Biology question on semi-permeable membrane?

Question

how does
simple diffusion,
carrier facilitated,
and active transport work?
How does it involve different concentrations of water? If you don't mind don't just send me to a site I want to hear it in someones own words. Thanks!

Answer 1

The natural tendency is for compounds to move from areas where their concentration is high to areas where their concentration is low. It is the same as the movement of gas molecules from high to low pressure or for charge from high to low potential. In fact for the diffusion of compounds, we say that the driving force is the difference in the chemical potentials (which for a given compount, temperature and pressure means a difference in concentration).

Therefore if there are no limits in the movement of molecules, they will tend to diffuse everywhere so that their concentrration is unifrom everywhere. Having a semi-permeable membrane, allows only certain molecules to move through the membrane in or out or the cell. For these molecules, the direction of their movement will depend on which side of the membrane has a lower concentration for that substance. If it is the inside of the cell, then that compound will diffuse passively into the cell, if it is the environment it will diffuse out of the cell. Water can move through the cell membrane. However it is a rather special case so I'll discuss it separately later on.

What happens to molecules that cannot move easily or at all through the membrane? For such a compound the cell has to use proteins to importor export the compound. If the cell wants to move the compound towards a certain direction and this action is energetically favourable (it moves the compounds towards the area of lower concentration) then the cell doesn't need to expend any energy and you have facilitated diffusion. Sometimes it has to move compounds uphill their gradient, that is to move them from an area of lower concentration to an area of higher concentration. Then they have to use energy in order to transport the compound. This energy comes either from the hydrolysis of ATP or by the membrane potential or by electrochemical gradients of other compounds. E.g. glucose can be imported by the Na+ - glucose symporter. This is a protein that takes advantage of the Na+ gradient (low inside high outside) to import glucose:
1 Na+ (if I remember) is imported together with 1 glucose molecule. The glucose molecule wouldn't have moved if there was no energy provided by the movement of Na+.

Now let's go back to water.
Water can move through the membrane and will tend to move through the semi-permeable membrane towards the area where its concentration is higher. This phenomenon is called osmosis (note that it is different to diffusion since you need a semi-permimeable membrane to have osmosis, whereas for diffusion you can have any heterogeneous system). Now osmosis depends on more things than the concentration of water. It depends on the water potential. Water will move from regions of higher to regions of lower potential. Water potential is a function of several things. Dissolving compounds lowers water potential. However water potential also depends on pressure. So if you have the same pressure on both sides of the membrane, water potential practically depends on the concentration of solute and water will move towards the direction of higher solute (lower water) concentration. If you have different pressure on the two sides of the membrane then you have to take into account the difference in pressure as well; e.g. plant cells can be exposed to hypotonic environments without bursting because the cell wall allows turgor pressure to build up to the extend that it equalizes the osmotic pressure, stops further entry of water into the cell and prevents bursting. Another example is reverse osmosis, where you force water to move from a solution of higher solute concentration to that of a lower by applying pressure.

I tried to be brief. I hope it was clear.