Question

Given that the interior of the cell is a very crowded place, hypothesize as to which form of transport may be more useful to the cell : active or facilitated transport. Explain.

Answer 1

The choice of transport between active or facilitated transport depends upon the specific needs of the cell. If the cell is performing absorption, such as intestinal epithelial whose function is to absorb glucose from the digested food, it will prefer active transport of glucose inside the cell because the system is designed to absorb glucose from the gut irrespective of it's concentration in the body. If the transport means were facilitated transport, i.e., a passive concentration dependent process, then when the glucose concentration in body had been higher, it would have been transported back to gut by the epithelial cells. There would be no concept of storage of glucose, it would quickly deplete short-term energy reserves of the body and the condition would become fatal and possibly even lethal. Thus, in this case, the extra energy expenditure and the costs associated with the active transport are worth the trouble for the cell in order to avoid a biologically wasteful situation.

On the other hand, if we take the example of red blood cells or eryhthrocytes glucose is transported inside through facilitated transport. This is because, the concentration of glucose in blood pretty much remains constant throughout because of homeostasis and as soon as the glucose is transported inside the cell, it is metabolised to release energy essential for performing cellular functions. As such, the concentration of glucose inside the cell is always less than it's concentration in surroundings, thus facilitated transport which depends on concentration gradient is the best choice for the cell and active transport is not preferred since the costs associated with it are wasteful in this case and not really required.

Thus, it does not matter how crowded the cell interior is, the mode of transport of any ion or compound or molecule will always depend on the specific needs of the cell and what functions it is performing. Same cell can prefer active transport for one ion, while another ion can be transported passively. So, it is the needs of cell that decide the mode of transport, not it's morphology.