Membrane Diffusion and Osmosis

Down to the last three lectures that need revising for now! Yay :) (Also there are only 3 more lectures left for the semester! Almost there!)

1. Understand the random movement of molecules down a concentration gradient.

As you should know by now, molecules diffuse from an area of high concentration to an area of low concentration. (To find out more, review my earlier post on diffusion and osmosis.)

One common misconception with diffusion is that particles move to an area of low concentration. This isn't strictly true. Molecules don't wake up in the morning and think "hmm, looks like there's less of us over there, better get moving then." They move around randomly, dispersing themselves over the available area. Throughout this process of dispersion, you get roughly even amounts of the molecule throughout the area. Hence the net movement of molecules is from a high concentration to a low concentration, but the motions of individual molecules is still random.

2. Understand the importance of molecular weight, lipid solubility and charge on membrane permeability.

Molecules with a lower molecular weight tend to diffuse more rapidly, probably because they don't have to "push aside" other molecules as much.

Lipid solubility is another important factor influencing membrane permeability. Molecules that are more lipid soluble are more permeable as they are soluble in the cell membrane (which is a lipid bilayer).

Charged particles, as they tend to be hydrophilic and lipid insoluble, have a very low permeability. They can exert their effects by binding to receptors on the cell surface (see my post on biochemical messengers) or by diffusing through ion channels (I'll talk about these later).

3. Define the concept of flux and its relationship to membrane permeability.

Flux is the movement of molecules across a given area. If you are looking at the flux across the membrane, you are essentially also looking at the membrane permeability (i.e. the ability of molecules to move across the membrane). Hence flux is directly proportional to membrane permeability.

4. Contrast the diffusion of solutes through a membrane and through a pore/channel.

I'm not really sure what there is to contrast here. While lipid-soluble substances diffuse through the membrane, ions and other charged substances often have to travel through channels. The general principles still apply, but sometimes the channels can open and close.

5. Understand the concepts of osmolarity/osmolality and tonicity.

Osmolarity is the number of particles dissolved in a litre of solution, while osmolality is the number of particles dissolved in a kilogram of solution. These terms are pretty much interchangeable on Earth.

Tonicity, on the other hand, is the ability of a solution to affect a cell. A hypotonic solution will cause a cell to burst while a hypertonic solution will cause a cell to lyse. Sometimes a hypotonic solution will also be hypoosmotic, but not always. Sometimes a solution might start out with the same osmolarity as the cell, but it contains certain particles that can diffuse into the cell, thus changing the osmolarities of the cell and the solution.

6. Describe the distribution of water and solute concentrations in the body compartments.

I'm not really sure what is required here. There's a diagram in the slides that shows that most water (roughly 25L) is in the cells, followed by extracellular fluid (roughly 13L), followed by blood plasma (roughly 3L). There's also another 1L marked "transcellular fluid," which, according to Google, is basically fluid that is between cells but separated off from the main interstitial fluid. All but the transcellular fluid has an osmolarity of around 290 mOsm (i.e. roughly the same amount of particles dissolved in all of the fluids in the body).

7. Understand the importance of osmolality and hydrostatic pressure in the regulation of fluid balance.

As blood travels to the tissues, water diffuses into the interstitial fluid via osmosis (a process that is partially counteracted by hydrostatic forces). In the capillaries in the arterial side, the blood pressure is much greater, causing more water to diffuse into the interstitial fluid than out of it (or at least that's my understanding). The opposite occurs in the venous capillaries. This process mostly "balances out" the movement of fluid into and out of the interstitial space, with any extra fluid picked up by the lymph system. If not enough water diffuses back into the blood on the venous side, however, water can build up in the interstitial space, resulting in swelling, or oedema.