Question

how do small water - soluble molecules and ions diffuse across the plasma membrane? how do lipid - soluble molecules diffuse across the plasma membrane? (figure 3 - 15)
○ diffuse directly through the plasma membrane; diffuse through membrane channels
○ diffuse by endocytosis; diffuse by pinocytosis
○ diffuse by carrier - mediated transport; diffuse by passive transport
○ diffuse by osmosis; diffuse by carrier - mediated transport
○ diffuse through membrane channels; diffuse directly through the plasma membrane

Explanation:

1. Recall the structure of the plasma membrane (a phospholipid bilayer, hydrophobic interior).
2. Small water - soluble molecules/ions: The hydrophobic core of the membrane prevents them from passing directly. They use membrane channels (ion channels or aquaporins for water) to diffuse.
3. Lipid - soluble molecules: Since the membrane is made of lipids, lipid - soluble molecules can dissolve in the lipid bilayer and diffuse directly through the plasma membrane.
4. Analyze the options:

• Option 1: Reverses the two processes.
• Option 2: Endocytosis and pinocytosis are for bulk transport, not simple diffusion of these molecules.
• Option 3: Carrier - mediated transport is for some molecules, but lipid - soluble don't need it; passive transport is a broad term, not specific enough.
• Option 4: Osmosis is for water across a selectively permeable membrane in a specific context, and carrier - mediated is not for lipid - soluble.
• Option 5: Matches the correct mechanisms (water - soluble/ions through channels, lipid - soluble directly through membrane).

Answer:

E. diffuse through membrane channels; diffuse directly through the plasma membrane