cell vol regulation

Question 1

Be able to determine which direction an uncharged substance will move across a membrane.

Answer 1

According to concentration gradient.

Question 2

Determine under a given set of conditions whether a cell will swell or shrink.

Answer 2

Look at the osmotic pressure inside and outside of the cell.

Question 3

List the three mechanisms that different cells have evolved to keep from swelling and
bursting.

Answer 3

A simple solution to the problem would be to make the cell membrane impermeable to water, as well as to the internal solute molecules (Fig. 2, left). However, this would create many new problems for the cell; growing cells, for example, must have a way of accumulating water as they grow (remember, cells are mostly water by volume), and therefore they must have surface membranes permeable to water. There are some special kinds of cell membranes that do exhibit extremely low water permeability (sweat glands, some cells in the kidney). In general, however, plasma membranes are quite highly permeable to water (although diffusion of water, and all substances we will consider, through even the most permeable membrane, is still thousands of times slower than diffusion in free solution).
Another solution (Fig. 2, middle) is to build a strong wall around the cell, and so keep the cell from swelling by brute force (that is, apply a hydrostatic force to counter the osmotic force drawing water into the cell). Plant cells, fungal cells, and bacteria do just this, by building tough cell walls outside their plasma membranes. But it's an expensive way to go, in the sense that the osmotic force is not trivial (as shown on the right, a small container of pure water would support a column of a 1 M solution, from which it is separated by a semi-permeable membrane, that is 900 feet high), so that cell walls require considerable metabolic resources (building materials, energy for synthesis, etc.), and greatly limit cell shape (try to imagine a neuron in the cerebral cortex, with all of its delicate dendrites surrounded by a cell wall!)
The solution in animal cells to the problem of volume control (Fig. 2, right) is to fight fire with fire, and balance the osmotic force osmotically, by having solute molecules in the ECF, in order to balance those in the ICF. Thus, the concentration (or, more accurately, the activity) of water is exactly the same in the ICF and ECF. How simple physiology can be.

Question 4

Know the difference between diffusion and osmosis.

Answer 4

The elementary force underlying this water movement is the random thermally-agitated movement of the molecules, that is, diffusion. This movement causes water molecules outside the cell to collide with the membrane; some will enter aqueous pores and so enter the cell. Similarly, some water molecules inside the cell will enter pores and so leave the cell. However, the presence of the nonpermeating molecules inside the cell will reduce the probability that a water molecule will leave the cell, and this means more water will enter than will leave. This net inward movement of water across a semi-permeable membrane (i.e., a membrane permeable to solvent, but not to solute) is called osmosis. In fact you can picture osmosis as diffusion of water.

Question 5

Know the effect of having a membrane with different, non-zero permeabilities (i.e.,
reflection coefficients less than 1) to different solutes .

Answer 5

Reflection coeffecient of 1: non-permeable
Reflection coeffecient of 0: same as water
A different reflection coefficient complicates matters, A molecule that crosses half as easily as water will exert half of the ideal osmotic pressure of a non-permeating solute. This has clinical implications. Shock involves having blood go to extremities, and not to the brain. You give them an IV of NaCl because it is non-permeable. Clysis. Fluid is given subcutaneously if you can’t find a vein. Glucose is less permeable, which temporarily sucks water out of cells before it gets into the cells and starts helping them absorb water.

Question 6

Know the definitions of molarity, osmolarity, equivalents, and tonicity, and know how to
convert between them.

Answer 6

Osmolarity: is the total concentration of solute particles: for example, a 1 M solution of CaCl2 gives a 3 osM solution (3 solute particles/molecule dissolved).
molarity = number of moles of solute per liter of solution
equivalents = number of “combining-weights” of an ion per liter; calculated by a two step process: for each ion – convert to mosM; multiply mosM by the valence of the ion
tonicity = effect of a solution on a cell; depends on the permeability of the membrane; solution that makes cell shrink is hypertonic; solution that makes a cell burst is hypotonic