If so, where does the energy come from? What happens to a vesicle during endocytosis? How is a vesicle formed during endocytosis? How are endocytosis and exocytosis similar? How are they different? Why is endocytosis important to cells? Is the white blood cells disposing of a worn-out red blood cell carried out through AP1 recruits specific cargo proteins to bring into the cell when the coated pits invaginate. Some details of receptor-mediated endocytosis are illustrated below.
In the illustration, substances to be internalized have bound to their cell membrane receptors. The receptors then cluster to form a coated pit. Assisted by the protein dynamin a GTPase , the coated pits invaginate. The final pinch-off of a coated vesicle requires GTP hydrolysis not shown. Once internalized, the coated vesicles lose their clathrin and associated adaptor protein coat. The uncoated vesicle fuses with an early endosome to form a sorting vesicle i.
Sorting vesicles separate imported content from the receptors that are recycled to the membrane. In the vesicle that remains, now a lysosome , digestive enzymes catalyze hydrolysis of the vesicle contents.
The digest products are then released for cellular use. A well-known example of receptor-mediated endocytosis is the uptake of cholesterol bound to low density lipoprotein LDL , a complex of phospholipid, protein and cholesterol illustrated below. A single LDL complex carries as many as 15, molecules of cholesterol. Maintaining cell size or volume seems to be a built-in component of the machinery of receptor-mediated endocytosis that balances endocytosis with membrane recycling.
However, exocytosis is also necessary to restore plasma membrane internalized by pinocytosis and phagocytosis, and for eliminating cellular waste products. Exocytosis is also the end-point of a complex process of packaging proteins destined for secretion or for insertion into the membrane themselves. The pathways of exocytosis and endocytosis share common features, as illustrated on the next page.
And I'm drawing all of these things in two dimension, but this would actually be happening in three dimension. So this wouldn't just be a circle, this would right over here would be a sphere. And this thing that has been pinched off and is now inside the cell, we call this a vesicle, which is just a general term for these membrane-bound compartments inside of cells. And this process where the cell has essentially drunk a bunch of fluid and the stuff that happens to be in the fluid, we call this Pinocytosis.
And pino comes from the Greek word "to drink". And I'm always fascinated by word roots. And I'm not a linguistic expert here, but it's neat because even in languages I'm familiar with like Hindi and Urdu, the word pina means "to drink", so maybe it's even related to the word pani which is in those words in those languages. I know all of these have a shared linguistic root, so it's always fascinating to see these linguistic connections. So this is Pinocytosis where the cell is drinking so to speak, but it's also getting the other stuff that's in that fluid.
This is Phagocytosis, the cell is eating. And these are both special cases of I guess the more general term of engulfing in this way which is called Endocytosis. So Phagocytosis is a form of Endocytosis, and Pinocytosis is a form of Endocytosis. Now the next question you might say is: OK I can get that this happens, this can be observed under a microscope, but how does it happen?
How does the cell wrap around and pinch around? And like I say in a lot of videos, people think that we understand some of it, but this is not fully fully understood. There is views that well the cytoskeleton must be involved in some way. It has to create space here for this thing to be able to pinch off and move in that direction.
It maybe will help actually the cell's membrane wrap around in some way, but these are all areas of active research. How does this Endocytosis actually occur?
0コメント