Questions: Ion Channels and Selective Permeability Mechanisms

Question 1

What structural feature of a potassium channel is primarily responsible for excluding sodium ions despite Na⁺ being smaller than K⁺?

• A. A gate that physically blocks Na⁺ from entering
• B. The selectivity filter, whose carbonyl oxygen atoms are spaced to coordinate K⁺ but not the smaller Na⁺
• C. A negatively charged residue that repels Na⁺ by electrostatic force
• D. The channel only opens when K⁺ concentration is high enough to displace Na⁺

Answer: (B)

The selectivity filter's carbonyl oxygens are precisely positioned to substitute for the water molecules that normally surround K⁺ in solution — snugly coordinating the larger K⁺ ion. Na⁺ is too small to be stabilized by the same oxygen positions, so it retains its hydration shell and cannot fit the filter efficiently. This is a classic example of precise molecular geometry overcoming the intuitive expectation that 'smaller fits through smaller holes.'

Question 2

Ion channels use ATP hydrolysis to drive ions across the membrane, similar to the sodium-potassium ATPase pump.

• T. True
• F. False

Answer: False

This is a fundamental misconception. Ion channels are passive transporters — they allow ions to flow down their electrochemical gradient without consuming energy. The Na⁺/K⁺-ATPase is an active transporter (pump) that uses ATP to move ions against their gradients. Ion channels are orders of magnitude faster than pumps (10⁶–10⁷ ions/s vs ~100–1000 ions/s for pumps) precisely because they don't perform active work.

Question 3

What is 'gating' in an ion channel, and what stimuli can trigger it?

Model answer: Gating is the opening and closing of an ion channel — the transition between a conducting (open) state and a non-conducting (closed or inactivated) state. Stimuli that trigger gating include changes in membrane voltage (voltage-gated channels, e.g., Nav, Kv), binding of a ligand (ligand-gated channels, e.g., nicotinic acetylcholine receptor), and mechanical deformation of the membrane (mechanosensitive channels, e.g., in hair cells).

The ability to gate — to switch rapidly between open and closed — is what allows ion channels to encode information and respond to stimuli. A permanently open channel would collapse ion gradients; a permanently closed one would be useless. Gating mechanisms provide exquisite temporal and spatial control, which is why loss-of-function or gain-of-function mutations in channel gating cause diseases like long QT syndrome, epilepsy, and cystic fibrosis.