Cell membranes and transport
Cell membrane structure
The fluid mosaic model: a phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) about 7 nm thick, in which proteins float (fluid) in a scattered pattern (mosaic). Channel and carrier proteins transport polar molecules and ions. Cholesterol sits between phospholipids, regulating fluidity and reducing permeability to water and ions. Glycoproteins and glycolipids act as receptors and antigens for cell recognition. In cell signalling, a hormone binds a complementary receptor on the membrane, triggering a cascade inside (often via a second messenger such as cyclic AMP). Higher temperatures increase permeability (phospholipids move more; proteins denature), shown by pigment leaking from beetroot discs.
≈ 40 minTransport across membranes
Diffusion: net movement down a concentration gradient (passive). Facilitated diffusion: polar molecules and ions through channel or carrier proteins (passive). Osmosis: net movement of water from higher to lower water potential (ψ) across a partially permeable membrane; pure water ψ = 0, solutions negative. Plant cells: ; in concentrated solutions they become plasmolysed; in water, turgid. Animal cells burst (lysis) or shrink (crenation). Active transport moves substances against a gradient using ATP and carrier proteins; co-transport (e.g. Na⁺ and glucose in the ileum) uses a gradient set up by active transport. Endocytosis/exocytosis move bulk material in vesicles. Rate of diffusion increases with surface area, concentration gradient and temperature, and decreases with distance.
≈ 50 min