Some cargo is far too big for any channel or pump: a whole bacterium, a drop of fluid, a load of hormone molecules. Cells move it by reshaping the membrane itself, using the vesicles you met in The Endomembrane System.
Key idea
In endocytosis the membrane folds inward around material outside and pinches off as a vesicle, bringing it in. In exocytosis a vesicle inside fuses with the membrane and releases its contents outside. Both use energy.
- Phagocytosis ("cell eating"): the cell wraps around a large particle, as a white blood cell engulfs a bacterium or an amoeba takes in food.
- Pinocytosis ("cell drinking"): the cell takes in small droplets of the fluid around it.
- Receptor-mediated endocytosis: receptors on the surface catch one particular substance first. Liver cells take in cholesterol-carrying particles this way.
- Exocytosis: pancreas cells release insulin and digestive enzymes; nerve cells release the signal molecules that cross to the next cell.
Why it works
The membrane is fluid, so a patch of it can bend, close into a bubble and seal itself without leaking; that is the fluid mosaic model at work. Moving and reshaping the membrane uses the cell's protein machinery, which runs on ATP. Exocytosis also adds membrane to the cell surface and endocytosis removes some, so the two keep each other in balance.
Worked example: In or out?
A white blood cell surrounds a bacterium with arm-like extensions of its membrane and closes them around it. What process is this?
- The material goes into the cell: endocytosis.
- A large particle is engulfed: phagocytosis.
Answer: Endocytosis (phagocytosis)
Worked example: Shipping out
A cell in the lining of the nose releases mucus. How does the mucus leave the cell?
- Mucus is made of large molecules built through the ER and Golgi.
- It is packed into vesicles that move to the membrane.
- The vesicles fuse with the membrane and spill the mucus outside: exocytosis.
Answer: Exocytosis
Common mistake
Mixing up the two directions. Endo means in: the membrane folds inward and a vesicle forms inside. Exo means out: a vesicle already inside joins the membrane and empties outside. Check by asking where the cargo starts.