Endocytosis and exocytosis are cellular processes that transport materials into and out of a cell, respectively. Both are examples of bulk transport, meaning they move large molecules or many small molecules en masse through the cell membrane. Both are forms of active transport, meaning they require energy, usually in the form of ATP.
Importance of Bulk Transport
Bulk transport is crucial for cells as it enables the intake of materials that are too large to pass through the cell membrane by passive diffusion, such as nutrients and signaling molecules, and the expulsion of waste products and molecules destined for other sites within the organism.
How Vesicles Work
In both endocytosis and exocytosis, vesicles – small, membrane-bound sacs within the cell – play a key role. They package and transport substances within the cell and facilitate their passage across the cell membrane. One of the main reasons the cell needs vesicles is because large polar molecules cannot pass through the hydrophobic part of the cell membrane.
Energy Requirement and Active Transport
Both endocytosis and exocytosis require energy and are considered forms of active transport. The energy is typically derived from adenosine triphosphate (ATP), a molecule that stores and transfers energy within cells.
Endocytosis
Definition
Endocytosis is the process by which cells engulf external materials, bringing them into the cell within a vesicle. Russian zoologist Élie Metchnikoff discovered the process in 1882, while Belgian scientist De Duve coined the term “endocytosis” in 1963.
Examples of Materials Transported
- Nutrients (e.g., iron)
- Signaling molecules
- Foreign particles (e.g. bacteria, viruses)
Functions in the Cell
- Nutrient uptake
- Immune response (ingestion of pathogens)
- Cell signaling
Types of Endocytosis
- Phagocytosis: Cellular “eating,” where large particles are engulfed.
- Pinocytosis: Cellular “drinking,” where the cell ingests extracellular fluid and its dissolved solutes.
- Receptor-Mediated Endocytosis: Specific molecules are ingested into the cell after they bind to receptors on the cell’s surface.
Steps of Endocytosis
- The cell membrane surrounds the material.
- The membrane forms a vesicle containing the ingested material.
- The vesicle breaks away from the membrane and moves into the cytoplasm.
- The vesicle may fuse with other organelles (e.g., a lysosome) for further processing.
Usage in the Body
- White blood cells use phagocytosis to ingest and destroy pathogens.
- Cells uptake cholesterol via receptor-mediated endocytosis.
- Plant cells intake fertilizer via pinocytosis.
- Cells intake water and dissolved extracellular nutrients using pinocytosis.
Exocytosis
Definition
Exocytosis is the process by which cells expel materials, enclosed in a vesicle, out of the cell. Belgian cytologist and biochemist De Duve coined the term in 1963.
Examples of Materials Transported
- Digestive enzymes
- Hormones
- Waste products
Functions in the Cell
- Secretion of substances
- Waste elimination
- Inserting membrane proteins and lipids into the cell membrane
Types of Exocytosis
In eukaryotic cells, there are two main types of exocytosis: regulated exocytosis and constitutive (non-regulated) exocytosis. Regulated exocytosis is a selective and stimulus-dependent (Ca2+ ion) process mainly involved in the release of signaling molecules. Constitutive exocytosis is a continuous and non-selective process that delivers materials to the cell membrane or extracellular space for cell growth, repair, and other functions.
- Regulated Exocytosis: Cells store bioactive substances in secretory vesicles and release them in response to specific signals or stimuli. Typically calcium ions (Ca2+) act as the signal.
- Constitutive or Non-Regulated Exocytosis: A continuous process that delivers newly synthesized lipids, proteins, and other materials from the Golgi apparatus to the cell membrane, or releases them to the extracellular space.
Steps of Exocytosis
- A vesicle containing the material to be expelled moves toward the cell membrane.
- The vesicle membrane fuses with the cell membrane.
- The contents of the vesicle are expelled outside the cell.
Usage in the Body
- Cells use the process to release waste and toxins.
- Pancreatic cells secrete digestive enzymes via exocytosis.
- Neurons release neurotransmitters via exocytosis.
- Macrophages release waste material after eliminating pathogens.
Endocytosis vs Exocytosis Summary Table
Here is a summary of the key points regarding endocytosis and exocytosis:
| Feature | Endocytosis | Exocytosis |
|---|---|---|
| Definition | Uptake of materials into the cell via vesicles | Expulsion of materials from the cell via vesicles |
| Functions | Nutrient uptake, immune response, cell signaling | Secretion, waste elimination |
| Types | Phagocytosis, pinocytosis, receptor-mediated endocytosis | Regulated (Ca2+) and non-regulated (non-Ca2+) |
| Examples | Uptake of cholesterol, ingestion of pathogens | Secretion of digestive enzymes, release of hormones, inserting molecules into the cell membrane |
References
- Falcone, S.; Cocucci, E.; et al. (2006). “Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events”. Journal of Cell Science. 119 (22): 4758–4769. doi:10.1242/jcs.03238
- McMahon, H.T.; Boucrot, E. (2011). “Molecular mechanism and physiological functions of clathrin-mediated endocytosis”. Nature Reviews. Molecular Cell Biology. 12 (8): 517–33. doi:10.1038/nrm3151
- Michaelis A.; Green, M.M.; Rieger, R. (1991). Glossary of Genetics: Classical and Molecular (5th ed.). Berlin: Springer-Verlag. ISBN 978-3-642-75333-6.
- Pang, Zhiping P; Südhof, Thomas C (August 2010). “Cell biology of Ca2+-triggered exocytosis”. Current Opinion in Cell Biology. 22 (4): 496–505. doi:10.1016/j.ceb.2010.05.001
- Shin, O. H. (2014). “Exocytosis and synaptic vesicle function.” Comprehensive Physiology. (1):149-75. Wiley. doi:10.1002/cphy.c130021