What substances can cross the semipermeable cell membrane?

What substances can cross the semipermeable cell membrane?

• A. Water, small hydrophilic molecules, and lipophilic molecules
• B. Large proteins and polysaccharides
• C. Ions only via channels
• D. Non-polar solvents only

Answer: (A)

Membrane permeability depends on a solute’s ability to dissolve in the lipid bilayer and its size. The hydrophobic interior of the membrane favors diffusion of lipophilic (nonpolar) substances, so small lipophilic molecules cross readily. Water, though polar, is tiny and moves through aquaporin channels or by limited diffusion, allowing it to cross efficiently. Small hydrophilic molecules can also cross, to some extent, depending on their size and whether channels or transporters are involved. In contrast, large proteins and polysaccharides are too big to pass through the lipid core, and ions are charged, requiring specific channels or transporters rather than simple diffusion. Non-polar solvents cross well, but that doesn’t exclude water and small hydrophilic molecules, so describing the membrane’s permeability as including water, small hydrophilic molecules, and lipophilic molecules best captures the range that can cross.

Membrane permeability depends on a solute’s ability to dissolve in the lipid bilayer and its size. The hydrophobic interior of the membrane favors diffusion of lipophilic (nonpolar) substances, so small lipophilic molecules cross readily. Water, though polar, is tiny and moves through aquaporin channels or by limited diffusion, allowing it to cross efficiently. Small hydrophilic molecules can also cross, to some extent, depending on their size and whether channels or transporters are involved. In contrast, large proteins and polysaccharides are too big to pass through the lipid core, and ions are charged, requiring specific channels or transporters rather than simple diffusion. Non-polar solvents cross well, but that doesn’t exclude water and small hydrophilic molecules, so describing the membrane’s permeability as including water, small hydrophilic molecules, and lipophilic molecules best captures the range that can cross.