Fluid mosaic membranes
Cambridge A-Level Biology (9700) · Unit 4: Cell membranes and transport · 7 flashcards
Fluid mosaic membranes is topic 4.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 4 — Cell membranes and transport , alongside Movement into and out of cells. In one line: Cholesterol regulates membrane fluidity by preventing the phospholipid fatty acid tails from packing too closely at low temperatures (increasing fluidity) and by stabilizing the membrane at high temperatures (decreasing fluidity).
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 3 definitions and 4 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What role does cholesterol play in the cell surface membrane
Cholesterol regulates membrane fluidity by preventing the phospholipid fatty acid tails from packing too closely at low temperatures (increasing fluidity) and by stabilizing the membrane at high temperatures (decreasing fluidity).
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe membrane structure using the fluid mosaic model, explaining how hydrophobic and hydrophilic interactions cause the phospholipid bilayer to form and determine where proteins sit.
- Describe where cholesterol, glycoproteins and glycolipids are positioned in cell surface membranes.
- Describe what proteins, glycoproteins, phospholipids, glycolipids and cholesterol each do in cell surface membranes, relating them to stability, fluidity and permeability, to transport (via channel and carrier proteins), to cell signalling (via cell surface receptors) and to cell recognition (via cell surface antigens, covered in 11.1.2).
- Outline the main steps of cell signalling that lead to a specific response: cells secrete particular chemicals (ligands), the ligands travel to target cells, and they bind to receptors on the target cells' surface.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Fluid mosaic membranes
- › Apply the 'fluid' part of the fluid mosaic model; proteins and phospholipids move laterally within the membrane layer.
- › Draw each phospholipid with exactly one polar phosphate head and two non-polar fatty acid tails to form the bilayer.
- › Use terms like 'hydrophobic' and 'hydrophilic' interactions to explain the orientation of molecules within the phospholipid bilayer.
- › Remember that every cell surface membrane is a phospholipid bilayer; therefore, a molecule must pass through two phospholipid layers for every membrane it crosses.
Describe the arrangement of phospholipids in the fluid mosaic model and explain the interactions that drive this arrangement.
Phospholipids form a bilayer with hydrophilic phosphate heads facing outwards, interacting with water, and hydrophobic fatty acid tails facing inwards, shielded from water. This arrangement is driven by the hydrophobic effect and hydrophilic interactions, minimizing free energy.
What role does cholesterol play in the cell surface membrane?
Cholesterol regulates membrane fluidity by preventing the phospholipid fatty acid tails from packing too closely at low temperatures (increasing fluidity) and by stabilizing the membrane at high temperatures (decreasing fluidity).
Distinguish between the roles of channel proteins and carrier proteins in membrane transport.
Channel proteins form pores that allow specific molecules to passively diffuse across the membrane, following the concentration gradient. Carrier proteins bind to specific molecules and undergo conformational changes to actively or passively transport them across the membrane.
How do glycoproteins and glycolipids contribute to cell recognition?
Glycoproteins and glycolipids, located on the cell surface, have carbohydrate chains that act as cell surface antigens. These antigens allow cells to identify each other, which is important in immune responses and tissue formation.
Outline the role of a cell surface receptor in cell signalling.
Cell surface receptors bind to specific ligands (
Explain how the fluid mosaic model contributes to the permeability of the cell membrane.
The fluid nature of the phospholipid bilayer allows small, nonpolar molecules (like oxygen and carbon dioxide) to easily diffuse across the membrane. Membrane proteins also contribute to permeability by selectively transporting larger or charged molecules.
Describe how proteins contribute to the stability of the fluid mosaic membrane.
Proteins, specifically integral proteins, can interact with the phospholipid heads and cytoskeleton, providing structural support and maintaining the membrane's integrity. These interactions contribute to overall membrane stability.
Review the material
Read full revision notes on Fluid mosaic membranes — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 4 — Cell membranes and transport
Fluid mosaic membranes sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Fluid mosaic membranes deck
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