In 1897, Italian biologist Camillo Golgi discovered a reticular organelle within nerve cells. Later, biologists also identified similar structures in other cells and named them after Camillo Golgi. Because its refractive index is close to cytoplasm, it’s difficult to observe even after staining, so it was once regarded as staining artifact. Although Golgi apparatus was discovered as early as the 19th century, its existence was debated for nearly half a century. It wasn't until the advent of electron microscopy and ultrathin sectioning techniques that the debate was finally settled.
These organelles are highly dynamic. Their number, morphology and sacs vary greatly in different cells and even at different stages of cell growth. Animal cells typically have dozens of Golgi bodies, while some plant cells may contain hundreds. Maybe it is because of plant cell walls. Although Golgi body itself is not the site for cellulose synthesis, it plays an auxiliary role. Hemicellulose, pectin and related proteins and enzymes are processed in Golgi apparatus and transported to cell wall.
Sacs or Cisternae, Vesicle
It contains a series of flat, smooth membrane-enclosed sacs that resembles disks, and are known as cisternae. Four to eight or more cisternae stack together in a Golgi apparatus. They are divided into three regions based on shape and function.
The cisternae near nucleus are concave towards the cell membrane. This region is called cis face or forming face whose outer side are some vesicles and incomplete tubular or reticular membranes. The main task of cis face is to receive vesicles budding from ER . Thinner membranes and tighter arrangements facilitate vesicles to fuse rapidly. Most proteins enter the next region, while a small portion with ER retention signals return to ER again. The central part of Golgi apparatus is called medial region where proteins are glycosylated.
The cisternae near plasma membrane are concave towards cell nucleus. This region is called trans face or mature face. On its outer side are also some reticular membranes. The cisternae are arranged more loosely here and contain larger vesicles, as proteins, lipids, and polysaccharides leave this region for plasma membrane or other parts of endomembrane system. Protein precursor hydrolysis occur here.
Substances move directionally within Golgi apparatus via 3 pathways. Numerous small vesicles surround all the cisternae. Cargo from ER is encased in vesicles and moves through each compartment towards the trans face. Recycled proteins move in the opposite direction from the trans face. Cargo can also rapidly shuttle between cisternae via small tubules connecting each compartment. Sometimes cisternae are considered non-static structures. The cis face is the fused ER vesicles. It matures into the trans cisterna as it passes through every compartment, eventually disintegrating into vesicles that disappear at their destination.
Glycosylation
Their primary function is to modify, sort, and package proteins, lipids, and carbohydrates from endoplasmic reticulum, and transport them to specific locations within cell or outside cell. It functions like a transportation hub. N-glycosylation begins in rough ER where the initial oligosaccharide usually contains 14 units. Here, they are either added or trimmed to form mature glycoproteins with long and complex side chains. The entire process of O-glycosylation takes place in Golgi apparatus. Monosaccharides are added one by one to the oxygen-containing side groups. Usually, the final product is quite simple and only contains a few monosaccharide units. Unlike proteins and genetic material, glycosylation is not template-dependent but occurs in all regions of Golgi apparatus through a series of enzymatic reactions and environmental factors. It facilitates protein correct folding and increases their stability. The rigid saccharide side chains limit other macromolecules to approach membrane proteins. The side chains provide a protective shell, but unlike the cell wall, it doesn’t restrict the cell shape and movement. For example, the highly glycosylated proteins in lysosome membrane prevent themself from being degraded by hydrolases. Monosaccharides are covalently linked to each other at any point, which gives them various combinations that carry more information than amino acids and nucleic acids. Just three glucose can form an astonishing 5x5x5x2÷2=125 combinations. Cell adhesion, signal transduction and pathogen recognition all depend on the different glycan chains structures.
Hydrolysis and More
Some polypeptides must be cleaved at the trans face to acquire biological activity. The terminal sequences are removed from proproteins to form mature products, such as insulin and collagen. Some precursors contain more than one peptide sequences, and multiple active peptides are produced after hydrolysis. These sequences can be identical or different. This approach has many advantages. Some neuropeptide has only five amino acids. Adding signal sequences one by one is cumbersome, so it's easier to place them all in one long chain. One the other hand, different cleavage patterns can also arrange short peptides into different proteins. By controlling more signaling molecules with as few genes as possible, it's similar to the compressed files in our computers. Most importantly, this prevents active substances from being triggered prematurely.
Some cargo processed in Golgi apparatus is not immediately sent to its destination. It is stored in secretory vesicles and only released upon receiving a signal, such as hormones and neurotransmitters. Other molecules are secreted immediately after synthesis, such as the lipids that make up endomembrane system. Lysosomal hydrolases are tagged with an MP6 marker in Golgi apparatus. It was once believed that the glycans in proteins served as sorting signals, but DNA recombination techniques have shown that proteins can still be correctly targeted even after these glycans are removed. It is now believed that most signal molecules are embedded within the amino acid sequences rather than modifications.
In addition, hydroxylation of amino acids and the production of proteoglycans are also occur in Golgi body. A well-known example is hyaluronic acid. It absorbs a large amount of water to fill up the whole extracellular matrix to resist external pressure. Lubrication and cushioning are also its functions. Thus, hyaluronic acid is widely used in skincare and cosmetic products. Skin will appear smoother and less wrinkled, when it is injected under the skin to replace the lost connective tissue.