Final answer:
Molecules competing for the same membrane carrier must have a structural similarity that allows them to bind to the same site on the carrier protein, enabling competition during the process of passive transport such as facilitated diffusion. This can lead to saturation of carrier proteins, limiting the transport rate.
Step-by-step explanation:
If two molecules compete for the same membrane carrier, they must be similar in the way they bind to the carrier protein. Carrier proteins are specific to the molecules they transport; they recognize and bind to these molecules through a specific binding site. The similarity should be in such a way that both competing molecules can bind to the same site on the carrier protein, which generally means they must have a similar structure, size, and charge. This is known as structural specificity. If two molecules bind to the same carrier protein, they will compete for transport across the cell membrane which follows a passive transport pathway, moving molecules down their concentration gradient without requiring energy.
Because carrier proteins can become saturated when all binding sites are occupied by their ligands, an increase in the concentration of one type of molecule can lead to competition, wherein one molecule is transported in preference over another. This is important in the process of facilitated diffusion, where a molecule is moved across a cell membrane from a region of higher concentration to a region of lower concentration through a carrier protein. The molecules move in this way until equilibrium is reached or the carrier proteins become saturated, and the rate of transport hits its maximum limit.