Overview
This lecture explains how potassium and sodium channels in cell membranes facilitate the selective movement of ions via facilitated diffusion, emphasizing differences in size, charge, and gating mechanisms.
Facilitated Diffusion and Channels
- Channels are protein tunnels in cell membranes that enable facilitated diffusion of molecules.
- Potassium and sodium are key ions with dedicated channels for cell transport.
Potassium vs. Sodium Ions
- Potassium (K⁺) is a relatively large ion with a molar mass of about 39 and often surrounded by water molecules due to its positive charge.
- Sodium (Na⁺) is smaller with a molar mass of about 22 and also surrounded by water molecules.
- Potassium cannot fit through sodium channels because it is too large.
- Sodium cannot pass through potassium channels because it cannot shed its water molecules.
Channel Selectivity Mechanisms
- Potassium channels have structures ("brushes") that strip water molecules from potassium ions, allowing them to pass through.
- Sodium ions, due to their size and hydration, cannot have their water molecules removed by potassium channels and therefore cannot pass through.
- Sodium channels have negatively charged interiors that attract sodium ions, pulling them in and removing attached water molecules for passage.
- Potassium ions are too large to pass through sodium channels even if attracted by the negative charges.
Gated Channels
- Both potassium and sodium channels can have gates, or doors, that block transport regardless of ion size or charge.
- Gated channels can open in response to specific stimuli such as mechanical force, enabling controlled ion flow.
- Opening many channels simultaneously allows rapid ion diffusion.
Key Terms & Definitions
- Facilitated diffusion — passive movement of molecules across membranes via proteins, without energy input.
- Channel protein — a tunnel-like membrane protein that enables specific ions or molecules to diffuse across the cell membrane.
- Ion — an atom or molecule with an electrical charge.
- Gated channel — a channel protein with a door that opens or closes in response to stimuli.
- Hydration shell — the water molecules that surround and interact with an ion in solution.
Action Items / Next Steps
- Review notes on carrier proteins as the next topic.
- Understand the differences in ion channel selectivity for potassium and sodium.