Golgi Apparatus Definition

Golgi Apparatus is involved in the packing of proteins and other molecules for secretion, modifying proteins through processes like glycosylation. These membrane-bound stacks are responsible for the production of lysosomes.

What is Golgi Apparatus?

The Golgi apparatus was discovered by an Italian physician Camilo Golgi in 1898. It is a part of the GERL complex and is present in eukaryotic organisms. It is also known as dictyosomes in plants and as the parabasal body in flagellate protozoa. All the Golgi apparatus of a cell is together termed as Golgi complex.

The membranous stacks of Golgi are cisternae that have their origin from the Endoplasmic Reticulum in form of vesicles. The stacks of cisternae in the case of mammals range from 40 to 100. The flattened and fused cisternae comprise stacks that often have microtubule connections in between them.

If these interconnecting tubules are removed then now the organelles consist of only individual disks. In plants, the microtubules in stacks are replaced by actin and its location in a cell varies.

It is found proximal to the nucleus especially in the centrosomal region in mammals. Unlike mammals, plants, are not concentrated in any specific region. The usual location of Golgi is near ER exit sites in almost all eukaryotic cells. The cisternal stack has 2 faces: a cis (entry face) and a trans face (exit face). Different enzymes are present in these 2 faces.

In the case of cis face, early modification enzymes are present while in the trans face of cisternae the enzymes required for the final processing of proteins are present. These enzymes are anchored to the membrane, whereas in ER they are anchored in the lumen.

Golgi Apparatus Function

The Golgi organelle is responsible for processing steps like glycosylation, packaging molecules into vesicles, production of lysosomes, and transport of lipids. In glycosylation, sugar monomers are attached to proteins in a series of steps.

Golgi Apparatus sorts the proteins emerging from ER and packs and tags them so that they can be allocated to their destinations.

The modified and tagged glycoprotein is packaged in form of vesicles for transport. They can be stored in form of vesicles for later use in the cell itself as in lysosomes.

Cells that have a secretory function have more Golgi apparatus and they are also larger in these mammalian cells. For instance in plasma B cells play a crucial role in secreting antibodies.

Biological Importance of Golgi Apparatus

The endoplasmic reticulum secretes proteins in form of vesicles for transport to Golgi where they are further modified. For instance, the oligosaccharides on lysosomal proteins are phosphorylated in cis cisternae of Golgi where the mannose residues are also removed. N-acetylglucosamine is added in the mid-cisternae stack.

In trans face, sialic acid and galactose are added. Other processes like sulfation of carbohydrates and tyrosines also occur here. Proteins are also tagged here to specify their destinations. For example, the proteins destined for the lysosome are labelled with mannose-6-phosphate.

The protein that is secreted by RER is enclosed in vesicles and transported to Golgi for maturation. The vesicles containing properly folded proteins pinch off from ER in this process.

They are then transported via cytoskeletons to the cis face for post-translational modifications. The vesicle fuses with Golgi and empties its contents into its lumen. Here the protein is processed, tagged with a signal sequence, and packed for export.

If the vesicle is to be exported to the extracellular space then it will fuse to the cell membrane for constitutive secretion. These vesicles fuse when they receive a signal, till then they are transiently stored. This process is termed as regulated secretion as in neurotransmitters released by neurons.

For hydrolytic enzymes like proteases, the vesicles are first shuttled to late endosomes and then transported to the lysosome.