Explain how vesicles are used to transport large molecules across the plasma membrane by exocytosis.

IB DP Biology Standard Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Large molecules are packaged into membrane‑bound vesicles within the cell.
The vesicle travels to the plasma membrane and fuses with it.
Fusion releases the vesicle contents outside the cell.
Exocytosis is an active process that requires ATP for vesicle movement and membrane fusion.

Examiner tips

Use the term ‘vesicle’ and ‘fusion’ to show understanding of the mechanism. Show the sequence: packaging → movement → fusion → release. Mention ATP to justify the active nature of the process.
Include all four points to hit the full 4 marks.

Common mistakes

Confusing exocytosis with endocytosis. Omitting the requirement for ATP. Using vague phrases like ‘cell releases material’ without specifying vesicle fusion.

Mark scheme (4 marks)

Large molecules are packaged into vesicles (membrane-bound sacs) within the cell.
The vesicle moves to and fuses with the plasma membrane.
Fusion releases the contents of the vesicle to the outside of the cell.
Exocytosis requires energy (ATP) as it is an active process.

Key terms in this question

exocytosis · vesicle · plasma membrane