Shared mechanisms in active transport and facilitated diffusion

Steps:

• Active transport uses ATP to move molecules against their concentration gradient via carrier proteins.
• Facilitated diffusion moves molecules down their gradient through channel or carrier proteins without energy input.
• Both processes require specific membrane proteins for transport and exhibit saturation at high substrate concentrations.
• Options identify correct shared features as 2 (protein involvement) and 3 (saturation kinetics), corresponding to choice C.

Why C is correct:

• Both active transport and facilitated diffusion rely on membrane proteins that can become saturated, as defined by Michaelis-Menten kinetics in carrier-mediated transport.

Why the others are wrong:

• A includes 1, which likely refers to energy requirement—true only for active transport, not facilitated diffusion.
• B includes 1, incorrectly pairing it with 3.
• D omits 2, ignoring the shared use of transport proteins.

Final answer: C