Help aout the cell membrane?

Question

I have to explain why the two sides of a membrane may differ in composition and function and why are membranes partially permeable?

Answer 1

On the side of the cell membrane that faces the out side of the cell may have protein receptors on them so that the cell can detect things in its environment (food molecules, other cells, antigens, etc.). On the inside is where the cytoskeleton attaches to move the cell and the organelles within the cell.

Cell membranes allow certain molecules that are very small to pass through (like water and gas molecules). Large molecules can't get in between the molecules that make up the membranes, so they can't just pass through (they would have to go through protein channels on the membrane). Also, charged paticles like ions cannot get through because the membrane has a layer that non-polar in the middle, which repels things that have charges on them. This is what it means to be semi-permeable. Some things can go though, but not others.

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Thanks gabe j! I never knew about aquaporins. I've wondered about how water molecules go though the lipid layer... now I know!

Answer 2

Basically, most membranes have a hydrophyllic layer ("water loving") and a hydrophobic ("afraid of water") lipid layer. This is especially true for animal cells. The reason for this division is to regulate fluid transfer within in the cells. too much fluid, and the cells rupture, too little and they shrink.
This can be demonstrated well with red blood cells. If you put some blood in a hypotonic solution of sodium chloride (less than 9 g/L concentration), you will note that the cells swell and rupture. If you place them in a hypertonic solution (> 9g/L concentration) the cells wrinkle up (or crenulate) and eventually lose cohesion.
Membranes serve as barriers to aid in metabolic processes, and to avoid contamination of the cells by viruses, bacteria and toxins.

Answer 3

in response to someone above: water does not go through the cell membrane because it can't since it is polar, it actually goes through aquaporins which are water channels.