B2.1 Membranes and membrane transportIB Biology HL: Revision notes

Amphipathic phospholipids form continuous bilayers in water. The hydrophobic core has low permeability to ions, polar molecules and large molecules, so it is a barrier. Small non-polar molecules (O₂, CO₂) cross by simple diffusion, which is not selective: it depends only on size and hydrophobic or hydrophilic properties. Osmosis is the net movement of water to a higher solute concentration, speeded by aquaporins.

Integral proteins are embedded in the bilayer; peripheral proteins are attached to a surface. Channel proteins allow facilitated diffusion of specific ions when open; pump proteins use ATP for active transport against a gradient. Together these give selective permeability. Glycoproteins and glycolipids carry carbohydrates on the extracellular side, for cell recognition and adhesion. The fluid mosaic model includes phospholipids, integral and peripheral proteins, glycoproteins and cholesterol.

Unsaturated fatty acids have kinked chains and lower melting points, keeping membranes fluid and flexible at the temperatures cells experience. Saturated fatty acids have higher melting points and make membranes stronger at higher temperatures. Example of adaptation to habitat: fish kept in cold water increase the proportion of unsaturated fatty acids in their membranes.

Cholesterol sits between phospholipids in the hydrophobic region, with its hydroxyl group near the heads. It is a modulator of fluidity: it stabilizes membranes at higher temperatures and prevents stiffening at lower temperatures.

Membrane fluidity lets membranes bend, fuse and pinch off. In endocytosis, the plasma membrane folds inwards to form a vesicle (e.g. phagocytosis of bacteria by white blood cells). In exocytosis, a vesicle fuses with the plasma membrane and releases its contents (e.g. secretion of digestive enzymes or release of neurotransmitters).

Neurotransmitter-gated channels open when a signalling chemical binds: e.g. the nicotinic acetylcholine receptor lets positive ions (mainly Na⁺) into the postsynaptic cell. Voltage-gated channels open in response to changes in membrane potential: sodium and potassium channels in neurons.

The sodium–potassium pump is an exchange transporter: each cycle uses one ATP to pump 3 Na⁺ out and 2 K⁺ in. This is important in generating membrane potentials.

Sodium-dependent glucose cotransporters carry out indirect active transport: Na⁺ diffusing into the cell down the gradient made by the Na⁺/K⁺ pump provides the energy to move glucose in against its gradient. They are important in glucose absorption in the small intestine and glucose reabsorption in the nephron.

Cell-adhesion molecules (CAMs) attach cells to each other to form tissues; different forms of CAM are used for different types of cell–cell junction.