas are great examples. This is known as simple diffusion. Also, it doesn’t take any energy to force these molecules in or out so this is known as passive transport.

Simple diffusion moves with the flow. Meaning, it moves with the concentration gradient. Molecules move from a high concentration to a low concentration.

That’s the natural way molecules like to move---from high to low---so when you hear someone saying it’s going with the gradient then that’s what they mean. Remember how we said the cell membrane is actually a pretty complex structure? Well, one thing we haven’t mentioned yet are proteins in the membrane, and some of them are transport proteins.

Some transport proteins act as channels. Some of these proteins actually change their shape to get items across. Some of them open and close based on a stimulus of some kind.

And these are good things, because it’s helping with molecules that may be too big to cross the membrane on their own or molecules that are polar---and therefore need the help of a transport protein. This is known as facilitated diffusion. It’s still diffusion, and it still moves with the concentration gradient of high to low.

It does not require energy so it is a type of passive tran sport. It’s just that the proteins are facilitating, or helping, things pass. Charged ions often require a protein channel in order to pass through.

Glucose needs the help of a transport protein to pass through. In osmosis, for water to travel at a fast rate across the membrane, it passes through protein channels called aquaporins. These are all examples of facilitated diffusion, which is a type of passive transport and moves with the concentration gradient of high to low concentration.

Now all the transport we’ve mentioned has been passive in nature, that means it’s going from high concentration to low concentration. But what if you want to go the other way? For example, the cells lining your gut need to take in glucose.

But what if the concentration of glucose in the cell is higher than the environment? We need to get the glucose in and it’s going to have to be forced against the regular gradient flow. Movement of molecules from low to high concentration takes energy because that’s against the flow.

Typically ATP energy. A reminder that ATP ---adenosine triphosphate---it has 3 phosphates. When the bond for the last phosphate is broken, it releases a great amount of energy.

It’s a pretty awesome little molecule. ATP can power Active Transport to force those molecules to go against their concentration gradient, and one way it can do that is actually energizing the transport protein itself. One of our favorite examples of active transport is the sodium-potassium pump so that’s definitely something worth checking out!

- There’s other times the cell needs to exert energy for transport – we’re still in active transport for now. But let’s say a cell needs a very large molecule---let’s say a big polysaccharide (if you check out our biomolecule video, that’s a large carbohydrate)---well you may need the cell membrane to fuse with the molecules it’s taking in to bring it inside. This is called Endocytosis--- think endo for “in.

” Often, this fusing of substances with the cell membrane will form vesicles that can be taken inside the cell. Endocytosis is a general term, but there are actual different types of endocytosis depending on how the cell is bringing substances inside. Amoebas for example rely on a form of endocytosis.

Pseudopods stretch out around what they want to engulf and then it gets pulled into a vacuole. There are other forms too such as the fancy receptor-mediated endocytosis---where cells can be very, very, very picky about what’s coming in because the incoming substances actually have to bind to receptors to even get in. Or pinocytosis---which allows the cell to take in fluids.

So to the Google to find out more details of the different types of endocytosis. Exocytosis is the reverse direction of endocytosis, so this is when molecules exit---think exo and exit. Exocytosis can also be used to get rid of cell waste but it’s also really important for getting important materials out that the cell has made.

Want a cool example? Thinking back to those polysaccharides---did you know that large carbohydrates are also really important for making plant cell walls? Cell walls are different from cell membranes----all cells have membranes but not all cells have a wall.

But if you are going to make a cell wall, you’re going to need to get those carbohydrates that are produced in the plant cell out of the cell to make the wall. So there’s a great example of when you’d need exocytosis right there. Well that’s it for the amoeba sisters and we remind you to stay curious!

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