Explain how the fluid mosaic model accounts for the ability of membrane proteins to carry out their functions within the phospholipid bilayer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Phospholipids form a bilayer with hydrophilic heads outward and hydrophobic tails inward, giving a stable yet flexible scaffold.
The bilayer is fluid because phospholipids can move laterally; this fluidity lets proteins diffuse within the membrane and reach the positions needed for their activity.
Membrane proteins are embedded in this mosaic – integral proteins span the bilayer, while peripheral proteins attach to one surface – so they can perform functions such as transport, signalling or catalysis.
Each protein’s tertiary structure is preserved in the lipid environment, allowing it to interact precisely with specific molecules (ligands, substrates) required for its function.
The bilayer is fluid because phospholipids can move laterally; this fluidity lets proteins diffuse within the membrane and reach the positions needed for their activity.
Membrane proteins are embedded in this mosaic – integral proteins span the bilayer, while peripheral proteins attach to one surface – so they can perform functions such as transport, signalling or catalysis.
Each protein’s tertiary structure is preserved in the lipid environment, allowing it to interact precisely with specific molecules (ligands, substrates) required for its function.
Examiner tips
- Use the term ‘fluid mosaic model’ early. Mention lateral movement of lipids. State both integral and peripheral proteins. Highlight maintenance of protein shape for function.
Common mistakes
- Confusing the bilayer arrangement with protein arrangement. Forgetting that peripheral proteins do not span the bilayer. Over‑generalising protein functions without linking to structure.
Mark scheme (4 marks)
- Phospholipids are arranged in a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward, providing a stable but flexible framework
- The 'fluid' nature of the bilayer arises because phospholipids can move laterally, allowing proteins to move within the membrane and adopt functional positions
- Proteins are embedded ('mosaic') in the bilayer either spanning the full width (intrinsic/integral proteins) or attached to one surface (peripheral proteins), enabling diverse functions such as transport, signalling, or enzymatic activity
- The specific tertiary structure / shape of each membrane protein is maintained within the bilayer, allowing precise interactions (e.g. with specific molecules / ligands / substrates) necessary for the protein's function
Key terms in this question
fluid mosaic model · phospholipid bilayer
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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