Endocytosis transports substances into a cell by forming membrane-bound vesicles, whereas exocytosis transports substances out of a cell when vesicles fuse with the plasma membrane. Both are energy-requiring forms of bulk transport enabled by membrane fluidity.
This is Additional Higher Level (AHL) content in B2.1 Membranes and membrane transport.
How the Processes Work
In endocytosis, part of the plasma membrane bends inward, or invaginates, around material outside the cell. The edges of the membrane join, and a vesicle containing the material pinches off into the cytoplasm.
For example, a phagocyte can engulf a bacterium by endocytosis. The resulting vesicle may fuse with a lysosome, allowing hydrolytic enzymes to digest the bacterium.
In exocytosis, a vesicle formed within the cell moves towards the plasma membrane. The vesicle membrane fuses with the plasma membrane, releasing its contents into the extracellular environment. For example, pancreatic cells secrete insulin by exocytosis, and presynaptic neurons release neurotransmitters into the synaptic cleft by exocytosis.
| Feature | Endocytosis | Exocytosis |
|---|---|---|
| Direction of transport | Into the cell | Out of the cell |
| Membrane mechanism | Plasma membrane invaginates and forms a vesicle | Vesicle membrane fuses with the plasma membrane |
| Effect on plasma membrane | Removes membrane from the cell surface | Adds membrane to the cell surface |
| Example | Engulfment of a bacterium by a phagocyte | Secretion of insulin or neurotransmitters |
| Energy requirement | Requires energy | Requires energy |
A common misconception is that these processes move substances directly through the phospholipid bilayer. They do not: materials remain enclosed by a membrane during vesicle transport.
Exam Technique
For an explain question, state the direction of transport and describe the membrane mechanism step by step. Use precise terms such as invagination, vesicle formation, membrane fusion and energy requirement; simply writing “endocytosis enters and exocytosis exits” may not earn all available marks.
