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Learn7402Paper 1 · Paper 1: Topics 1–4CellsCell membrane structure
Cells · Subtopic 1 of 6

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.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 40 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the fluid mosaic model.
  • State roles of each membrane component.
  • Describe cell signalling via receptors.
  • Explain effects of temperature and solvents on permeability.
Pause and think

Why can oxygen pass through a membrane easily but glucose cannot?

Section 2 of 14

Core idea

Start hereA border that moves

A cell membrane isn't a wall. It's a moving film of lipids with proteins drifting through it.

The big idea

The phospholipid bilayer blocks polar molecules; proteins let specific substances through and receive signals.

Membrane components

Each part has a role.

Worked exampleHormone specificity

Why does insulin affect liver and muscle cells but not red blood cells?

Goal
Explain.
Strategy
Receptors.
  1. insulin+complementary receptor\text{insulin}+\text{complementary receptor}
    Hormones bind specific receptors.
  2. no receptor⇒no response\text{no receptor}\Rightarrow\text{no response}
    Red cells lack insulin receptors.
AnswerOnly target cells have complementary receptors.
✓ CheckSame idea for any hormone ✓.

Summary

  • Bilayer + proteins.
  • Cholesterol, glycoproteins.
  • Heat increases permeability.
Key words
Fluid mosaic
Model of a membrane with proteins moving in a phospholipid bilayer.
Receptor
Protein that binds a specific signalling molecule.
HistorySinger and Nicolson, 1972. The fluid mosaic model replaced an older idea of a protein–lipid "sandwich" after freeze-fracture images showed proteins embedded in the bilayer.
Fun factMembranes are thin. A membrane is about 7 nm thick: stack 10 000 of them and they'd still be thinner than a sheet of paper.
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