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 cannot cross phospholipid bilayers.
○ simple diffusion
○ solutes may use active transport, facilitated diffusion, or simple diffusion, depending on the solute.
○ facilitated diffusion
○ active transport

Explanation:

1. Analyze each option:

• Option 1: Incorrect. Some solutes (like small non - polar molecules) can cross phospholipid bilayers.
• Option 2: Simple diffusion is the movement of solutes from an area of higher concentration to lower concentration across a phospholipid bilayer without the need for a protein (channel or carrier). This matches the description in the question.
• Option 3: Active transport requires energy and moves solutes against the concentration gradient (from low to high), and facilitated diffusion requires a protein. So this option is incorrect as the question states no protein is involved and the movement is down the concentration gradient.
• Option 4: Facilitated diffusion requires a protein (channel or carrier) to help the solute cross the membrane, so this is incorrect as the question says no protein assistance.
• Option 5: Active transport moves solutes from low to high concentration (against the gradient) and requires energy, which does not match the description in the question.

Answer:

B. simple diffusion