EK Biology

[browncomputer]

What does EK mean when it says this:

"Although there are many compartments in a cell, it can be divided into two sides: the cytosol and the ER Lumen. In order to reach the cytosol a substance must cross a membrane via passive or facilitated diffusion, or active transport, but it can reach the ER lumen via endocytosis without ever transporting across a membrane." (EK Bio, page 76)

I have questions about the above statement.
1. Substances can come via endocytosis into the cytosol also...phagocytosis, pino, receptor mediated. So a substance does not need to do passive/active transport?
2. Why is the ER lumen only considered? Why is the inside of the golgi or nucleus not being considered?

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[Odi]

In that passage they don't explicitly state that endocytosis is not a means of active transport--nor do they state that the ER is the *only* destination for incoming molecules via endocyctosis.

Not sure if VR or BS lol

 

[Czarcasm]

As far as I know, vesicular transport is *not* an active process. However, while transporting the vesicle to its destination, it may utilize certain motor proteins to aid in the the transportation, so you can say it has an indirect effect on the utilization of ATP. Even some hormones (1 example below) require a special form of endocytosis, but utilize ATP indirectly to propogate its message throughout the cell.

There are 3 types of Endocytosis:

- Phagocytosis: Is a function of only certain types of cells. This occurs when cells like neutrophils and macrophages engulf microbes such as bacteria and dust, surrounding that debris to form a large vesicle called a phagosome, which then combines with a lysosome (which then works on destroying the substances inside the vesicle).

- Pinocytosis: Unlike the case for phagocytosis (which functions in certain cell types), all of our cells do this. This essentially means small drinking, and conceptually what happens is, ECF molecules exterior to the plasma membrane push inward forming a vesicle. This process is unspecialized and generally has no control of what substances it uptakes, but its a great way to obtain additional nutrients from the ECF.

- Receptor Mediated Endocytosis: This is a specialized process the requires the uptake of certain nutrients. Most textbooks give the example of LDL. Basically, certain regions of the plasma membrane (facing ECF), have a protein called clathrin. These proteins specifically bind to certain substances. LDL is one of them. When LDL binds collectively to these clathrin proteins, it folds inwards forming a clathrin coated pit, that eventually forms a vesicle inside the cytoplasmic region of the cell. The clathrin proteins themselves help to redirect the vesicles to its destination, whether it's the ER or some other region of the cell. LDL is one way we can obtain cholesterol which has a variety of functions in our body, particularly as a precursor for the synthesis of some cholesterol-based hormones and maintaining membrane fluidity.

Another good example I recall from class is insulin. Being a large polarized molecule, it cannot diffuse through the membrane (because it's polar) and its too big to utilize active transport via transmembrane proteins. So how does it pass through our blood stream, through the surrounding endothelial cells, and exits the endothelial cell (exocytosis) towards the appropriate tissues?

Well, it functions the same way LDL does in that it utilizes clathrin proteins, where it binds to clathrin proteins, forming a vesicle that enters on one end (from blood plasma) and exits on the other end (towards tissues). Once it exits, it then could bind to the appropriate receptor and propogate its message.