MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A researcher is studying the transport of a small, uncharged molecule across a cell membrane. They observe that the rate of transport increases with the concentration gradient but eventually plateaus, even when the concentration gradient continues to rise. This transport mechanism does not require direct ATP hydrolysis. Which of the following transport mechanisms is most consistent with these observations?
- ASimple diffusion
- BSecondary active transport
- CPrimary active transport
- DFacilitated diffusion
Show answer & explanationAnswer & explanation
Correct answer: D. Facilitated diffusion
The key observations are: transport increases with concentration gradient (suggesting passive transport), it eventually plateaus (indicating saturation of a carrier mechanism), and it does not require direct ATP hydrolysis (ruling out primary active transport). Facilitated diffusion uses a carrier protein or channel, which can become saturated, and moves molecules down their concentration gradient without direct energy input.
Why the other options are wrong
- A. Simple diffusion increases linearly with concentration gradient and does not plateau because it does not involve carrier proteins.
- B. Secondary active transport also moves molecules against their concentration gradient (or one molecule against while another moves down), and while it doesn't directly use ATP, it relies on an electrochemical gradient established by primary active transport. It's usually against the net gradient for the transported molecule, unlike the observation here.
- C. Primary active transport requires direct ATP hydrolysis and moves molecules against their concentration gradient, which contradicts the observation of increasing with the gradient.
Facilitated Diffusion
A type of passive transport that uses integral membrane proteins (channels or carriers) to move substances across the cell membrane down their concentration gradient, without direct energy expenditure.
- Passive transport (down gradient).
- Uses carrier proteins/channels.
- Exhibits saturation kinetics.
- No direct ATP required.
Memory trick: Facilitated: Fast, Free, but Finite!