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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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