Golgi apparatus

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Definition

noun

Gol·gi ap·pa·rat·us, [ˈgɔl dʒi ˌæpəˈreɪtəs]

(cell biology) An organelle that is comprised of membrane-bound stacks and is involved in glycosylation, packaging of molecules for secretion, transporting of lipids within the cell, and giving rise to lysosomes

Details

Overview

Organelle literally means "little organs". As the body is composed of various organs, the cell, too, has "little organs" that perform special functions. Some references are strict in their definition of an organelle. An organelle is a structure surrounded by lipid bilayers. In this regard, nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid) are regarded as organelles whereas ribosomes and nucleosomes are not. In the same way, lysosomes and vacuoles, would not qualify as an organelle because they are single-membrane bounded cytoplasmic structures. Other references, though, are less restrictive. An organelle is one that which acts as a specialized subunit inside the cell that performs a specific function. In this case, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles. Examples of membrane-bound organelles are nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes, and vacuoles. Examples of non-membrane-bound organelles are ribosomes, spliceosome, vault, proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, photosystem I, ATP synthase, nucleosome, centriole, microtubule-organizing center, cytoskeleton, flagellum, nucleolus, stress granule, etc.


Characteristics

The Golgi apparatus, sometimes referred to as simply Golgi, is an organelle that is identified in 1898 by the Italian physician Camilo Golgi. It is also one of the components of the GERL complex. The Golgi apparatus is present in eukaryotic cells where many substances such as proteins are secreted. In plants, this organelle is referred to as dictyosome. In flagellate protozoa, it is called parabasal body. The collective term for all the Golgi apparatus in the cell is Golgi complex.


The Golgi apparatus is made up of cisternae. These cisternae originate from the vesicles that bud off from the endoplasmic reticulum. In mammals, there are about 40 to 100 stacks of cisternae. The stacks are comprised of fused, flattened cisternae that are linked together by tubular (microtubules) connections. Without these microtubules, the Golgi becomes comprised of individual disks. In plants, the stacks are connected not by microtubules but by actin. Its location inside the cell varies. In mammals, it is usually adjacent to the nucleus, particularly in the centrosomal region. In plants, it is not concentrated at the centrosomal region. Nevertheless, most Golgi across eukaryotic cells is near to the endoplasmic reticulum exit sites.[1] Furthermore, in all eukaryotes, the Golgi cisternal stack has a cis and a trans faces. The cis is the entry face whereas the trans is the exit face. They contain differing metabolic enzymes. In the cis face cisternae, the enzymes present are those involved in the early modifications of proteins whereas in the trans face, the enzymes are involved in the final protein modification. The location of the enzymes of the Golgi differs from that of the enzymes in the endoplasmic reticulum; the enzymes in the Golgi are anchored in the membrane whereas the enzymes of the endoplasmic reticulum are in its lumen. Thus, during protein modification processes, the enzymatic reactions occur adjacent to the membrane surfaces of the Golgi where the enzymes are located.


Biological functions

The functions of Golgi include glycosylation (i.e. adding carbohydrate to a protein via the local glycosylation enzymes that aid in the attachment of sugar monomers to proteins that come from the endoplasmic reticulum), packaging of molecules like proteins into vesicles for secretion, transport of lipids around the cell, and the creation of lysosomes. With regard to it functioning as the packaging center of the cell, the organelle sorts the proteins secreted by the endoplasmic reticulum into packages, and then marks them to be sent to the appropriate destination. The glycoprotein that is to be transported outside the cell will be excreted as vesicles. If the glycoprotein is for later use, it may be stored in the vesicles within the cell (e.g. as lysosomes).


In mammals, the cells that contain greater and larger Golgi apparatus are those that are for secretion, e.g. plasma B cells that secrete antibodies.


Common biological reactions

Post-translation modification of proteins

Proteins from the endoplasmic reticulum for post-translation modifications are transported (as vesicles) to the Golgi apparatus. For example, oligosaccharides on lysosomal proteins undergo phosphorylation. This step occurs in the cis cisternae. Another post-translation modification that occurs in cis cisternae is the removal of mannose residues. As for the addition of N-acetylglucosamine, it occurs in the mid-portion of the cisternal stack. The addition of galactose and sialic acid, in turn, transpires in trans cisternae. Sulfation of tyrosines and carbohydrates also occurs in it. The Golgi is also responsible for the labeling of proteins to annotate their destination. For instance, the Golgi would add mannose-6-phosphate to proteins that are destined for lysosomes.


Vesicular transport

The protein processed by the rough endoplasmic reticulum for transport to the Golgi apparatus would be packed inside a transport vesicle. The vesicle containing the properly-folded protein is pinched off from the endoplasmic reticulum and then shuttled to the Golgi apparatus where the protein will undergo maturation (by post-translational modifications). The vesicle is transported from the endoplasmic reticulum along the cytoskeleton to the cis face of the Golgi. The contents of the vesicle are emptied into the lumen of the Golgi. Inside the lumen, the protein is modified, labeled with a signal sequence, and packed in a vesicle for transport to the next destination. Vesicles containing proteins meant for extracellular release would pinch off from the Golgi and then fuse to the plasma membrane for constitutive secretion. An example of this is a plasma B cell releasing antibodies. Vesicles that contain proteins for later extracellular release bud off from the Golgi and are transiently stored inside the cell. Upon a signal, the vesicle then fuses to the plasma membrane to release the proteins outside the cell (the process is referred to as regulated secretion). This is what happens when a neuron releases neurotransmitters upon stimulation. Vesicles that contain digestive proteins (e.g. proteases) and other lysosomal proteins would have to be shuttled to the late endosome first before the contents are transported to the lysosome.


Supplementary

Etymology

  • from Camillo Golgi, Italian physician and scientist

Synonym(s)

  • Golgi body
  • dictyosome
  • Golgi complex
  • Golgi vesicle


Further reading

See also


Reference

  1. Suda Y, Nakano A (April 2012). "The yeast Golgi apparatus". Traffic. 13 (4): 505–10.



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