Explain how vesicle fusion during exocytosis results in the secretion of proteins from a cell and describe one consequence of this process for the plasma membrane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Secretory vesicles containing proteins move along the cytoskeleton to the plasma membrane and fuse with it. The vesicle membrane merges with the plasma membrane, creating a pore that opens the vesicle lumen to the extracellular space and releases the protein contents. The phospholipid bilayer of the vesicle is added to the plasma membrane, increasing its surface area. Membrane proteins that were in the vesicle membrane become inserted into the plasma membrane, potentially altering its composition or function.
Examiner tips
- Use the word ‘fuse’ to show the merging of membranes; mention the pore opening and protein release. Include the increase in surface area and the insertion of vesicle proteins. Keep the answer concise and use correct terminology such as ‘phospholipid bilayer’ and ‘membrane proteins’.
- Structure the answer in two parts: (1) the fusion and release mechanism, (2) the consequence for the plasma membrane. This matches the mark scheme and ensures all points are covered.
Common mistakes
- Failing to mention that the vesicle membrane merges with the plasma membrane, only describing protein release. Not stating that the vesicle phospholipids are added to the plasma membrane, thus missing the surface area increase. Using vague terms like ‘cell surface changes’ without specifying membrane proteins or composition.
Mark scheme (4 marks)
- Secretory vesicles (containing proteins) move towards and fuse with the plasma membrane
- The vesicle membrane merges with the plasma membrane, opening the vesicle lumen to the extracellular space, releasing the protein contents
- The phospholipid bilayer of the vesicle membrane is added to the plasma membrane, increasing its surface area
- Membrane proteins originally in the vesicle membrane become inserted into the plasma membrane, potentially altering its composition or function
Key terms in this question
exocytosis · vesicle fusion · plasma membrane
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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