Channels, carriers and pumps move individual small solutes across a membrane. Cells also transport membrane components, macromolecules and large particles by moving pieces of membrane itself. This is vesicular transport.

A vesicle is a small membrane-bounded compartment that can bud from one membrane and later fuse with another.

Endocytosis

Endocytosis brings material into the cell by bending the plasma membrane inward and pinching off a vesicle.

Forms include:

• phagocytosis, uptake of large particles by specialized cells;
• pinocytosis, uptake of extracellular fluid and dissolved material;
• receptor-mediated endocytosis, in which membrane receptors bind selected cargo and concentrate it into forming vesicles.

The cargo does not cross the hydrophobic bilayer interior molecule by molecule. It remains enclosed in an aqueous compartment whose surrounding membrane changes location.

Exocytosis

Exocytosis reverses that membrane rearrangement: an intracellular vesicle fuses with the plasma membrane and releases its soluble contents outside the cell. The vesicle membrane becomes part of the plasma membrane.

Cells use exocytosis to secrete proteins and other molecules and to deliver newly synthesized lipids and membrane proteins to the cell surface.

Intracellular trafficking

Vesicles also move cargo between compartments of the endomembrane system. Budding from one organelle and fusion with another must be selective so that cargo reaches the correct destination and organelles preserve distinct compositions.

Vesicular transport therefore requires more than membrane fluidity. Proteins control cargo selection, membrane curvature, vesicle movement, target recognition and membrane fusion.

Membrane orientation is preserved

During budding and fusion, the side of a membrane that faces the cytosol continues to face the cytosol, while the opposite surface continues to face either an organelle lumen or the extracellular space.

This explains how membrane proteins can retain their sided orientation while being moved through the endomembrane system.

Vesicular transport is therefore fundamentally different from transmembrane solute transport: the cargo moves with a membrane-bounded compartment rather than crossing through a molecular pore or carrier.