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

Phospholipids are amphipathic: they have a hydrophilic phosphate head and hydrophobic hydrocarbon tails. In water they naturally form continuous sheet-like bilayers, with heads facing the water on each side and tails facing inwards.

The hydrophobic core has low permeability to large molecules and to hydrophilic particles, including ions and polar molecules, so membranes are effective barriers between two aqueous solutions.

Simple diffusion is the net movement of particles from a higher to a lower concentration, passing between phospholipids. Small non-polar molecules such as oxygen and carbon dioxide diffuse this way.

Osmosis is the net movement of water across a membrane from a lower to a higher solute concentration. Water molecules move randomly in both directions, but the membrane is impermeable to the solutes, so more water moves towards the side with more solute. Aquaporins are channel proteins that greatly increase the rate at which water crosses.

Integral proteins are embedded in one or both lipid layers; peripheral proteins are attached to one surface. Membrane proteins have diverse structures, locations and functions.

Channel proteins allow facilitated diffusion of specific ions down their concentration gradient. They are selective because of their pore, and ions pass only when the channel is open.

Pump proteins carry out active transport: they use energy from ATP to move specific particles, and can move them against a concentration gradient.

Simple diffusion is not selective: it depends only on size and whether particles are hydrophobic or hydrophilic. Facilitated diffusion and active transport make membranes selectively permeable, because each channel or pump transports only specific particles.

Glycoproteins and glycolipids have carbohydrate chains attached to proteins or lipids, always on the extracellular side. They function in cell adhesion and cell recognition (e.g. ABO blood group antigens).

The fluid mosaic model describes a phospholipid bilayer with integral and peripheral proteins, glycoproteins and cholesterol in it. When drawing it, label the hydrophilic heads and hydrophobic tails.