A membrane is two layers of oil, five nanometres thick. Everything a cell is depends on what it lets across — and what it makes work to cross.
Drop phospholipids in water and they have no choice: heads out, tails in. The bilayer is not built — it happens.
A phosphate head is hydrophilic; two fatty-acid tails are hydrophobic. Water pushes the tails together and pulls the heads outward — a bilayer is the lowest-energy way to hide every tail.
The tails make a 5 nm layer of oil. Ions and polar molecules can't dissolve in it, so they can't cross on their own. Small non-polar molecules slip through freely.
Fluid: phospholipids and proteins drift sideways. Mosaic: proteins are scattered through the bilayer, not layered on top of it. Singer & Nicolson, 1972.
Self-assembly. Too few lipids and they sit at the surface. More, and they hide their tails in a ball. But a ball has no inside for water — the only arrangement that shields every tail and encloses a watery space is two layers back to back. That is a cell's membrane, and it forms by itself.
Tap a component. The cell is on the bottom; the outside world, with its sugar coat, is on top.
Tap the phospholipid heads, tails, integral proteins (channel, carrier), the peripheral protein, glycolipid, glycoprotein or cholesterol.
Size and charge decide. Small and non-polar walks in; anything charged or bulky needs a protein — and the cell decides which proteins to install.
Selectivity is a protein list. The bilayer is the same everywhere; what differs between a neuron and a gut cell is which channels, carriers and pumps sit in it. Water crosses slowly on its own but fast through aquaporins — a cell can tune its water permeability by adding or removing them.
Pick a substance and watch it try to cross. Set the gradient, then see which routes move it, which way, and what it costs.
Reading it. Simple and facilitated diffusion both stop at equilibrium — they can only run down the gradient, the protein just opens a door. The pump keeps going after equilibrium and builds a gradient the wrong way, and the ATP counter is the bill.
Two are passive and free, one is passive but needs a door, one costs ATP. Learn them by what limits their rate.
Net movement down a gradient through the bilayer itself. Rate rises with the gradient, temperature, and the surface area; falls with distance and particle size.
Diffusion of water across a membrane, from the less concentrated to the more concentrated solution. Aquaporins are channels that make it fast. Full treatment in D2.3.
Down a gradient but through a channel protein — a hydrophilic pore, often specific to one ion, sometimes gated. No ATP. Rate saturates when every channel is busy.
Against a gradient, by a pump protein that changes shape when ATP is hydrolysed. Specific, controllable, and how a cell keeps its inside different from its outside.
Same graph, three fingerprints. A straight line means the bilayer alone. A plateau means a finite number of proteins. A flat line that ignores the gradient — and stops when respiration is poisoned — means pumps.
Tails that pack tightly make a stiff membrane; kinks and warmth loosen it. Cholesterol pushes both ways. Turn the dials.
A C=C double bond bends the tail. Kinked tails can't stack, so the bilayer stays fluid in the cold — fish and winter plants swap in unsaturated fatty acids.
Wedged between tails: at high temperature it restrains movement (less fluid); at low temperature it stops tails packing (more fluid). Animal cells only — plants use other sterols.
Because the bilayer is fluid, it can break and reseal. Bulk cargo rides in a bubble of membrane — energy is spent, but no one crosses the oil.
Phagocytosis (solids: a phagocyte eating a bacterium) and pinocytosis (fluid). The vesicle's inside was the outside a second ago.
Neurotransmitter release, saliva, insulin, digestive enzymes. The vesicle's membrane becomes plasma membrane — which is how a cell grows its surface.
Uses ATP for the cytoskeleton and membrane reshaping, but nothing moves against a gradient through a protein. Call it an active process, not active transport.
Three sodium out, two potassium in, one ATP. Every animal cell runs it, and neurons run it hardest. Step through one cycle.
Two substances, opposite directions, one protein. The pump is an antiporter powered directly by ATP — primary active transport. Net export of positive charge: the inside ends up negative.
Ligand-gated: the nicotinic acetylcholine receptor opens when ACh binds, letting Na⁺ in. Voltage-gated: K⁺ channels open when the membrane depolarises. Facilitated diffusion, switched on and off.
The pump builds the gradients; the gated channels spend them in milliseconds to make a nerve impulse. Pump = battery, channels = switch.
Once a gradient exists it is stored energy. Cotransporters cash it in to move something else uphill.
Indirect, because the ATP is spent elsewhere. The cotransporter never touches ATP; it uses the sodium gradient the pump built. Phloem loading in plants runs the same trick with a proton gradient.
One carrier moves Na⁺ down and glucose up in the same direction. Kidney proximal tubule and gut epithelium — the cells with the microvilli.
Membrane glycoproteins — cadherins, integrins, selectins — that bind neighbouring cells or the extracellular matrix. Different CAMs for different tissues; that's how cells stay sorted.
1935: protein sheets sandwiching lipid. Overturned by freeze-fracture (bumps in the middle), protein biochemistry (globular, hydrophobic patches) and antibody tagging (proteins mix within 40 min of fusing two cells).
Tap a node to light up its links. The pink dashed link is why one membrane can be a barrier and a gate at once.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
Single best answer. You get the reasoning as soon as you commit. HL items are marked.