Which transport process moves substances down their concentration gradient through channel proteins without energy input?

Which transport process moves substances down their concentration gradient through channel proteins without energy input?

• A. Active Transport
• B. Osmosis
• C. Facilitated Diffusion
• D. Phagocytosis

Answer: (C)

Movement down a concentration gradient with help from membrane proteins is diffusion that uses a passageway provided by the cell membrane. In facilitated diffusion, specific channel or carrier proteins form a path that allows certain molecules or ions to cross the membrane without using cellular energy. The gradient itself provides the driving force, so substances move from higher to lower concentration passively. This fits the idea of “through channel proteins without energy input” because the protein creates a doorway for the solute, making crossing possible even though the lipid bilayer would be impenetrable to many polar or charged particles. Examples include ions moving through ion channels or sugars moving via carrier proteins. While osmosis also happens passively and can involve aquaporin channels for water, the question targets solutes moving through channel proteins down their gradient, which is best described by facilitated diffusion. Phagocytosis, on the other hand, is active and involves engulfing large particles, requiring energy.

Movement down a concentration gradient with help from membrane proteins is diffusion that uses a passageway provided by the cell membrane. In facilitated diffusion, specific channel or carrier proteins form a path that allows certain molecules or ions to cross the membrane without using cellular energy. The gradient itself provides the driving force, so substances move from higher to lower concentration passively.

This fits the idea of “through channel proteins without energy input” because the protein creates a doorway for the solute, making crossing possible even though the lipid bilayer would be impenetrable to many polar or charged particles. Examples include ions moving through ion channels or sugars moving via carrier proteins. While osmosis also happens passively and can involve aquaporin channels for water, the question targets solutes moving through channel proteins down their gradient, which is best described by facilitated diffusion. Phagocytosis, on the other hand, is active and involves engulfing large particles, requiring energy.