The fluid mosaic membrane (HL)

The fluid mosaic model (Singer & Nicolson) replaced the older Davson-Danielli sandwich model. Proteins are embedded within and across a fluid phospholipid bilayer, not merely coating it.

• Integral proteins span the bilayer (channels, carriers, pumps).
• Cholesterol regulates fluidity; glycoproteins and glycolipids handle recognition and signalling.

Membrane transport

Passive (no ATP): simple diffusion, facilitated diffusion, osmosis — all down a gradient.

Active (needs ATP): protein pumps move substances against the gradient (e.g. the sodium-potassium pump); endocytosis and exocytosis move bulk cargo in vesicles.

Your turn — calculate

Organelles and compartmentalisation (HL)

Membrane-bound organelles let a cell run many processes at once. Benefits of compartmentalisation include:

• keeping enzymes and substrates concentrated together;
• separating incompatible reactions (e.g. digestion inside lysosomes);
• maintaining different local conditions (pH, ions).

Surface area for reactions is boosted by folded membranes (cristae, thylakoids).

Your turn — calculate

Sort each item into its group

Sort each item by transport type or organelle function.

Cell specialisation and stem cells

Cells differentiate by expressing only some genes. Stem cells vary in potency:

• Totipotent: can form any cell type, including placenta.
• Pluripotent: can form all body cell types (not placenta).
• Multipotent: limited to a related family of cells.

Stem cells live in niches that maintain them until repair or growth is needed.

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Match each item to its partner

Match each membrane component to its role.

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The big ideas to take away

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