Paper 2 · November 2016 · Cell Structure and Membranes
In a cell surface membrane, water-soluble (polar) substances cannot diffuse freely across the lipid bilayer. Which part of the membrane structure is mainly responsible for restricting this movement?
AThe glycocalyx carbohydrate chains attached to the outer surface of the membrane
BThe peripheral proteins loosely attached to the inner face of the membrane
CThe hydrophilic phosphate heads, which face the aqueous cytoplasm and are thought to actively draw polar substances inward
DThe hydrophobic fatty acid tails, which form a non-polar core that repels polar molecules
Explanation
The phospholipid bilayer's hydrophobic fatty-acid tails form a non-polar core. Water-soluble/polar substances are repelled by this core and so cannot simply diffuse across it; they instead require hydrophilic channels or carrier proteins.
Derived from ZIMSEC Biology Paper 2, November 2016, Q1