The Golgi apparatus is also known as Golgi body and Golgi complex. They were first discovered in 1897 by Italian biologist Camillo Golgi in nerve cells. Later, biologists found these organelles in other cells as well and named them after Camillo Golgi. Due to their close refractive index to the cytoplasm, they are not easily observable even after staining, and was once thought to be an artifact of staining. Although Golgi apparatus was discovered over a century ago, it took several decades of debate for their true existence, and was finally resolved with the advent of electron microscopy.
The Golgi apparatus is composed of a series of flattened, smooth membrane-bound sacs called cisternae that resemble concave circular trays. Since the cisternae are separated from each other, numerous small vesicles surround them for transportation. The main body of the Golgi apparatus consists of 4-8 or more of these stacked cisternae. Animal cells typically have several Golgi apparatuses, while some plant cells even can have hundreds. The Golgi apparatus can be divided into three regions based on the shape of cisternae: cis region, medial region and trans region.
The flow of substances within the Golgi apparatus is directional. They usually enter from the end close to the nucleus and exit from the end close to the plasma membrane. The cisternae near the nucleus are concave towards the outside of the cell, which is called the cis region. These cisternae are thinner and closely packed. They are responsible for receiving transport vesicles from the endoplasmic reticulum and sorting the substances within the vesicles. Most of the substances move to the medial region, while a small portion returns to the endoplasmic reticulum. Proteins are also processed in the cis region, such as acetylation and phosphorylation.
The middle region of the Golgi apparatus is called the medial region, where glycosylation, synthesis of glycolipids and some polysaccharides occur.
The cisternae in the trans region concave towards the cell nucleus. They are thicker and more dispersed. They are responsible for receiving substances transported from the medial region and tagging them for sorting. Finally, these substances are packaged into vesicles for secretion or transportation to lysosomes. The trans region also performs some processing of proteins, such as sulfation and sialylation.
There are numerous small vesicles concentrated around the cisternae, particularly in the cis and trans regions. The vesicles near the cis region are smaller. They bud from the nearby endoplasmic reticulum, transporting membrane proteins, polysaccharides and lipids to the Golgi apparatus. The trans region is a dynamic area where large vesicles bud off to transport sorted proteins, lipids and polysaccharides to their destinations, such as the plasma membrane, organelles, or the extracellular space. It's worth noting that the size, number, and morphology of the Golgi apparatus, as well as the vesicles, can vary in different cells and can be influenced by cellular metabolism and the external environment.
Function of Golgi apparatus
The Golgi apparatus processes polypeptide synthesized in the endoplasmic reticulum, including glycosylation, phosphorylation, acetylation and glycan modifications (where some parts of the glycans may be replaced by other compounds). These added groups are either components of proteins or serve as labels for protein sorting. Some partially processed proteins are also hydrolyzed to generate active forms.Lipids synthesized in the endoplasmic reticulum are modified in the Golgi apparatus by adding oligosaccharide side chains.
Some polysaccharides are synthesized in the Golgi apparatus. For example, hemicellulose and pectin are components of plant cell walls. In animal cells, one of the polysaccharide is hyaluronic acid, which is a component of the extracellular matrix and plays a role in hydration, lubrication and buffering.