Revision sheet
The plasma membrane and how things cross it
Why the membrane holds together at all, and the four ways a molecule can get through it.
A fluid mosaic, held together by water
5 nm thick. Nothing bonds the phospholipids to each other: they hold together because their fatty-acid tails are hydrophobic and water drives them inwards, away from itself, while the phosphate heads stay in contact with the water on both faces. Break the membrane and it reseals for the same reason.
Proteins float in that bilayer — integral proteins crossing it, peripheral proteins attached to one face — and cholesterol wedges between the phospholipids, keeping the membrane fluid at low temperature and stiffer at high.
Sugars attached to the outer face form the glycocalyx, the cell's identity card, which is where the immunology unit will pick the story up again.
- The bilayer is held by the hydrophobic effect, not by chemical bonds between lipids.
- Cholesterol buffers fluidity in both directions.
- Membrane asymmetry is absolute: sugars are always on the outer face.
Passive transport: down the gradient, free of charge
Small non-polar molecules — O₂, CO₂, steroids — dissolve straight through the lipid and cross by simple diffusion, always down their concentration gradient and without any protein.
Water follows its own gradient by osmosis, moving towards the more concentrated solution, greatly accelerated by aquaporins.
Ions and polar molecules cannot cross the lipid core and need a protein: a channel that opens as a pore, or a carrier that changes shape around the solute. This is facilitated diffusion — still passive, still down the gradient, but saturable, because there is a finite number of proteins.
- Simple diffusion: no protein, no ATP.
- Facilitated diffusion: protein, no ATP, saturable.
- Osmosis moves water towards the more concentrated compartment.
Active transport: paying to go uphill
Active transport moves a solute against its gradient and therefore costs energy.
Primary active transport hydrolyses ATP directly: the Na⁺/K⁺-ATPase pushes 3 Na⁺ out and 2 K⁺ in per ATP, which is what keeps the cell's interior electrically negative and provides the gradient the whole of neurophysiology depends on.
Secondary active transport spends no ATP itself; it lets Na⁺ fall back down the gradient the pump built and couples that fall to another solute travelling uphill — glucose into the intestinal cell, for example. The ATP was still paid, only one step upstream.
- Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in, 1 ATP — electrogenic.
- Secondary transport is uphill for one solute, downhill for Na⁺.
Bulk transport
What is too large for any protein moves by membrane instead. Endocytosis invaginates the membrane and pinches off a vesicle: phagocytosis for particles and whole cells, pinocytosis for extracellular fluid, and receptor-mediated endocytosis for a specific ligand captured in a clathrin-coated pit.
Exocytosis is the reverse: a secretory vesicle fuses with the plasma membrane and empties outward. Both cost ATP, and both explain why a cell that secretes heavily has to recycle membrane constantly.