Plasma Layer

[blindrhino]

I'm reading a basic bio text at the moment and am a little confused about the plasma layer. It seems that water can cross the plasma layer of a cell (which is why plant cells have rigid walls, so they don't burst?). But, given the construction of the bilayer, with the hydrophobic bits on the inside and outside, I would have thought that water would be prevented from passing through the bilayer. I kind of thought that was the point of the bilayer. So I have several questions:

1. Is the plasma layer in fact a semi-permeable membrane, allowing water and some other smaller molecules through but preventing larger water soluable molecules from passing (except through specific channels).

2. If it is true that the membrane is semi-permeable, what exactly is allowed through by diffusion? Anything other than water?

3. How is it that water can get through the membrane if the bilayer is so hyrophobic on the outside?

4. If question 1 is not true, does water get into the cell through specific channels that only allow water? If so, the quantity seems somewhat unregulated and untriggered if it can burst a plant cell. Are the other molecules that are allowed into the cell in the same way?

Any help in clearing this up for me would be much appreciated. Thanks!

[biostudent84]

Water is the only thing that can move directly through the cell membrane. Turgor pressure is able to force water through the membrane even though the membrane resists. The bilayer is hydrophilic on the outside...hydrophobic on the inside. This, however, is mostly an ability developed to give the bilayer structure and keep it all together.

[thank.darwin]

1.Yes water is small enough to get through the membrane and in some cells there are specialized channels through which water can pass

2. Yes there are other things allowed through by the membrane- small nonpolar molecules can pass through such as oxygen but polar or charged molecules can't due to the phospholipid tails

3. The hydophilic is on the outside and the hydrophobic is on the inside

4. Some cell vacuoles pump out access water but I dodn't know if I can answer the last part of your question

[mith]

If so, the quantity seems somewhat unregulated and untriggered if it can burst a plant cell. Are the other molecules that are allowed into the cell in the same way?

Any help in clearing this up for me would be much appreciated. Thanks!

Notice that the water flows into the cell because of the unequal equilibrium and this pressure is unceasing. Now picture that you have a tiny water pump powered by two munchkins. This is you cellular pump. Your call

[thank.darwin]

Water is the only thing that can move directly through the cell membrane. Turgor pressure is able to force water through the membrane even though the membrane resists.

What about small non-polar molecules? Can't they get directly through the cell membrane?

[jacooper1993]

isn't it ozmosis that draws water in and out of a cell? i didnt think that it had anything to do with turgor pressure.

[pielover1234]

Each and every cell has a plasma membrane. the plasma membrane is semi- permeable, which means that it only lets certain things through the membrane and into the cell. If an active ingredient such as alcohol breaks through the membrane, it will kill the organelles there for killing the cell.