Question

which of the following accurately describes why specific material uses a protein to cross the membrane?
○ the membrane will allow any substance to cross it as long as it moves from a high concentration to a low concentration.
○ the hydrophobic core of a membrane will only allow hydrophobic (i.e., non - polar) substances to cross without a membrane protein.
○ material can only cross the membrane via a protein.
○ the hydrophilic core of a membrane will only allow hydrophilic (i.e., polar) substances to cross without a membrane protein.
○ the membrane will allow any substance to cross it if the concentration of solute is high enough on one side of the membrane.

Explanation:

To solve this, we analyze each option based on cell membrane structure (phospholipid bilayer with hydrophobic core):

• Option 1: Membranes are selective, not all substances cross with concentration gradient (e.g., polar molecules need proteins), so wrong.
• Option 2: The cell membrane's core is hydrophobic (non - polar). Hydrophobic substances can pass through the lipid bilayer without proteins, while polar/hydrophilic substances need membrane proteins (like channels or carriers) to cross. This matches the concept of membrane permeability.
• Option 3: Small non - polar molecules (e.g., O₂, CO₂) can cross without proteins, so "only via protein" is wrong.
• Option 4: The membrane core is hydrophobic, not hydrophilic, so this statement about the core and substance crossing is incorrect.
• Option 5: Membranes are selective; concentration alone doesn't allow any substance to cross (e.g., large polar molecules need proteins even with high concentration), so wrong.

Answer:

B. The hydrophobic core of a membrane will only allow hydrophobic (i.e., non - polar) substances to cross without a membrane protein.