Chapter 4

1) Membranes are responsible for cellular and subcellular compartmentalization, maintenance of intracellular chemical balance, regulation of intracellular metabolism, metabolic activities associated with the membrane, sensing and transduction of extracellular chemical signals, processing of electrical signals, endocytosis and exocytosis.

2) The evidence given in the book is the membranes are able to block some dyes but are permeable to others. They also mention the fact that cells swell in hypotonic water and shrink in hypertonic water. Nageli did this work.

3) Too much to list... There is a "spotlight" insert devoted to this on page 98. Gorter and Grendel took red blood cells and allowed them to burst in a hypotonic solution. They then observed that the lipid layer over the water was twice the surface area for the red blood cells. This suggests that the membrane is a bilayer. Because they oriented themselves with the lipid layer in the air, on top of the water, it suggest that part is hydrophobic and part hydrophilic.

4) Multiple techniques have been used to show this. Immunocytochemistry (this is a process of staining a tissue to visualize the presence of the molecules such as proteins) and fractionation were used to determine the composition of the bilayer. This was also shown through labeling studies that could differentiate which end of a protein is facing which side of the membrane. Also, electron microscopy was used to visualize the difference between protein-digested membranes and normal membranes.

5) This is a matter of definition. To solutions are isosmotic if a membrane that is only permeable to water separates them and their effective concentrations are equal. Thus there is no movement of solute or water. An isotonic cell or tissue will behave very different because of inherent abilities to allow some solutes to diffuse through the membrane. The main difference is that isosmotic does not care what ions are on the outside or inside, because no solute diffusion will happen. Whereas for tonicity it is a central issue because the membrane is not perfect and it will allow some diffusion. So for a cell that is immersed in a solution that is isosmotic but has an ion that can diffuse into or out of the cell then if diffusion works, they are not isotonic. Basically, a cell can be placed in a solution that is isosmotic and swell or shrink if the ions in the two solutions are differnt.

6) Electrolytes generally have very low membrane permeabilities. The central factor is their solubility coefficient. Their permeability given channels is effected by size, charge, and ability to shed water. Additionally, each cell membrane will have its own unique permeability to a given solute. Nonelectrolytes are in general much more permeable to membranes than that of electrolytes. This is easily understood because the core of the membrane supports nonpolar molecules better and thus only highly polar, large molecules will have a hard time passing through. Nonelectrolytes are also dependant on there partition coefficient. Basically, the more soluble in lipid the easier it can pass through the membrane.

7) The text suggests that small pores open in the membrane allowing a stream of water to enter. Small ions come along with the water to enter the cell. Water can also cross the membrane by using water channels made from proteins.

8) Nonpolar molecules diffuse more easily due to the entropy of the interaction. Hydrophilic interactions push the nonpolar substances into the membrane. This is because when a nonpolar molecule is in water it forces the water to become very ordered around it. This ordering is obviously not favored by entropy. Therefor when the nonpolar molecule enters the lipid the water can take on the entropy favored, random orientation in solution. This is all reversed for molecules with charge, that is they want to stay in the water and not the lipid. Or, simply put, most of the membrane is lipid and that is why lipid gets through. Like dissolves like.

9) In epithelial tissue, the ends of the cells are joined by tight juctions, creating a narrow cleft between the cells. Salt solutions will diffuse down there gradient into the cell and are then pumped into the cleft, creating a very concentrated solution. Water is then drawn into the cells and out of the cells into the cleft due to the high conc in the cleft. Thus, even though the bulk of the solution on one side is more dilute than the other, immediately around the cells it is reversed, and water moves down its concentration gradient.

10) Simple diffusion is an act of molecules that independently move down their gradient. Facilitated diffusion is where a protein helps a molecule move down its gradient.

11) This is very similar to enzymes. Here the rate limiting steps can be either transporter concentration, substrate concentration or decreased gradient.

12) Active transport will take energy directly from ATP while facilitated transport does not. It is important to remember that facilitated transport may require energy, it is just that facilitated gets this energy from a energy stored in a gradient.

13) This can be considered a common source of energy because so many transporter use this gradient (symporters, antiporters). Remember that the Na/K pump sets up the gradient and that this is an energy requiring process. Why though would these Na gradient moochers just use ATP instead? It would seem to me that maybe the energy required by a facilitated transporter might be so small that using a hole ATP might be a "waste" of energy.

14) The membrane does this by size and the ions ability to shed water.

15) If metabolism shuts down then there will be no ATP. If there is no ATP then the active transporters will stop working. After some time then the Na will build up due to leak and various symporters. This will lead to enlargement of the cell and eventually to death.

16) Na/K ATPase pumps.

17) A gap junction is range of membrane between to cells that is studded with hexameric proteins that form pores between the two opposing cytoplasms. These proteins are attached to identical proteins on the opposing surface that creates pores. It is these pores that allow signals to pass from one cell to the next. A tight junction is junction where the membranes between two cells touch to not allow any solution the pass between cells.

18) To answer this question one must realize that the Donnan equilibrium is applicable to this situation (eq 4-9). Based on Donnan's observations, there will be a reciprical relationship between K and Cl. Therefore, as K's ratio is 25, Cl's will be 1/25.

19) Substances are transported throught the cell by placing different transporter on opposing sides. Thus, a substance might flow down its gradient on one side (facilitated diffusion) and then get pumped out the other side (active transport).

20) Huf first did this by taking frog skin and placing it between to chamber of solution. They the placed Na isotopes in the bath to track the movement of Na. They were able to conclude that Na was actively transported since they could even see net movement when the experiment was set up against a gradient. This experiment is detailed on page120.

21) First, ouabain blocks Na/K pumps and can inhibit epithelial transport only when applied to the serosal side. Secondly, K is required on the serosal side on the membrane for proper function. This is because the pump must pump K into the cell. Finally, Na transport exhibits saturation kinetics. See page 121 for figure. The pump must bind both Na an K to function. Na saturates on th mucosal side.