III. Membrane Transport Mechanisms
Transport across the membrane can involve a single molecule (uniport) or the cotransport of two different molecules in the same direction (symport) or opposite directions (antiport).
- Passive Transport
This process does not require energy, as molecules move down a concentration or electrochemical gradient.
- Simple Diffusion: The movement of small nonpolar molecules (e.g., O₂, N₂) and small, uncharged polar molecules (e.g., H₂O, CO₂) directly across the bilayer.
- Facilitated Diffusion: A faster process that uses protein channels or carriers to transport ions and large polar molecules.
- Ion Channel Proteins: Form aqueous pores for specific small molecules and ions. Aquaporins are specialized channels for the rapid transport of water.
- Carrier Proteins: Undergo conformational changes to transport specific molecules.
- Active Transport
This energy-requiring process uses carrier proteins to move molecules against an electrochemical gradient.
- Na⁺–K⁺ Pump: An antiport system mediated by Na⁺–K⁺ ATPase. It pumps three Na⁺ ions out of the cell and two K⁺ ions into the cell, powered by the hydrolysis of one ATP molecule. Its primary function is to maintain constant cell volume.
- Glucose Transport: A symport system that often uses the electrochemical Na⁺ gradient to drive glucose into the cell.
- ATP-Binding Cassette (ABC) Transporters: Use ATP to export substances like toxins and drugs from the cytoplasm. Multidrug-resistant (MDR) proteins are a type of ABC-transporter found in cancer cells that can expel cytotoxic drugs.
- Ion Transport Systems
- Selective Ion Channels:
- K⁺ Leak Channels: Ungated channels that are the most common type; their leaking of K⁺ is primarily responsible for the membrane’s potential difference.
- Gated Ion Channels: Open transiently in response to specific stimuli.
- Voltage-gated channels: Respond to changes in membrane potential (e.g., Na⁺ channels in action potentials).
- Mechanically gated channels: Respond to mechanical stimuli (e.g., in inner ear hair cells).
- Ligand-gated channels: Respond to the binding of a signaling molecule (e.g., neurotransmitter receptors).
- Ionophores: Lipid-miscible molecules that bind to ions and transport them across the membrane, either by acting as mobile carriers or by forming channels.