Membrane Transport

Simple diffusion. Net movement of particles from a region of higher concentration to lower concentration, down their concentration gradient, until equilibrium. Driven by random kinetic motion.

Crosses the bilayer directly: small non-polar molecules — O₂, CO₂, urea, lipid-soluble vitamins.

Facilitated diffusion. Passive (no ATP), but mediated by membrane proteins:

• Channel proteins form a hydrophilic pore (e.g. aquaporins for water; ion channels).
• Carrier proteins bind the molecule and change shape (e.g. GLUT transporters for glucose).

Used for polar molecules and ions too large or charged to cross the bilayer.

Osmosis. Net movement of water across a partially-permeable membrane from a region of higher water potential ($\psi$) to lower water potential.

• Hypotonic solution: lower solute, higher water potential than cell → water moves IN.
• Isotonic solution: equal water potential → no NET movement.
• Hypertonic solution: higher solute, lower water potential than cell → water moves OUT.

In plant cells with a wall: turgid (full), flaccid (limp), plasmolysed (membrane pulls from wall in hypertonic). Animal cells lyse in hypotonic (no wall to resist).

Active transport. Uses ATP to move solutes AGAINST their concentration gradient.

Worked example — sodium-potassium pump (Na⁺/K⁺ ATPase):

1. Three Na⁺ bind to the pump from inside the cell.
2. ATP is hydrolysed; the pump phosphorylates and changes shape.
3. Na⁺ released outside; two K⁺ bind from outside.
4. Dephosphorylation; pump returns to original shape and releases K⁺ inside.

Net: 3 Na⁺ out, 2 K⁺ in, per ATP. Maintains the resting membrane potential of neurons.

Endocytosis. Membrane folds inward, pinching off a vesicle containing extracellular material.

• Phagocytosis — solid particles (white blood cell engulfing a bacterium).
• Pinocytosis — fluid droplets.
• Receptor-mediated — specific molecules bind a receptor and are internalised (e.g. LDL uptake into cells).

Exocytosis. Reverse process. A vesicle from the Golgi fuses with the plasma membrane and releases its contents outside. Examples: neurotransmitter release at synapses; hormone secretion; enzyme export.

Bulk transport (endo/exo) requires energy because membrane reshaping and vesicle fusion use ATP via the cytoskeleton.

Summary table.

Mechanism	Energy	Direction	Examples
Simple diffusion	None	Down gradient	O₂, CO₂
Facilitated diffusion	None	Down gradient	Glucose, Na⁺
Osmosis	None	High → low ψ	Water
Active transport	ATP	Against gradient	Na⁺/K⁺ pump
Endocytosis	ATP	Bulk into cell	LDL, bacteria
Exocytosis	ATP	Bulk out of cell	Hormones