B2.1 Membranes and membrane transport

Every cell is wrapped in a membrane barely ten nanometres thick, yet this delicate film decides what enters and leaves, holds the cell together, and lets it sense its surroundings. The membrane is not a static wall but a restless, self-healing fluid in which proteins drift like boats on a sea of lipid — the famous fluid mosaic model. For B2.1 the central skill is linking the chemical nature of phospholipids to the structure of the bilayer, and then using that structure to predict exactly how each kind of substance crosses. Master that logic and the transport questions, which look varied, all collapse to a few simple rules.

Phospholipids and the fluid mosaic model

A phospholipid has two regions with opposite affinities for water. The phosphate head is charged and hydrophilic, while the two fatty acid tails are non-polar and hydrophobic. A molecule with both properties is described as amphipathic. When surrounded by water, phospholipids spontaneously arrange themselves so the heads face the watery solutions on either side and the tails huddle together away from water, forming a bilayer. This self-assembly happens because it is the lowest-energy arrangement — no enzyme or energy input is needed.

The fluid mosaic model (Singer and Nicolson) describes the result: a two-dimensional fluid of phospholipids in which a mosaic of proteins is embedded. The bilayer is fluid because individual phospholipids are not bonded to one another and can move sideways within their layer. This fluidity lets membranes bend, fuse and self-seal, and allows proteins to move to where they are needed.

Cholesterol sits between the phospholipids in animal membranes and acts as a fluidity buffer: it restrains movement at higher temperatures and prevents tight packing at lower temperatures, keeping the membrane stable across a range of conditions.

Membrane proteins and their functions

Proteins give the membrane most of its functional versatility. They are classified by position. Integral (intrinsic) proteins are embedded in the hydrophobic core, often spanning the whole bilayer (transmembrane); their membrane-buried regions have hydrophobic surfaces that interact with the fatty acid tails. Peripheral (extrinsic) proteins are attached to one surface and have hydrophilic surfaces.

The syllabus expects you to link specific proteins to specific jobs:

• Channel proteins form hydrophilic pores that let specific ions or polar molecules diffuse through.
• Carrier (pump) proteins change shape to move substances, used in facilitated diffusion and active transport.
• Receptors bind signalling molecules such as hormones.
• Enzymes catalyse reactions at the membrane surface.
• Glycoproteins (with carbohydrate chains) act in cell recognition and adhesion.

Passive transport: diffusion and osmosis

Passive transport moves substances down a concentration gradient and requires no ATP — the energy comes from the random kinetic motion of particles.

Simple diffusion is the net movement of particles from a region of higher to lower concentration, directly through the bilayer. Only small, non-polar molecules — such as oxygen and carbon dioxide — cross freely this way, because the hydrophobic core repels charged and large polar particles.

Facilitated diffusion moves polar molecules and ions down their gradient through proteins: channel proteins for ions, carrier proteins for molecules such as glucose. It is still passive, but the rate depends on the number of transport proteins, so it can saturate.

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of lower solute concentration (higher water potential) to higher solute concentration (lower water potential). Water crosses both directly and through channel proteins called aquaporins, which greatly speed the process in cells such as kidney tubules.

Active transport and bulk transport

Active transport moves substances against a concentration gradient, from low to high concentration, and therefore requires energy from ATP. It is carried out by pump proteins that bind the substance, change shape using the energy released by ATP hydrolysis, and release it on the other side. The classic example is the sodium–potassium pump, which exports sodium ions and imports potassium ions against their gradients — essential for nerve impulses and for maintaining cell volume.

Because each pump is specific and limited in number, active transport also saturates, and it stops if the ATP supply is cut (for example by a respiratory poison) — a useful way to tell it apart from passive processes in data questions.

Large quantities of material cross the membrane by bulk transport, which uses membrane fluidity and ATP. In endocytosis the membrane folds inwards and pinches off a vesicle to take material in (phagocytosis for solids, pinocytosis for fluids); in exocytosis a vesicle fuses with the membrane to release contents such as secreted proteins.