What is exocytosis?
The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes 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 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 (see image below). 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.
In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. Image credit: modification of work by Mariana Ruiz Villareal
Summarizing the Methods of Transport Used by Cells
The table below is a summary of the methods of transport used by cells. We have looked at these different methods in the past few lessons. In general, the transport methods include diffusion, osmosis, facilitated transport/diffusion, primary active transport, secondary active transport, phagocytosis, pinocytosis and potocytosis, as well as receptor-mediated endocytosis.
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 |