2.4.5Inquiry Process
Vesicle Formation — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 59 of 160
Section 1 of 6What Vesicles Are
Vesicles link the compartments of the endomembrane system.
Definition
- A vesicle is a small, spherical, membrane-bound sac that carries substances through the cell.
Origin
- Vesicles bud from the endoplasmic reticulum, the Golgi apparatus and the plasma membrane.
Cargo
- They transport proteins, lipids, enzymes or neurotransmitters, enclosed away from the cytoplasm.
Membrane
- The vesicle membrane is a phospholipid bilayer like others in the cell.
| Source | Destination | Cargo |
|---|---|---|
| Rough ER | Golgi | New proteins |
| Golgi | Plasma membrane | Secretory products |
| Golgi | Lysosome | Hydrolytic enzymes |
| Plasma membrane | Cytoplasm | Engulfed material |
Section 2 of 6How a Vesicle Forms
Vesicle formation is an active process.
1
A region of membrane bends outwards, often helped by coat proteins such as clathrin.
2
The curved region grows into a bud containing the cargo.
3
The neck of the bud narrows until the membrane pinches off.
4
The membranes seal, making a closed vesicle with the cargo inside.
5
The vesicle moves along the cytoskeleton to its target using energy from ATP.
Section 3 of 6Fusion
Fusion
Recognition
- Proteins on the vesicle and the target membrane recognise each other, so the cargo reaches the right place.
Membrane merging
- The two bilayers fuse, which needs a fluid membrane.
Cargo release
- Contents enter the lumen of the target compartment or leave the cell.
Membrane recycling
- The vesicle membrane becomes part of the target membrane, which keeps the membrane area balanced.
Section 4 of 6Link to Bulk Transport
Endocytosis
- Plasma membrane forms vesicles that bring in food particles, fluids or receptors.
Exocytosis
- Vesicles carry material to the plasma membrane and release it outside the cell.
Fluidity
- Cholesterol and unsaturated fatty acids maintain fluidity needed for bending and fusion.
Energy
- Both forming and moving vesicles require ATP.
Section 5 of 6Worked Example
Question
- Place these events of exocytosis in order: vesicle fuses with plasma membrane;
- protein modified in Golgi;
- vesicle buds from Golgi;
- contents released outside the cell.
- The Golgi first modifies the protein.
- Next a vesicle buds off the Golgi carrying the protein.
- The vesicle moves to and fuses with the plasma membrane, and then the contents are released.
Answer Protein modified in Golgi, vesicle buds, vesicle fuses, contents released.
Section 6 of 6Exam Checklist & Tips
2.4.5 – Key Points for the Exam
- Vesicle A small membrane-bound sac used to transport materials.
- Budding Formation of a vesicle by outgrowth and pinching off of a membrane.
- Clathrin A protein that forms a coat on vesicles during formation.
- Exocytosis Fusion of a vesicle with the plasma membrane to release contents.
Common Mistakes
- Saying vesicles are made of protein only; they have phospholipid bilayers.
- Thinking vesicle fusion needs no energy.
- Mixing the direction of endocytosis and exocytosis.
- Forgetting that fluid membranes make budding possible.
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