Question

• A migrating fish would be rapidly transferring substances across cell membranes. These substances would include sodium, water, carbon dioxide, oxygen, and steroids. Which of those would be most likely to be moved via active transport? Why?

Answer 1

Active transport of molecules is the transport of molecules from a region of lower concentration to higher concentration with help of a protein carrier.

Notice that the transport occurs against the concentration gradient, this is why the transport needs a carrier where the protein utilizes the energy produced due to respiration in the cell.

The given scenario is that of a fish migrating between various water bodies. The chief difference in water bodies is the amount of salt dissolved i.e you have fresh water and salt water that have varying amounts of salt.

Different fresh water bodies have varying salt concentrations based on the amount of silt present but the salt concentrations are very extremely less compared to salt water bodies.

Oxygen and CO₂ have little varying concentrations in the water. O₂ rapidly diffuses into the fish gills and does not require any active transport and the CO₂ has the same rapid diffusion out of the fish after respiration.

Amongst all the given molecules, sodium content will have the highest gradients and the steroids formed in fish have a negative gradient w.r.t to water body to fish gradient.

Steroids will most definitely diffuse through the fish with the gradient, whereas the sodium content is a different scenario.

Fishes migrating from fresh waters to salt waters will have an intake of sodium which has to be excreted to maintain the balance of sodium ions in the body.

But as the concentration of sodium in salt waters is significantly much higher, the sodium ions have to be transported against the gradient and therefore require active transport.