Bulk Transport

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes we discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the plasma membrane's interior. This fusion opens the membranous envelope on the cell's exterior, and the waste material expels into the extracellular space (Figure). Other examples of cells releasing molecules via exocytosis include extracellular matrix protein secretion and neurotransmitter secretion into the synaptic cleft by synaptic vesicles.

This illustration shows vesicles fusing with the plasma membrane and releasing their contents to the extracellular fluid.
In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents then release to the cell's exterior. (credit: modification of work by Mariana Ruiz Villareal)
Methods of Transport, Energy Requirements, and Types of Transported Material
Transport MethodActive/PassiveMaterial Transported
DiffusionPassiveSmall-molecular weight material
OsmosisPassiveWater
Facilitated transport/diffusionPassiveSodium, potassium, calcium, glucose
Primary active transportActiveSodium, potassium, calcium
Secondary active transportActiveAmino acids, lactose
PhagocytosisActiveLarge macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosisActiveSmall molecules (liquids/water)
Receptor-mediated endocytosisActiveLarge quantities of macromolecules