How does water get through the membrane?

Answer 1

Water molecules cross the bi-lipid layer via simple Diffusion.

There are 3 types of Osmotic Forces: Isotonic, Hypotonic (the Cell swells), and Hypertonic (the Cell shrinks). The Cell wants an even amount of solutes and water in the liquid surrounding it. Therefore, if a Cell is placed in a liquid with a high amount of solutes, it tries to even it out by excreting water, making the Cell shrink.

Answer 2

Awesome question! Truth is the phosopholipid bilayer is a semi-permeable membrane (SPM), as you have already pointed out, and each SPM will have different characteristics that determine what goes in and what stays out, and at what rate the diffusion occurs.
We have already established that things move down their concentration gradient given the opprotunity ie. wind blows from high to low pressure zones, objects fall from high potential to low etc.
So we must clarify what the cell membrane allows in and out. First and foremost it allows small non-polar molecules to diffuse freely (O2, CO2), this is of utmost importance because we are aerobic organisms and without the free movement of O2 in and CO2 out, respiration would not be possible. The only source of energy (ATP) would be from glycolysis and this produces 2 ATP/glucose, whereas we get ~36 ATP from respiration, we would not be able to sustain life without respiration. Anyway water is also a byproduct of glucose metabolism C6H12O6 + 6O2 --> 6CO2 + 6H20 + energy, so there is some water made inside the cell, which of course needs to get out. But how you ask? Well the selective permeability of the cell membrane may not favor polar molecules but that does not mean it does not allow them to pass.
The conundrum arrises when we consider water first as a polar molecule and second as small particle. Im not sure where you are in your studies but if your proficcient in chemistry you should know that water is polar because its VESPR arrangement is bent. Most small molecules like CO2 and O2 are symmetric and therefore the charge difference on one side is negated by the charge on the other. The oxygen in H2O has 2 pairs of unbound electrons which result in a bent shape and polarity. While small polar molecules are not uncommon, they are relatively rare in the biological system. Obviously, the larger the molecule the greater the chance of it being non-symmetric and therefore polar.
What it boils down to is that water is small enough that the fact that it is polar can be negated. It simply flows in and out with little beef from the membrane. It does not need a transport protein or an aquaporin. Aquaporins, discovered in 2003, are water pores that allow water in but separate the ions from it. These are very common in the kidneys, but the major mover of water is still osmosis. So in a nutshell, small molecules (polar or not) go though no problem! Small charged moleules (ions) are usually able to cross through protein channels in the membrane, some larger charged molecules cross the membrane with help from carrier proteins.

Answer 3

Water passes through the cell membrane by using a process called osmosis.

Answer 4

Osmosis