Selective permeability of the plasma membrane The barrier properties of the plasma membrane depend primarily on the presence of which feature?

Introduction / Context:
Cells must regulate what enters and leaves. The lipid bilayer provides a hydrophobic barrier, but selectivity and controlled rates come from protein transport systems embedded in the membrane.

Given Data / Assumptions:

• Lipids form a barrier to polar and charged solutes.
• Transport proteins allow regulated passage of ions and molecules.
• Carbohydrates are involved mainly in recognition, not translocation.

Concept / Approach:
Selective permeability emerges from channels, carriers, and pumps—each an integral protein with substrate specificity and gating/regulatory features. This contrasts with nonselective diffusion of hydrophobic solutes through lipids.

Step-by-Step Solution:
Identify the need: selectivity and regulation.Associate with mechanism: specific transport proteins (ion channels, uniporters, symporters, antiporters, ATPases).Eliminate options lacking transmembrane transport capabilities.

Verification / Alternative check:
Patch-clamp and transporter kinetics demonstrate substrate-specific conductance and saturable transport, respectively.

Why Other Options Are Wrong:

• Amino acids / DNA: Not structural transport elements.
• Carbohydrates: Key for cell–cell recognition but not for solute translocation.

Common Pitfalls:
Assuming the lipid bilayer alone confers selectivity; it mainly restricts polar solutes without specificity.

Final Answer:
Specific transport proteins.