Lesson 4 of 4 / Membranes and transport
Endocytosis and exocytosis
How does a cell move material too large for a carrier?
I can explain vesicle-mediated bulk transport and membrane continuity.
I can explain surface and internal membrane functions.Syllabus
Syllabus 9477, Core 1(k). Explain surface and internal membrane functions.
I can explain simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.Syllabus
Syllabus 9477, Core 1(l). Explain simple and facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
About 5 min
Make a guess. You are not marked.
A white blood cell engulfs a whole bacterium. Does the bacterium pass through the cell membrane?
Show the answer
The membrane wraps around it into a vesicle.
In phagocytosis the membrane folds around the bacterium and pinches off. It never crosses the bilayer itself.
The key idea
Endocytosis encloses external material in a vesicle; exocytosis fuses a vesicle with the surface membrane to release material.
Explore the idea
Follow the membrane as well as the cargo
External cargo approaches the cell surface. The carbohydrate chain of the marked membrane protein faces outside.
Gold circles are cargo. The purple branch marks a membrane protein's carbohydrate chain. Track which space it faces: membrane budding and fusion preserve orientation. These energy-requiring bulk processes differ from transport through a carrier.
Original schematic. Molecular sizes and distances are illustrative, not to scale.
Explanation
During endocytosis, the cell surface membrane bends inward around material and pinches off as an internal vesicle. Large particles can be engulfed by phagocytosis, while other forms take in fluid or selected receptor-bound molecules. The material crosses into a membrane-bound compartment without passing through the hydrophobic bilayer itself.
During exocytosis, a vesicle moves to and fuses with the cell surface membrane. Its lumen becomes continuous with the extracellular space and its contents are released. The vesicle membrane becomes part of the surface membrane, delivering lipids and membrane proteins as well as secreted products.
These processes require cellular energy and coordinated membrane/cytoskeleton machinery. They differ from a carrier changing conformation to move a small solute across a membrane. A protein secreted by exocytosis need not unfold and squeeze through a glucose transporter.
Vesicle budding and fusion preserve membrane orientation: the vesicle interior corresponds topologically to the extracellular side. This helps explain why carbohydrate chains processed within the endoplasmic reticulum and Golgi body end up on the outer surface of the plasma membrane.
Step by step
- 1
Follow the membrane
Track invagination or fusion rather than drawing a pore.
- 2
Follow the cargo
Keep cargo inside a vesicle until release or processing.
- 3
Account for the boundary
Explain where the vesicle membrane goes.
Worked example
Work through the evidence
After repeated secretion, why need a cell not simply lose all its vesicle membranes?
One way to explain it
Vesicle membranes fuse into the surface membrane during exocytosis; endocytosis and membrane trafficking can retrieve and recycle components.
Why this answer works
- Cargo release and membrane fate are separate observations.
- Cells balance multiple membrane trafficking processes.
Watch out for this
A student says: "Exocytosis pushes a vesicle through an intact plasma membrane." What is wrong with this?
Show the answer
The membranes fuse; the vesicle does not pass bodily through an unchanged bilayer.
Check your understanding
What happens to a secretory vesicle's lumen during exocytosis?
Show the answer
It opens to the extracellular space when membranes fuse.
Cargo can then be released outside.
Quick recall
How does a cell move material too large for a carrier?