Introduction to Biochemistry - Quiz 2.5.5

It is the fluorescently-labeled lipids.

At $ t = 0 $, the laser has bleached the lipid dye at this spot.

At $ t = 100 $, the lateral diffusion of the bleached and fluorescent lipids caused the dark spot to be mixed in.

Membrane protein serves their functions, so depending of the function of the cell or organelle, the percent mass of lipid versus protein will vary.

Red blood cell travels around the body, it is easy to imagine it need to perform many different functions, and therefore it has a lot of different proteins on its membrane. Photoreceptor, however, serve one purpose, to sense light, so it is reasonable to predict there is fewer types of protein on it.

Here is the answer.

The protein composition of a membrane reflects its function.

For example, while red blood cells and photoreceptors both have a high density of protein in their plasma membrane, red blood cells have a greater variety of protein while photoreceptor cells have essentially one type of protein in their membranes.

Basically integral protein can go through the membrane while peripheral protein are on one side, either outside or inside, so it is not hard to figure out:

Integral membrane protein must have hydrophobic coils that pass through the core of the lipid bilayer.
Peripheral membrane protein can be covalently linked to a lipid that is inserted in the lipid bilayer
Integral membrane protein can transduce an extracellular signal across the lipid bilayer.
Both membrane protein can serve functions on the intracellular and extracellular surfaces of the lipid bilayer.

The last one is tricky, I was tricked.

The first one is correct, water is very concentrated in the body - and the high concentration make a high concentration gradient if diffusion is blocked.

Many molecule are polar and can hydrogen bond, even a whole protein molecule can, this is wrong, being polar and can hydrogen bond alone cannot lead to diffusion.

Water molecule is small, and small molecule can easily go around barrier, this is correct.

While it is true that water is needed in the cytosol (for example, for protein folding), this is not a reason, so this is wrong.

Water is not hydrophobic and it is not bulky at all, so this is wrong.

The concentration gradient for these ions are crucial for neural signalling, they are maintained through pumps. These pumps are integral membrane proteins, so the answer is.

The ions require integral membrane proteins to cross.