Membrane fluidity lets vesicles bud, move, and fuse during endocytosis, exocytosis, and intracellular transport of materials; These events connect membrane structure to uptake, secretion, and intracellular movement.
Coverage 2013–2023 · Updated 15 Jul 2026
Membrane fluidity allows bilayers to bend, bud, pinch off and fuse, enabling vesicle formation, endocytosis and exocytosis.
Lipids move laterally as proteins and cytoskeletal forces reshape the bilayer. The membrane remains sealed around aqueous cargo, preserving compartmentalization during budding and fusion.
| Process | Membrane event | Example |
|---|---|---|
| Endocytosis | Plasma membrane surrounds material and pinches inward as a vesicle | Uptake of particles or receptor-bound molecules |
| Exocytosis | Internal vesicle fuses with plasma membrane and releases cargo | Secretion of a hormone or neurotransmitter |
A secretory vesicle can fuse with the plasma membrane, add its lipids to the surface and release a water-soluble protein outside the cell.
Vesicle transport is not uncontrolled leakage. Cargo remains enclosed by a continuous bilayer until a regulated budding or fusion event.
This objective is assessed through structured response, commonly using Describe / Identify / State.
Describe / Identify / State / Explain
Build the answer around this relationship: Fluid membranes can change shape to form vesicles.
Naming endocytosis without describing membrane invagination and vesicle formation.
Representative question
Explain how vesicles are used by cells to move materials.
The HL extension asks how membrane structure becomes dynamic cell behaviour. Fatty acid saturation and cholesterol tune fluidity. Fluid membranes form and fuse vesicles. Gated ion channels and sodium-potassium pumps create nerve-cell gradients and electrical responses. Sodium-dependent glucose cotransport uses a sodium gradient to move glucose indirectly against its gradient. Adhesion molecules organize tissues.
