2.1.4Topic 2 · Genes and Health

Cell Membranes — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 35 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision

0%read

Lesson summary

2.1.4 Cell Membranes — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the fluid mosaic model of membrane structure and explain the roles of its components and the functions of membranes in cells.Cell surface membranes and the membranes around organelles share a common stru…

phospholipid bilayer
Compartmentalisation
Surface for reactions
Cell signalling and recognition
endocytosis
exocytosis

Part 1 of 7The fluid mosaic model

Cell surface membranes and the membranes around organelles share a common structure described by the fluid mosaic model.

Phospholipid bilayer

The membrane is about 7 nm thick and is built from a phospholipid bilayer in which proteins are embedded.

It is 'fluid' because phospholipids and many proteins can move sideways within their layer, and 'mosaic' because the proteins are scattered through the bilayer like tiles in a mosaic.

Part 2 of 7Components and their roles

A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails.

Components and their roles

In water the heads face outwards towards the aqueous cytoplasm and tissue fluid, and the tails face inwards, forming a bilayer.

The hydrophobic core is a barrier to water-soluble substances such as ions, glucose and amino acids, but small non-polar molecules (oxygen, carbon dioxide, steroid hormones) pass through.

Component	Role
Phospholipid bilayer	Basic structure; barrier to polar molecules and ions
Channel proteins	Water-filled pores allowing specific ions or molecules through by facilitated diffusion
Carrier proteins	Bind a molecule and change shape to move it across; used in facilitated diffusion and active transport
Cholesterol	Sits between phospholipids, reducing fluidity at high temperature and stopping the tails packing too tightly at low temperature; makes the membrane more stable
Glycoproteins	Cell recognition, receptors for hormones and drugs, cell adhesion
Glycolipids	Cell recognition and stability; act as antigens

Part 3 of 7Functions of membranes

Functions of membranes

Boundary

Boundary of the cell: controls what enters and leaves, keeping a different internal environment.

Compartmentalisation

Compartmentalisation membranes surround organelles such as the nucleus, mitochondria and lysosomes so that reactions with different conditions can occur separately.

Surface for reactions

Surface for reactions enzymes are embedded in the inner mitochondrial membrane, thylakoid membranes and rough endoplasmic reticulum.

Cell signalling and recognition

Cell signalling and recognition receptor proteins bind hormones or neurotransmitters; antigens identify cells.

Transport

Transport membrane proteins carry out facilitated diffusion and active transport; vesicles carry out endocytosis and exocytosis.

Part 4 of 7Factors affecting structure

Temperature changes membrane fluidity.

Factors affecting structure

At high temperatures phospholipids move more, and the bilayer becomes more fluid and leaky; above about 45 °C the proteins denature and gaps appear, so permeability rises sharply.

At very low temperatures phospholipids pack closely and the membrane becomes rigid, and ice crystals may rupture it.

Organic solvents such as ethanol dissolve the lipid bilayer, increasing permeability.

Membranes also differ in the proportion of unsaturated fatty acids: kinks in unsaturated tails stop close packing, keeping membranes fluid in cold conditions.

Part 5 of 7Evidence and links

Electron microscopy shows a membrane as two dark lines (the phosphate heads) with a lighter gap between them. The fluid mosaic model replaced earlier 'sandwich' models once freeze-fracture images showed proteins within the bilayer. The next lessons apply this structure to a practical investigation of permeability and to the movement of substances by diffusion and facilitated diffusion.

Part 6 of 7Transport across the membrane: a preview

The membrane's selective permeability depends on the combination of its parts.

Lipid-soluble molecules diffuse through the bilayer; ions and polar molecules need proteins; and the cell can move substances against a gradient using carrier proteins that couple transport to ATP hydrolysis.

Endocytosis

Large particles are taken in by endocytosis, where a region of the surface membrane folds inwards and pinches off a vesicle, and are released by exocytosis, where a vesicle fuses with the surface membrane.

Both processes use ATP and rely on the membrane's fluidity.

Cell signalling depends on receptor proteins.

Transport across the membrane: a preview

A hormone such as insulin binds to a receptor with a complementary shape on a target cell, causing a change inside the cell, while cells without the receptor do not respond.

Glycoproteins also let the immune system distinguish self from non-self.

Questions about membranes often reward statements that link a structure (for example a protein with a specific shape) to a function (for example specificity of binding).

Remember also that membrane thickness and composition vary between cell types and organelles: the inner mitochondrial membrane has a high protein content because it carries the electron transport chain, whereas myelin is rich in lipid because it insulates. Exam answers that mention a specific membrane and its specific proteins gain more credit than general statements.

Part 7 of 7Exam-style questions

Describe the fluid mosaic model of membrane structure. [4]4 marks
Explain the role of cholesterol in cell membranes. [3]3 marks
Explain why oxygen can cross a membrane without a protein but a sodium ion cannot. [3]3 marks

Key terms

Term	Meaning
Phospholipid	A lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails.
Bilayer	Two layers of phospholipids with tails facing inwards.
Hydrophilic	Attracted to water.
Hydrophobic	Repelled by water.
Glycoprotein	A protein with a carbohydrate chain, used in recognition and receptors.
Cholesterol	A lipid in animal membranes that regulates fluidity.
Channel protein	A membrane protein forming a pore for facilitated diffusion.
Compartmentalisation	Division of the cell into membrane-bound areas with different conditions.

Avoid these mistakes

Saying the membrane is a rigid barrier. It is fluid and dynamic.

Stating that cholesterol makes membranes fluid. It regulates fluidity: it lowers it at high temperature and prevents solidifying in the cold.

Describing phospholipid heads as hydrophobic. Heads are hydrophilic, tails are hydrophobic.

Writing that proteins are only on the surface. Many span the whole bilayer.

Last updated: 18 October 2026