Permeability of the membrane
This topic covers how substances cross the plasma membrane: passive and active transport of ions and small molecules, and vesicular transport by endocytosis and exocytosis.
This topic covers how ions and small molecules cross the plasma membrane. Transport is passive when substances move down their electrochemical gradient without metabolic energy, and active when they move against it at the cost of energy. Transport of large particles in vesicles (endocytosis and exocytosis) is covered in separate topics.
The hydrophobic core of the bilayer makes the membrane selectively permeable. The smaller and more lipid-soluble a molecule is, the faster it diffuses across:
- Small nonpolar molecules (O₂, CO₂, N₂) and lipid-soluble substances (steroid hormones) cross easily.
- Small uncharged polar molecules (water, urea, glycerol) cross slowly.
- Larger polar molecules (glucose, amino acids) and ions (Na⁺, K⁺, Ca²⁺, Cl⁻) practically cannot cross the bilayer and need membrane transport proteins: channels and carriers (transporters).
Passive transport
Simple diffusion
Molecules pass directly through the lipid bilayer from higher to lower concentration. The net flux J is proportional to the membrane permeability coefficient P, the membrane area A and the concentration difference (Fick's law):
The rate rises linearly with the concentration difference and shows no saturation.
Facilitated diffusion
- Channel proteins form water-filled pores. They are very fast (millions of ions per second) and selective for particular ions. Most are gated: voltage-gated (Na⁺ and K⁺ channels of nerve cells), ligand-gated (nicotinic acetylcholine receptor) or mechanically gated (hair cells of the inner ear); K⁺ leak channels stay mostly open. Aquaporins are channels for water.
- Carrier proteins bind the solute and change conformation to release it on the other side. Carrier transport is specific, saturable (it has a maximum rate, Vmax) and can be competitively inhibited. Example: the GLUT glucose transporters – GLUT1 in erythrocytes and the blood–brain barrier, insulin-dependent GLUT4 in skeletal muscle and adipose tissue.
Osmosis
Osmosis is the passive movement of water across a semipermeable membrane toward the side with the higher concentration of impermeant solutes. In an isotonic solution (such as 0.9% NaCl) cells keep their volume; in a hypotonic solution they swell and may burst (hemolysis of erythrocytes); in a hypertonic solution they shrink (crenation).