Which statement correctly describes facilitated diffusion?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Facilitated diffusion is a passive transport mechanism in which specific integral membrane proteins—channel proteins and carrier proteins—enable the movement of polar or charged solutes across the phospholipid bilayer down their electrochemical gradient, without any input of metabolic energy (ATP). The interior of the bilayer consists of the nonpolar hydrocarbon tails of phospholipids, generating a hydrophobic core with a low dielectric constant. Polar molecules and ions—such as glucose, amino acids, K⁺, Na⁺, and Cl⁻—cannot freely cross this region because doing so would strip away their hydration shells and place partial charges (or full ionic charges) into an environment lacking hydrogen-bond partners or charge-stabilizing dipoles, an energetically unfavorable thermodynamic state. Channel proteins solve this by forming a water-filled pore lined with hydrophilic residues that rehydrate the solute during transit, while carrier proteins undergo substrate-induced conformational shifts (alternating access) to shield the solute from the lipid interior. The driving force is the electrochemical gradient itself: the combination of a concentration difference and, for ions, a membrane potential difference. Because the gradient represents stored free energy (ΔG < 0 for downhill movement), no external energy source is required. Transport is saturable—once all protein binding sites or channels are occupied, the rate plateaus—distinguishing it from simple diffusion through the lipid phase.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

A correct description of facilitated diffusion must therefore include three non-negotiable criteria: (1) movement occurs down the concentration (or electrochemical) gradient, from the region of higher free energy to lower free energy; (2) a specific transmembrane protein mediates the crossing; and (3) no ATP hydrolysis or other metabolic energy input powers the process. The canonical correct statement will affirm passive, protein-mediated movement along the gradient. When evaluating any candidate statement, verify each criterion. For example, the transport of glucose into a red blood cell via the GLUT1 carrier proceeds from the extracellular space (higher glucose concentration after a meal) into the cytosol (lower concentration as glycolysis consumes glucose). GLUT1 alternates between outward-facing and inward-facing conformations triggered by glucose binding—classic carrier-type facilitated diffusion. No Na⁺ gradient, proton gradient, or phosphorylation event is involved, reinforcing that the free-energy drop across the membrane itself supplies all necessary energy.

PILLAR 3 — DISTRACTOR ANALYSIS

Incorrect options typically violate one of the three criteria above. A common distractor conflates facilitated diffusion with active transport, claiming movement occurs against the concentration gradient; this mis-models the thermodynamic directionality and ignores the necessity of ATP (or a coupled ion gradient) for uphill work. Another frequent trap states that facilitated diffusion requires direct ATP hydrolysis, confusing carrier-mediated passive transport with primary active transport (e.g., Na⁺/K⁺-ATPase, which hydrolyzes ATP to pump ions against their gradients). A third distractor may describe transport occurring directly through the phospholipid bilayer without protein involvement, which is simple diffusion—the correct pathway only for small, nonpolar molecules like O₂ or CO₂, not for glucose or ions. A fourth option might invoke vesicular or bulk transport mechanisms (endocytosis, exocytosis), conflating macromolecular engulfment with single-molecule protein-channel transit. Finally, some options incorrectly state that facilitated diffusion can continue indefinitely at a linear rate without saturation, misrepresenting the kinetics that distinguish protein-mediated transport from the unconstrained flux of simple diffusion.