Crossing the cell membrane.

A fluid-mosaic bilayer with drifting proteins: small non-polar molecules diffuse, polar ones need carriers, and the Na+/K+ pump spends ATP to push ions uphill.

Move Na+ out of the cell against a 10:1 gradient, then show what stops when ATP is cut.

All transport across the membrane needs energy.

• 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.

Teaching this idea? Put the simulation on your own page or LMS with this code.