Explain in detail the structural organisation and functions of Golgi-Complex.

Structural Organization of the Golgi Complex

The Golgi complex is a dynamic organelle characterized by a distinct polarized structure. It is composed of a series of flattened, membrane-bound sacs called cisternae, arranged in stacks. These stacks are typically 3-8 cisternae in number. The Golgi is divided into three functionally distinct regions:

• Cis-Golgi Network (CGN): This is the entry face of the Golgi, located closest to the ER. It receives transport vesicles from the ER containing newly synthesized proteins and lipids.
• Medial-Golgi: This is the central region where much of the processing and modification of proteins and lipids occurs.
• Trans-Golgi Network (TGN): This is the exit face of the Golgi, responsible for packaging modified proteins and lipids into vesicles for transport to their final destinations.

Components of the Golgi Complex

Besides cisternae, the Golgi complex contains several other important components:

• Golgi Matrix: A network of proteins that provides structural support and helps maintain the shape of the Golgi.
• Vesicles: Small, membrane-bound sacs that bud off from the cisternae and transport molecules to other parts of the cell.
• Golgi Enzymes: A variety of enzymes involved in the modification and processing of proteins and lipids.

Cisternal Maturation Model

The prevailing model explaining the movement of molecules through the Golgi is the cisternal maturation model. This model proposes that the cisternae themselves are dynamic structures that mature and move through the Golgi stack. New cisternae are continuously formed at the cis face by the fusion of ER-derived vesicles. As a cisterna matures, it moves towards the trans face, undergoing changes in its enzyme composition and function. Eventually, the trans cisternae disassemble into vesicles that bud off from the TGN.

Functions of the Golgi Complex

1. Protein Processing and Modification

The Golgi complex plays a crucial role in modifying proteins synthesized in the ER. These modifications include:

• Glycosylation: The addition of carbohydrate chains to proteins, forming glycoproteins. This process is essential for protein folding, stability, and targeting.
• Phosphorylation: The addition of phosphate groups to proteins, which can regulate their activity.
• Sulfation: The addition of sulfate groups to proteins.

2. Lipid Metabolism

The Golgi complex is involved in the synthesis and modification of lipids, including sphingolipids and glycolipids. These lipids are important components of cell membranes.

3. Packaging and Sorting

The TGN sorts and packages modified proteins and lipids into vesicles for transport to their final destinations. These destinations include:

• Plasma Membrane: For proteins destined to be secreted or embedded in the cell membrane.
• Lysosomes: For proteins destined to become lysosomal enzymes.
• Other Organelles: For proteins destined to function in other organelles.

4. Lysosome Formation

The Golgi complex is involved in the formation of lysosomes, which are organelles responsible for degrading cellular waste products. Lysosomal enzymes are synthesized in the ER, modified in the Golgi, and packaged into vesicles that become lysosomes.

Golgi and Disease

Dysfunction of the Golgi complex has been implicated in a number of diseases, including:

• Congenital Disorders of Glycosylation (CDGs): These are genetic disorders caused by defects in glycosylation pathways, often affecting Golgi function.
• Neurodegenerative Diseases: Abnormal Golgi morphology and function have been observed in neurodegenerative diseases such as Alzheimer's and Parkinson's disease.