The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed in the last section 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 interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure 1). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

This illustration shows vesicles fusing with the plasma membrane and releasing their contents to the extracellular fluid.

Figure 1. In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal)

A summary of the cellular transport methods discussed is contained in Table 1, which also includes the energy requirements and materials transported by each.

Table 1. Methods of Transport, Energy Requirements, and Types of Material Transported
Transport Method Active/Passive Material Transported
Diffusion Passive Small-molecular weight material
Osmosis Passive Water
Facilitated transport/diffusion Passive Sodium, potassium, calcium, glucose
Primary active transport Active Sodium, potassium, calcium
Secondary active transport Active Amino acids, lactose
Phagocytosis Active Large macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosis Active Small molecules (liquids/water)
Receptor-mediated endocytosis Active Large quantities of macromolecules
Exocytosis Active Waste materials, proteins for the extracellular matrix, neurotransmitters

In Summary: Exocytosis

Exocytosis in many ways is the reverse process from endocytosis. Here cells expel material through the fusion of vesicles with the plasma membrane and subsequent dumping of their content into the extracellular fluid.