Answer:
The process of passage of different molecules, solutes, and liquids, through the phospholipid bilayer in human cells, and really in all animal cells, is highly dependent on a tight coordination between chemical, and thermodynamic balances, that will collaborate in these elements being able to pass, or not pass, through a cell´s membrane, and activate other mechanisms within the cell when their passage is not possible. Unlike what was once believed, that transport proteins were like buses parked at the membrane and waiting to be loaded with molecules to later remove themselves from the membrane and carry their load into the cytoplasm, scientific research has found that this is definitely so, and that transport proteins do not come off the layer to transport molecules. They are permanently anchored to the membrane and through a series of second messenger systems, energy produced by the passage of certain ions like potassium and sodium, and other such processes, these transport proteins become activated, allow the passage of molecules and change them in such a way that they can be taken into the cell in vesicles, or, they will anchor them to second messengers, who will be responsible for carrying the molecule inside.
From the list of words given and the two sentences down below, which are two reasons why the earlier believed models for transport proteins are not correct would be:
1. Integral membrane proteins are embedded stably in the membrane and protrude from one or both side based on their hydrophobic, or hydrophilic, regions. These sides will not switch because of the disbalance that would be created if the two sides had to be switched chemically to allow them to pass to the opposie sides.
2. For protein to traverse a membrane, movement of its hydrophilic regions through the interior of the membrane would be required, which would be highly endergonic and hence thermodynamically improbable.
