The Golgi apparatus

The Golgi apparatus

It contains multiple classes of cisternae that differ in structure, composition, and function, but there is no consensus about the number and the definition of these classes. A useful way to classify Golgi cisternae is according to the trafficking the pathways by which the cisternae import and export components. By this criterion, we propose that Golgi cisternae can be divided into the three classes that correspond to functional stages of maturation. First, cisternae at the cisternal assembly the stage receive the COPII vesicles from the ER and recycle components to the ER in COPI vesicles. At this stage, the new cisternae are generated. Second, cisternae at the carbohydrate synthesis stage exchange material with the one another via COPI vesicles. At this stage, the most of the glycosylation and polysaccharide synthesis reactions occur. Third, cisternae at the carrier formation stage produce the clathrin-coated vesicles and exchange material with endosomes. At this stage, the biosynthetic cargo proteins are packaged into various transport carriers, and the cisternae ultimately disassemble. The discrete transitions occur as a cisterna matures from one stage to the next. Within each stage, the structure and composition of a cistern can evolve, but the trafficking pathways remain unchanged. This model offers the unified framework for understanding the properties of the Golgi in the diverse organisms. The Golgi apparatus is the most elaborate organelles in the cell. It consists of flattened membrane sacs called the cisternae, which are usually but not always organized into the polarized stacks. Depending on the organism and cell type, the Golgi stack contain as few as three or as many as 20 cisternae. Most organisms have individual Golgi stacks, but in the vertebrate cells, the Golgi stacks are linked by the lateral connections to form a continuous ribbon. Despite of this variable morphology, the Golgi operates by the conserved principles in diverse eukaryotes. Within a Golgi stack, the cisternae differ in the structure, composition, and function. Biosynthetic cargo proteins enter the Golgi at the cis face of the stack and depart from the trans face. During passage through the stack, biosynthetic cargo proteins undergo glycan remodeling and other modifications. Complex polysaccharides are also synthesized within the Golgi. The trans-most cisternae are designated the trans-Golgi network and are responsible for packaging biosynthetic cargo proteins and polysaccharides into the transport carriers for the delivery to downstream destinations. However, no consensus yet exists about the number of the such classes or the molecular events that define them. We suggest that the most useful way to classify Golgi cisternae is to consider the trafficking pathways. According to this view, cisternae in a given class employ the same mechanisms for importing and exporting the components, and these trafficking pathways reflect conserved core functions. This approach will enable us to integrate experimental findings of Golgi organization in a variety of organisms.

Golgi organization and models for Golgi traffic

The properties of the Golgi cisternae are linked to the pathways of Golgi membrane traffic, a topic that is vigorously debated. Currently, the predominant model is the cisternal maturation, which postulates that Golgi cisternae form de novo and then progressively mature into the TGN cisternae. In the simplest version of the maturation model, a TGN cisterna is merely an older version of the cis-cisterna, and the Golgi can be viewed as the set of cisternae on a maturation continuum. A more nuanced view is that the maturation occurs in the several discrete steps, with the different classes of Golgi cisternae representing successive stages of maturation. The maturation model can accommodate most of the experimental data from a variety of cell types. According to this model, the resident Golgi proteins recycle from the older to younger cisternae, thereby staying within the organelle while the biosynthetic cargo moves forward. Still uncertain is the mechanism by which resident Golgi proteins recycle. The one possibility is that Golgi membrane proteins recycle in COPI vesicles, which bud from Golgi cisternae. There is good evidence that the COPI vesicles contain at least some Golgi membrane proteins, but conflicting data have been obtained about the presence of the glycosylation enzymes in COPI vesicles. In addition to COPI vesicles, transient tubular connections between the cisternae have been postulated to allow the recycling of Golgi membrane proteins Alternative models for Golgi traffic views the cisternae in a different light. The rapid partitioning model proposes that the Golgi is the continuous structure, with the glycosylation and export occurring at all levels of the stack. This model treats the Golgi as the single intermixed compartment and makes no distinction between the various cisternae. The cisternal progenitor and rim progression models propose that the Golgi cisternae are long-lived structures and that large portions of the cisternae undergo the fission and subsequent fusion to carry the biosynthetic cargo forward while resident Golgi enzymes remain in the static portions of the cisternae.

Functions

• Packaging and storage of materials.
• Cells plate formation
• Synthesized some special carbohydrates like sialic acid and galactose found in the plasma membrane.
• Formation of acrosome in sperms during spermatogenesis.

References

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Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.