Characteristics of Facilitated Diffusion

Steps:

• Define facilitated diffusion as passive transport of molecules across membranes via protein channels or carriers, without energy input.
• Check statement 1: It uses channels but relies on concentration gradients, not ATP (that's active transport).
• Check statement 2: Correct, as it moves substances down their concentration gradient from high to low.
• Check statement 3: Correct, as diffusion rate depends on channel availability and reaches a saturation maximum.

Why D is correct:

• Facilitated diffusion follows the passive transport law, moving solutes down gradients via channels with rates limited by carrier saturation (Michaelis-Menten kinetics).

Why the others are wrong:

• A includes 1, which incorrectly adds ATP energy.
• B includes 1, confusing it with active transport.
• C includes 1, misstating the energy source.

Final answer: D