Question

complete the table to compare the three types of passive transport. simple diffusion facilitated diffusion osmosis molecules cross the membrane through the phospholipids molecules cross the membrane through proteins molecules move down their concentration gradient molecules move without using cellular energy

Explanation:

• Simple diffusion: Small non - polar molecules (e.g., oxygen, carbon dioxide) cross the membrane through the phospholipids. All passive transport (simple diffusion, facilitated diffusion, osmosis) involves molecules moving down their concentration gradient and without using cellular energy.
• Facilitated diffusion: Polar or large molecules (e.g., glucose) cross the membrane through proteins (channel or carrier proteins).
• Osmosis: It is the diffusion of water. Water can cross the membrane through the phospholipids (to a limited extent, but also through aquaporins which are proteins). But the key for osmosis in the context of the first row (phospholipids) is that water can pass through the membrane (as it is small and uncharged to some degree through the phospholipid bilayer). And like other passive transport, it is down the concentration gradient and no cellular energy is used.

Answer:

• Molecules cross the membrane through the phospholipids: simple diffusion (yes), facilitated diffusion (no), osmosis (yes)
• Molecules cross the membrane through proteins: simple diffusion (no), facilitated diffusion (yes), osmosis (yes - through aquaporins which are proteins, although water can also pass through phospholipids to a small extent)
• Molecules move down their concentration gradient: simple diffusion (yes), facilitated diffusion (yes), osmosis (yes)
• Molecules move without using cellular energy: simple diffusion (yes), facilitated diffusion (yes), osmosis (yes)