IB Biology HLOrganelles & CompartmentsPaper 1 & 2~10 min read
Vesicle Formation
A vesicle is the cell’s delivery van: a small bag of membrane that carries cargo from one compartment to another. The interesting question is how a flat sheet of membrane becomes a sealed sphere, and the answer is a protein called clathrin.
📚 What you need to know
Vesicles are membrane-bound sacs used for transport and storage.
Peroxisomes contain enzymes that digest fatty acids.
Lysosomes contain lytic enzymes that digest cellular waste and harmful substances.
Transport vesicles move molecules between compartments inside the cell.
Secretory vesicles carry substances out of the cell by exocytosis.
Clathrins are proteins that help form vesicles, lining those that carry molecules between membrane-bound compartments.
The process runs: a clathrin-coated pit forms, receptor proteins bind target molecules, cytoskeleton proteins deepen and seal the pit, and a vesicle is formed.
Vesicles are not vacuoles: vacuoles are larger, and their membrane cannot fuse with other cellular membranes.
Four kinds of vesicle
Lysosomes and peroxisomes are the ones people forget are vesicles at all. They are: membrane-bound sacs holding a set of enzymes.
Clathrin and how a vesicle forms
Clathrins are proteins that help with the formation of vesicles. They assemble into a curved scaffold on the cytoplasmic face of the membrane, and as more of them join, that scaffold pulls the flat membrane into a rounded pit. They line the vesicles that transport molecules between membrane-bound compartments.
Because receptors do the catching, this is selective. A cell takes in the molecules it has receptors for, and leaves the rest outside.
🧩 The four steps in words
A clathrin-coated pit forms on the surface of the cell membrane.
Receptor proteins on the cell surface bind to the target molecules.
Once enough target molecules are attached, cytoskeleton proteins help the clathrin pit to deepen and eventually seal off, trapping the target molecules inside.
A vesicle is now formed, and it can move away through the cytoplasm.
Notice how neatly this joins up with earlier pages. The receptors are membrane proteins. The pinching-off only works because the bilayer is fluid. And what has just happened is endocytosis — this page is the close-up view of a process you already met.
Vesicles are not vacuoles
This distinction is worth getting right, because the words get used loosely in everyday biology.
Feature
Vesicle
Vacuole
Size
Small
Larger
Can its membrane fuse with other membranes?
Yes — this is how it delivers its cargo
No
Typical role
Transport and short-term storage
Storage, and support in plant cells
Example
A secretory vesicle carrying insulin
The large permanent vacuole of a plant cell
The fusion point is the important one. A vesicle’s whole purpose is to merge with a target membrane and hand over what it is carrying — with the Golgi, with a lysosome, or with the plasma membrane during exocytosis. A vacuole cannot do that.
🧠
Clathrin as scaffolding
Think of clathrin as scaffolding poles clipping together on the inside of a flat wall. Each new piece added forces a little more curvature, until the wall has been bent into a ball. Then the scaffolding comes off and is used again.
Worked examples
WE 1
Describing the process
Describe the role of clathrin in the formation of a vesicle at the cell surface membrane. (3 marks)
Point 1: where it sits
Clathrin proteins gather on the cytoplasmic side of the membrane and form a coated pit.
Point 2: what they do
They line the developing vesicle and cause the membrane to curve inwards.
Point 3: what finishes the job
Cytoskeleton proteins then deepen the pit and seal it off, trapping the target molecules inside a completed vesicle.
clathrin shapes the pit; the cytoskeleton closes itbe clear that clathrin is on the inside — it never touches the extracellular side
WE 2
Explaining selectivity
Explain how this process allows a cell to take in a specific molecule from the extracellular fluid while leaving others outside. (3 marks)
Point 1: the receptorsReceptor proteins on the cell surface bind to the target molecule.
Point 2: why that is selective
Each receptor has a binding site of a particular shape, so it is specific to one molecule and will not bind others.
Point 3: the consequence
Only molecules bound to those receptors are trapped when the pit seals, so everything else stays outside.
specific receptors decide what gets into the vesiclespecificity always comes back to complementary shape — same idea as enzymes
WE 3
Predicting the effect of a fault
A mutation prevents a cell from producing functional clathrin. Suggest two effects this would have on the cell. (3 marks)
Effect 1: taking things in
The cell could not form coated pits, so endocytosis of specific molecules would be greatly reduced and it could not take up the substances it needs.
Effect 2: internal transport
Vesicles carrying molecules between compartments, such as from the ER to the Golgi, would not form properly.
The knock-on
Proteins destined for secretion or for lysosomes could not be delivered, so secretion would fail.
no coated pits means no receptor-mediated uptake and no internal delivery“suggest two effects” — give one at the cell surface and one inside, not two versions of the same thing
💡 Exam tips
Define a vesicle as a membrane-bound sac. That phrase covers lysosomes and peroxisomes as well.
Learn the four formation steps in order. Sequence questions are marked on the order, not just the content.
Say clathrin lines the vesicle from the cytoplasmic side.
Give the cytoskeleton its credit for sealing the neck — clathrin alone does not finish the job.
Vesicle membranes fuse; vacuole membranes do not. That is the distinction examiners test.
Link vesicle formation back to membrane fluidity; a rigid bilayer could not do any of this.
⚠ Common mistakes
Saying clathrin is on the outside of the membrane. It coats the cytoplasmic face.
Using vesicle and vacuole interchangeably. They differ in size and in whether the membrane can fuse.
Forgetting the receptors. Without them the process would not be selective, and selectivity is the point.
Calling lysosomes organelles but not vesicles. They are both: a lysosome is a membrane-bound sac of enzymes.
Saying vesicle formation is passive. It requires ATP, like all bulk transport.
Describing peroxisomes as containing lytic enzymes. That is a lysosome; peroxisomes digest fatty acids.
That completes Organelles & Compartments. Read the five pages back as one argument: a membrane creates a compartment, a compartment concentrates and protects what is inside it, two organelles took that idea so far they built compartments within compartments, four organelles pass a protein along a production line, and vesicles are how anything moves between them. Every one of those steps depends on the membrane you met at the start of the previous sub-topic.
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