Eukaryotic cells and the cell membrane

Eukaryotes include all protists, plants, animals and fungi. Their cytoplasm is divided into compartments by membrane-bound organelles, and they have a well-organised nucleus with a nuclear envelope.

Plant cells have a cell wall, plastids and a large central vacuole, all absent from animal cells; animal cells have centrioles, which are absent from almost all plant cells.

The membrane's fine structure could only be studied once electron microscopes were available in the 1950s; before that, chemical analysis, chiefly of human red blood cells, showed it is built mainly of lipids and proteins, with carbohydrate also present.

The main lipids are phospholipids arranged in a bilayer with polar heads facing outward and hydrophobic tails facing inward, shielding the non-polar tails from the watery surroundings. Cholesterol is also present.

In the human RBC membrane, protein makes up roughly 52 per cent and lipid roughly 40 per cent; the protein to lipid ratio varies between cell types.

Membrane proteins are classed as peripheral (on the surface) or integral (partly or fully buried in the bilayer).

The fluid mosaic model of Singer and Nicolson (1972) says the lipid layer is quasi-fluid, so proteins can move sideways within the bilayer. This fluidity is needed for cell growth, forming intercellular junctions, secretion, endocytosis and cell division.

The membrane is selectively permeable. Movement without energy along a concentration gradient is passive transport; water moving by diffusion is osmosis. Polar molecules need carrier proteins to cross.

Movement against a concentration gradient uses ATP and is active transport; the Na⁺/K⁺ pump is the standard example.