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Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.
Just as you need to eat and drink, your cells need to eat and drink. And, just like you are covered with skin, your cells are covered by a cell membrane. And, just like your skin, it is the cell membrane's job to keep things outside of it. Okay, the process of diffusion does let some things cross the cell membrane, like water, small molecules, and molecules without a charge. I admit the analogy falls apart here, but stay with me anyway.
But, what if the cell wants that large molecule that is outside of it to be inside of it? If you see a cheeseburger outside of you, and you want it inside of you, you eat it! But, a cell does not have a mouth. So now what? This is where the process of endocytosis comes into play.
Endocytosis is from the Greek roots endon, meaning within; kytos, meaning cell; and -osis, meaning process. So, it is the process by which a substance is brought inside a cell without having to pass through the cell membrane. By the way, exocytosis is the opposite of endocytosis, the process by which substances exit the cell without having to pass through the cell membrane.
I should make a point here. Endocytosis, unlike diffusion, requires energy from the cell. To keep with the analogy of your skin, you can passively soak up the sun's rays. You just have to lie there, and the tanning can happen while you're asleep. However, to eat a cheeseburger, you have to expend energy to grab it, bite it, chew, and digest it. Same with the cell. Water diffuses through the membrane passively; the cell does not have to expend energy. However, if the cell wants that large molecule, it has to expend energy to grab it and bring it in.
There are three types of endocytosis: pinocytosis, phagocytosis, and receptor mediated endocytosis.
The Greek word pino means to drink. Pinocytosis is the process by which the cell takes in fluids (as well as any small molecules dissolved in those fluids). You can remember that pinocytosis is cell drinking by thinking about the wine that we drink, pinot grigio.
In the process of pinocytosis, the cell membrane folds in on itself, creating a small pocket, or pouch. The cell membrane closes around this little pocket, forming what's called a vesicle. Any liquids or small molecules that were trapped in that pocket are taken into the cell. The vesicle fuses with a lysosome, whose digestive enzymes break down the molecules so that its parts can be recycled. Lyse, by the way, is from the Greek lusis, meaning 'loosen' or 'break up.'
Phago- comes from the Greek word phagein, meaning to eat. Phagocytosis is the process by which cells engulf and digest large molecules. The cell sends out projections of its cytoplasm called pseudopodia, Greek for 'false feet.' The pseudopodia engulf the molecule, and the cell membrane fuses around, trapping it inside a cellular vesicle. The vesicle again joins with a lysosome, and the molecule is broken down.
Not all cells go through this process. Amoebae, white blood cells, and other cells that are able to change their shape are examples of cells that do conduct phagocytosis.
Receptor mediated endocytosis is utilized when there are very specific substances that the cell requires. It is similar to pinocytosis, but the in-folding of the cell membrane is triggered only when the target molecules bind to receptors on the outside of the cell.
Receptor mediated endocytosis is the primary way that our body processes LDL, low-density lipoprotein, or the 'bad' cholesterol (called 'bad' since too much can lead to blocked arteries). Receptors on the outside of the cell membrane bind to a protein in the membrane of LDL. This binding initiates endocytosis. Once the LDL has been digested by the enzymes of the lysosome, the amino acids, cholesterol, and fatty acids are released into the cytoplasm. Once there, they can either be recycled or excreted from the cell.
While some molecules can easily pass in and out of the cell membrane through diffusion, the cell must make an effort to acquire other molecules. One of the ways the cell does this is through endocytosis, the process of bringing molecules into the cell without having them cross the cell membrane.
There are three types of endocytosis: pinocytosis, phagocytosis, and receptor mediated endocytosis. Each type of endocytosis involves encapsulating the target molecule in a pocket of cell membrane called a vesicle and bringing it to the lysosome so that it can be broken down.
Pinocytosis (cell drinking), traps liquids and any small molecules dissolved in the liquid. Phagocytosis (cell eating), involves wrapping projections of cell cytoplasm (pseudopodia) around large molecules. Only cells that have the ability to change shape conduct phagocytosis. In receptor mediated endocytosis, endocytosis only occurs after target molecules have bonded with receptors in the cell membrane.
 |
| Main Terms | Definitions |
|---|---|
| Endocytosis | one way in which a cell membrane allows some molecules into a cell |
| Vesicle | a pocket in the cell membrane that targets a molecule and brings it down to the lysosome |
| Pinocytosis | captures liquids and molecules broken up in liquids |
| Phagocytosis | involves wrapping cell cytoplasm around large molecules |
| Receptor Mediated Endocytosis | this type of endocytosis happens after specific molecules have bonded with receptors |
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