Lesson 5 of 5 / Membranes and transport
Move bulk material in vesicles
How does a cell take in something too large for a channel?
I can explain endocytosis and exocytosis as energy-requiring membrane processes.
I can explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.Syllabus
Syllabus 8876, Core 1(j). Explain diffusion, osmosis, facilitated diffusion, active transport, endocytosis and exocytosis.
About 5 min
Make a guess. You are not marked.
When a vesicle releases its contents by exocytosis, does it leave a hole in the cell membrane?
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The two membranes fuse.
The two bilayers fuse, so the boundary stays continuous as contents are released.
The key idea
Endocytosis forms inward vesicles from the surface membrane. Exocytosis fuses internal vesicles with the surface membrane to release contents.
Explore the idea
Trace the cargo and the membrane
Before exocytosis: a membrane vesicle carries cargo inside the cell. Energy-dependent fusion will open its lumen to the exterior.
Before/after compartment model. It omits the intermediate membrane-bending and fusion geometry; the arrow shows cargo direction, not a channel.
Explanation
Large particles and macromolecular cargo cannot simply pass through the hydrophobic bilayer or an ordinary transport channel. In endocytosis, the cell surface membrane surrounds external material and pinches off to form an internal vesicle. The cargo enters while remaining enclosed by membrane.
Phagocytosis takes in large particles; pinocytosis takes in extracellular fluid and dissolved material. Receptor-mediated uptake can selectively concentrate cargo. These examples use membrane deformation and cellular energy rather than diffusion through a pore.
In exocytosis, an internal vesicle moves to and fuses with the cell surface membrane. Its lumen opens to the outside and the contents are released. The vesicle membrane becomes part of the cell surface membrane. Protein secretion is a useful example.
These processes transfer material across the cell boundary without forcing that cargo through the bilayer interior. Their energy requirement does not mean the cargo must always be moving against its concentration gradient; bulk transport is a distinct category.
Step by step
- 1
Choose the direction
Endocytosis brings material in; exocytosis releases it out.
- 2
Describe membrane change
Inward budding forms a vesicle; fusion opens a vesicle to the exterior.
- 3
Keep the cargo enclosed
Cargo moves in a membrane compartment rather than through a channel.
Worked example
Secreting a protein
A protein reaches a vesicle near the cell surface. Explain how it leaves without passing through the lipid interior.
One way to explain it
The vesicle membrane fuses with the cell surface membrane. The vesicle lumen becomes continuous with the extracellular space, releasing the protein by exocytosis; the protein does not dissolve through the bilayer.
Why this answer works
- Identify membrane fusion.
- Trace the compartment rather than imagining a pore through the protein.
Watch out for this
A student says: "Exocytosis destroys the vesicle membrane and makes a permanent hole in the cell." What is wrong with this?
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Membranes fuse and reorganise. The cell retains a continuous boundary as contents are released.
Check your understanding
Which process releases a secreted protein from a vesicle?
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Exocytosis.
The vesicle fuses with the surface membrane and opens to the exterior.
Exam-style questions
Check your understanding
Which statement about exocytosis is NOT correct?
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The vesicle contents pass through a channel protein
In exocytosis the vesicle membrane fuses with the cell membrane; contents are not passed through a channel.
Quick recall
How does a cell take in something too large for a channel?