Question

if a solute is moving through a phospholipid bilayer from an area of higher concentration to an area of lower concentration without the assistance of a protein, the manner of transport must be ______.
○ solutes may use active transport, facilitated diffusion, or simple diffusion, depending on the solute.
○ solutes cannot cross phospholipid bilayers.
○ active transport
○ simple diffusion
○ facilitated diffusion

Explanation:

1. Analyze each option:

• Option "Solutes may use active transport, facilitated diffusion, or simple diffusion, depending on the solute": Active transport requires energy and moves against concentration gradient (from low to high), facilitated diffusion needs a protein (carrier or channel), so this is incorrect as the problem states no protein and movement from high to low (passive, not active).
• Option "Solutes cannot cross phospholipid bilayers": Some solutes (like small non - polar molecules) can cross via simple diffusion, so this is wrong.
• Option "active transport": Active transport moves from low to high concentration and often needs energy, and the problem is from high to low without protein, so this is incorrect.
• Option "simple diffusion": Simple diffusion is the movement of solutes (usually small, non - polar or lipid - soluble) through the phospholipid bilayer from high to low concentration without the need for a protein. This matches the description in the question.
• Option "facilitated diffusion": Facilitated diffusion requires a protein (carrier or channel protein) to help the solute cross the membrane, but the question states no protein assistance, so this is incorrect.

Answer:

D. simple diffusion