A two-molecule-thick layer of fat keeps the inside in. How membranes work — and how cells get past them.
Every sub-topic below feeds at least one of these questions.
How do molecules of lipid and protein assemble into biological membranes?
What determines whether a substance can pass through a biological membrane?
The required syllabus content for B2.1, in order. Each card is one lesson-sized checkpoint.
Phospholipids and other amphipathic lipids naturally form continuous sheet-like bilayers in water.
Lipid bilayers as barriers
Simple diffusion across membranes
Integral and peripheral proteins in membranes
Movement of water molecules across membranes by osmosis and the role of aquaporins
Pump proteins for active transport
Selectivity in membrane permeability
Structure and function of glycoproteins and glycolipids
Fluid mosaic model of membrane structure
Phospholipids spontaneously form bilayers in water. The same self-assembly underlies every cellular membrane on Earth.
A phospholipid is amphipathic: hydrophilic phosphate head, two hydrophobic fatty acid tails. Drop them in water and the molecules self-arrange — heads outward in contact with water, tails inward huddled together. The result is a bilayer about 5 nm thick.
Phosphate head = circle. Two fatty acid tails = two lines extending from it. Bilayer = two rows of phospholipids facing each other tail-to-tail. Tails meet in the middle.
The hydrophobic core of the bilayer is impermeable to large molecules and hydrophilic particles — which makes the membrane a selective barrier.
The fatty acid tails in the middle of the bilayer are hydrophobic. They don't dissolve charged ions or large polar molecules. So only small, uncharged, hydrophobic particles can pass directly through the bilayer:
This selective barrier is what allows cells to maintain internal conditions different from their environment — to do chemistry at all.
For molecules that can cross the bilayer, movement happens passively — kinetic energy drives them from high to low concentration.
Kinetic theory: particles are always in motion, moving in random directions. Where there are more particles, more move outward than inward (statistically). Net movement: high → low concentration. No cellular energy required — hence "passive".
Canonical examples:
Membrane proteins do most of the membrane's specific work — transport, signalling, recognition, anchoring. Two categories based on how they're attached.
Permanently part of the membrane. Have a hydrophobic central region (matching the fatty acid tails) and hydrophilic regions at the surfaces. Often transmembrane — spanning the entire bilayer. Examples: channel proteins, pumps, transmembrane glycoproteins.
Temporarily attached to one side of the membrane by electrostatic interactions (with phosphate heads or integral protein surfaces). Don't penetrate the bilayer. Hydrophilic on all sides. Examples: receptor proteins, enzymes.
Osmosis is the passive movement of water across a semipermeable membrane from a region of low solute concentration to high. Water gets where the solutes can't.
Water is small and (barely) polar — it can cross the bilayer directly, but very slowly. Solutes can't cross the bilayer. So if solute concentrations differ on the two sides, water (but not solute) redistributes — diluting the high-solute side and concentrating the low-solute side. Net movement: low solute → high solute.
Direct osmosis through the bilayer is too slow for many tissues' needs. Aquaporins are integral channel proteins that allow water — and only water — to pass through rapidly. Osmosis through aquaporins is a form of facilitated diffusion. Essential in kidney tubules, plant root cells, red blood cells.
Charged or polar particles cross membranes via channel proteins — each protein specific for a particular ion or molecule. Selectively permeable membrane achieved.
A channel protein is a transmembrane protein with a central pore. The pore is lined with hydrophilic R-groups, suited to letting specific charged particles pass through. Channels are passive — they don't add energy — but they are selective.
Many channels are gated: they open and close in response to a signal. Voltage-gated sodium and potassium channels respond to changes in membrane potential and drive nerve impulses (covered in detail in C2.2).
Cells often need to move substances against their concentration gradient. That requires a protein pump and ATP energy. Five-step mechanism.
Three transport modes, three levels of selectivity.
Anything small and hydrophobic enough to cross the bilayer can. No protein involvement, no specificity.
Channel/carrier proteins are specific to particular ions/molecules. Only what fits the channel can pass.
Each pump moves a specific substance, and uses ATP to drive movement against the gradient.
Membrane proteins and phospholipids with attached carbohydrate chains face the cell exterior. The carbohydrates serve as recognition signals — for hormones, neurotransmitters, the immune system, and neighbouring cells.
Glycoproteins = protein + carbohydrate chain. Glycolipids = phospholipid + carbohydrate chain. Both have the carbohydrate facing the extracellular side. Together they form the glycocalyx — the cell's outer carbohydrate coat.
Singer and Nicolson (1972). The current model of membrane structure: a fluid phospholipid bilayer with proteins distributed throughout like a mosaic of tiles.
You should be able to draw a 2D fluid mosaic membrane showing:
An extra 7 sub-topics for HL — same syllabus, deeper mechanism.
Channel proteins for facilitated diffusion
Relationships between fatty acid composition of lipid bilayers and their fluidity
Cholesterol and membrane fluidity in animal cells
Gated ion channels in neurons
Sodium–potassium pumps as an example of exchange transporters
Sodium-dependent glucose cotransporters as an example of indirect active transport
Sodium-dependent glucose cotransporters as an example of indirect active transport
The kinds of fatty acids in a membrane's phospholipids control how fluid the membrane is. Different fluidity is needed at different temperatures.
No C=C double bonds → straight fatty acid tails. Phospholipids pack closely together. Higher melting point — membrane is more stable at high temperatures, less fluid at low temperatures.
One or more C=C bonds create kinks. Phospholipids can't pack as tightly. Lower melting point — membrane stays fluid even at low temperatures.
In steelhead trout, as ambient temperature drops, the proportion of unsaturated fatty acids in membrane phospholipids increases. This keeps the membranes fluid at low temperatures (preventing crystallisation). At higher temperatures, more saturated fatty acids increase membrane stability.
Cholesterol sits between the fatty acid tails of phospholipids. It modulates membrane fluidity in both directions — stabilising at high temperatures, preventing stiffening at low.
Cholesterol is amphipathic in a small way: most of the molecule is hydrophobic (sits between fatty acid tails), but the single –OH group on cholesterol is hydrophilic (hydrogen-bonds with the phosphate head of a phospholipid).
Gated ion channels open and close in response to specific signals. The IB names two types in neurons: voltage-gated and ligand-gated.
Open or close in response to changes in the potential difference (voltage) across the membrane. Sodium and potassium voltage-gated channels drive action potentials along neurons. Calcium voltage-gated channels trigger neurotransmitter release at synapses.
Open when a specific ligand (molecule) binds to the channel. Nicotinic acetylcholine receptor at the neuromuscular junction: acetylcholine binds → sodium channel opens → Na⁺ flows into the postsynaptic membrane → depolarisation → action potential.
The Na⁺/K⁺ pump is the classic exchange transporter: it moves Na⁺ out and K⁺ in, against both gradients, using ATP. Without it, no neuron could fire.
Net effect per cycle: 3 Na⁺ out, 2 K⁺ in. One ATP consumed. Maintains the unequal distribution of ions across the neuronal membrane — the basis of the resting potential (~−70 mV).
Cotransporters couple the downhill movement of one substance to the uphill movement of another. The Na⁺/glucose cotransporter is the IB-named example.
In epithelial cells of the small intestine and nephron, glucose is moved against its concentration gradient — but without directly using ATP. Instead:
Hence indirect active transport — ATP is consumed somewhere else (making the Na⁺ gradient), not directly by the glucose pump.
Cell adhesion molecules (CAMs) and an extracellular matrix join cells into tissues. Different junction types serve different mechanical and communication needs.
Form a watertight seal between adjacent cells, preventing leakage between them. Critical in intestinal epithelium and the blood-brain barrier.
Channels connecting the cytoplasm of adjacent cells. Small molecules and ions pass directly. Important for fast cell-cell communication (e.g. cardiac muscle synchronisation).
Protein complexes physically tether adjacent cells together, providing mechanical adhesion.
Especially strong protein-complex junctions providing structural integrity. Critical in tissues subject to stretch — skin, heart muscle.
If you can't define one of these in a sentence, that's where to revise next.
“What processes depend on active transport in biological systems?”
“What are the roles of cell membranes in the interaction of a cell with its environment?”