Biology › Membranes and transport across cells

Membranes and transport across cells

Unit B04 · 3 lessons.

This unit is about how a cell controls what crosses its surface. The first lesson builds the membrane, a phospholipid bilayer with proteins set into it. The second covers diffusion and osmosis, and introduces water potential, the quantity to use in place of a concentration of water. The third covers transport that costs the cell energy, from carrier proteins that hydrolyse ATP to endocytosis and exocytosis. Each route in the later lessons depends on a feature of the structure in the first, so take them in order.

Start with lipids and protein structure and function, which give you the phospholipid and the shape of a carrier protein. Water potential comes back in plant transport. Every specification mapped here examines the unit as AS content: AQA in 3.2.3, OCR A in 2.1.5, Cambridge International in topic 4, Pearson Edexcel Biology A in Topic 2 and Biology B in Topic 4.

Notes

Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.

• Membrane structure and the fluid mosaic model
  The fluid mosaic model, why a phospholipid bilayer assembles itself, intrinsic and extrinsic proteins, channel and carrier proteins, cholesterol at…
• Diffusion and osmosis
  Simple and facilitated diffusion, the factors in Fick's law and the relationship between them, osmosis defined by water potential in kilopascals, the…
• Active transport, co-transport and bulk transport
  Active transport against a concentration gradient using carrier proteins and ATP, the sodium-potassium pump and its three-to-two stoichiometry…

Practical work

The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.

• Investigating membrane permeability
• Water relations of plant tissue: osmosis and transpiration
• Non-living models of diffusion, osmosis and surface area