Briefly explain why cells control water balance across membr

Briefly explain why cells control water balance across membranes by transporting solutes like sucrose rather than just pumping water directly? Cystic fibrosis (CF) is a disease caused by a non-functional chloride ion channel. Chloride ions (Cl-) get trapped inside the cell and this turn prevents sodium ions (Na+) from leaving the cell easily, altering osmosis of water across the membrane. Why do you think the sodium ions do not diffuse freely, even with a sodium channel present in CF? Why do some cells have water channels (aquaporin\'s) and others do not? How do you think this difference will influence osmosis?

Solution

Please find the answers below:

Answer Part 10: The cells are physically made up of water more than 70%. Since optimum amount of water is absolutely required to manage the hydration status of the cells and homeostasis, cell have developed multiply types of methodologies to conserve and modulate movement of water across them. The most important and signficant method is osmosis and translocation of water. Since the matrix of cells is also composed of water, cells cannot simply diffuse water in or out of them in order to maintain hydration. If it had been possible, the water potential across the membrane would have been the same and no change in concentration of solutes would be possible. However, since movement of water is also dependent upon concentration of solutes and different kind of solutes are present inside and outside the cells, movement of water is often assisted by solute concentrations such as sodium chloride/salt or glucose/sugars.

When the solute concentration is high inside the cells, the cells become hypertonic in nature and movement of solvent/water takes place from outside to inside the cells. Reverse of this direction of movement takes place when solute concentration is higher in the extracellular matrix.

This explains that cells manage their intracellular and extracellular water potential by the means of movement of salts/solutes such as sugars.

Answer part 11: It is a very well known fact that sodium and chloride ions are antagonistic to each other and sequential movement of these ions across the plasma membrane is absolutely required to maintain the action potential along with polarization and depolarization status of the membrane. The movement of sodium ion outside the cells causes membrane to depolarize and generate action potential, whereas a subsequent movement of chloride ions inside the cells causes the membrane to repolarize. Since the movement of chloride ions is impaired in CF, the subsequent activation of sodium ion movement is also impaired. This phenomenon is termed as ANTIPORT. Biologically, these ions are bound to move inside/outside the cell by ATP-independent mechanisms and thus manage the membrane potential. This is why failure of sodium ions movement occurs in patients with CF along with failure of chloride ion translocation.

Answer part 12: The water channels, aquaporins are specialized ATP dependent channels located in the plasma membrane of cells actively engaged in fluid movement across them such as lung cells, brain cells etc. These cells open or close in response to extracellular/intracellular water potential in order to maintain fluid balance and clearance. Importantly, these channels are found only on some of the specialized types of cells and not all the cells of the body. This is because the fact that only these specialized types of cells are highly prone to pathological damage owing to faulty water movement across them and thus creation of local edema. Thus, presence of these channels in cells of these organs play highly significant role in managing water movement along with osmosis. For example, aquaporin 4 and 5 are found in lungs and brain and protects them from pulmonary and cerebral edema formation but altogether absent in cardiac cells. The optimum function of these channels is impaired under stress such as hypoxia, infection, increased blood pressure, hypertension, renal distrubances etc.