A level Biology B · Edexcel 9BI0 · Topic 4B
Cell membranes and transport
Key points
- The fluid mosaic model: a phospholipid bilayer with proteins scattered through it, plus cholesterol, glycoproteins and glycolipids.
- Diffusion: net movement down a concentration gradient. Facilitated diffusion uses channel or carrier proteins for polar or charged particles; both are passive.
- Osmosis: net movement of water from higher (less negative) to lower water potential across a partially permeable membrane. Pure water has .
- Active transport uses ATP and carrier proteins to move substances against a concentration gradient.
- Endocytosis and exocytosis move large amounts of material in vesicles, also using ATP.
- Membrane permeability increases at high temperature (proteins denature) or with solvents such as ethanol, shown with beetroot pigment leaking out.
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The cell surface membrane controls what enters and leaves the cell. Its structure explains why some molecules pass freely, why others need proteins to help them, and why some transport costs energy.
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