Question

how does the structure of the phospholipid bilayer help the membrane function as a selective barrier?
a the dna in the bilayer blocks toxic molecules
b the hydrophobic tails attract water, allowing all molecules to pass
c the hydrophilic heads allow only large proteins through
d the arrangement prevents most substances from passing freely

Explanation:

• Option A: Phospholipid bilayer does not contain DNA. DNA is mainly in the nucleus (in eukaryotes) or nucleoid (in prokaryotes).
• Option B: Hydrophobic tails repel water. And the membrane is not permeable to all molecules.
• Option C: Hydrophilic heads are part of the phospholipid structure at the surface of the bilayer. They do not selectively allow only large proteins through. The membrane has other mechanisms (like protein channels) for transporting molecules.
• Option D: The phospholipid bilayer has a hydrophobic interior (due to fatty acid tails) and hydrophilic exteriors (due to phosphate - containing heads). This arrangement prevents most polar and charged substances (which are hydrophilic) from passing freely through the hydrophobic core of the bilayer.

Answer:

D. The arrangement prevents most substances from passing freely