Describe endocytosis and exocytosis
Membrane vesicles bring large material into cells by endocytosis or release it outside by exocytosis.
Section 4.2 · Movement into and out of cells
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which process moves material into a cell by membrane folding and vesicle formation?
What you will learn
- Distinguish endocytosis from exocytosis by direction and membrane changes
- Explain how vesicles move material that cannot cross through a channel
Endocytosis brings material into a cell
During endocytosis, the cell-surface membrane folds around material outside the cell and pinches off to form an internal vesicle. The material is enclosed inside the cell.
This process can move larger particles or groups of molecules that do not pass directly through a channel or carrier. It involves membrane rearrangement and cellular energy.
Exocytosis releases material outside
During exocytosis, an intracellular vesicle moves to and fuses with the cell-surface membrane. The vesicle contents are released outside the cell, while vesicle membrane becomes part of the cell surface.
The direction is the key distinction: endocytosis moves material into the cell; exocytosis exports material. Do not confuse secretion by exocytosis with diffusion of a small molecule through the bilayer.
Pause and explain
What happens to an intracellular vesicle during exocytosis?
Worked example
A secretory vesicle fuses with the cell-surface membrane and releases its contents. Name the process and state what happens to the membrane.
Show the worked solution
- Fusion and release indicate exocytosis.
- Vesicle membrane joins the cell-surface membrane.
Answer This is exocytosis. The vesicle fuses with the cell-surface membrane, releases its contents outside the cell and contributes membrane to the cell surface.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Distinguish endocytosis from exocytosis.
Give me a hint
Compare direction and membrane action.
Compare my reasoning
- Endocytosis encloses external material and forms a vesicle inside.
- Exocytosis fuses an internal vesicle with the surface and releases contents.
Endocytosis moves material into the cell when the surface membrane folds around it and forms an internal vesicle. Exocytosis moves material out when an internal vesicle fuses with the surface membrane and releases its contents.
Look for these in your work
- Directions contrasted
- Membrane changes included
A cell packages a large secretion in a vesicle. Explain how the contents reach the extracellular space.
Give me a hint
Name the vesicle-membrane event.
Compare my reasoning
- The vesicle moves to the cell surface.
- Fusion releases contents outside by exocytosis.
The vesicle fuses with the cell-surface membrane and releases its contents into the extracellular space by exocytosis.
Look for these in your work
- Vesicle fusion stated
- External release named
Why is vesicle transport suitable for material too large to pass through a membrane channel?
Give me a hint
Contrast a vesicle enclosing bulk material with a channel pore.
Compare my reasoning
- A vesicle can enclose a larger particle or group of molecules.
- The material does not need to pass through a narrow protein pore.
Endocytosis can enclose larger material in a membrane-bound vesicle, rather than requiring the material to pass through a selective channel pore.
Look for these in your work
- Size advantage explained
- Channel contrast made
Common mistakes
- Reversing endocytosis and exocytosis
- Describing vesicle transport as diffusion through a protein channel
- Forgetting membrane fusion during exocytosis
What can you explain now?
Which process moves material into a cell by membrane folding and vesicle formation?
Compare with the explanation
Endocytosis
The membrane encloses external material in an internal vesicle.
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Make it stick
How do the direction and membrane changes distinguish endocytosis from exocytosis?
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