Question

In which of the following cases can transport saturation be seen?

• A. Simple diffusion
• B. Facilitated diffusion
• C. Active transport
• D. Both b and c

Answer 1

The correct option is D Both b and c

Simple diffusion is the random movement of molecules from their higher concentration to their lower concentration (along the concentration gradient) until equilibrium is attained. It is a slow process. Substances that are being transported across the plasma membrane must be lipid soluble. It occurs only for short distances. It is a passive process, i.e. there is no consumption of ATP. Movement of gases in plants occurs through simple diffusion.

Facilitated diffusion is the passive (there is no consumption of ATP) transport of molecules across a concentration gradient that is performed with the help of specialized proteins. Movement of lipid insoluble substances across the plasma membrane occurs through facilitated diffusion. It is highly specific to the molecules being transported. The specialized proteins transport a few molecules at a time across the membrane rather than a bulk transport. It is sensitive to inhibition.

Active transport is highly specific to the molecules that are being transported. ATP (energy currency of the cell) is consumed in this process as the transportation occurs against the concentration gradient (from a region of higher concentration to a region of lower concentration). It is sensitive to inhibition (inhibitors usually target the side chain of proteins). It generally occurs with the help of pumps. Pumps are ATP consuming membrane transport proteins that transport molecules across the membrane against their concentration gradient.

Rate of transport reaches a maximum when all transport proteins are occupied. Beyond this point, the transport of proteins does not occur.
Since facilitated diffusion and active transport both use transport proteins, transport saturation can be seen here.