Lecture- Structure and functions of Golgi apparatus1.pdf

Golgi complex
Structure and Function

1. Golgi complex
Structure and Function


Dr. Manikandan Kathirvel M.Sc., Ph.D., (NET)
Assistant Professor,
Department of Life Sciences,
Kristu Jayanti College (Autonomous),
Reaccredited with "A++" Grade by NAAC
K. Narayanapura, Kothanur (PO)
Bengaluru 560077
Email: [email protected]
ORCID ID: 0000000270066334

2. Golgi Apparatus definition
The Golgi apparatus or the Golgi body or Golgi complex or simply Golgi is a cellular
organelle present in most of the cells of the eukaryotic organisms.
It is referred to as the manufacturing and the shipping center of the cell.
Golgi is involved in the packaging of the protein molecules before they are sent to their
destination. These organelles help in processing and packaging the macromolecules like
proteins and lipids that are synthesized by the cell and hence act as the ‘post office’
of the cell.
Golgi apparatus was discovered in the year 1898 by an Italian biologist Camillo Golgi. In
plant it is called Dictyosome.

Figure: Diagram of Golgi Apparatus
Structure of Golgi apparatus
Under the electron microscope, the Golgi apparatus is seen to be composed of stacks of
flattened structures that contain numerous vesicles containing secretory granules.
The Golgi apparatus is morphologically very similar in both plant and animal cells.
However, it is extremely pleomorphic: in some cell types it appears compact and limited,
in others spread out and reticular (net-like).
Typically, however, Golgi apparatus appears as a complex array of interconnecting
tubules, vesicles, and cisternae.

A. Cisternae
It is the simplest unit of the Golgi apparatus is the cisterna.
Cisternae (about 1 μm in diameter) are central, flattened, plate-like or saucer-like closed
A Golgi complex has two faces – Cis
Face & Trans face.
1. Cis face is always facing
towards nucleus while the trans
face is facing towards PM.
2. It has zone of clear cytoplasm
known as zone of exclusion.
3. Golgi stacks are actually Golgi
cisternae which looks like
deflated balloons.
4. A liver cell may have up to 50
cisternae.
5. It constitutes about 2% of
total cell
volume.

3. compartments that are held in parallel bundles or stacks one above the other.
In each stack, cisternae are separated by a space of 20 to 30 nm which may contain rodlike
elements or fibers.
Each stack of cisternae forms a dictyosome which may contain 5 to 6 Golgi cisternae
in animal cells or 20 or more cisternae in plant cells.
Each cisterna is bounded by a smooth unit membrane (7.5 nm thick), having a lumen
varying in width from about 500 to 1000 nm.
The margins of each cisterna are gently curved so that the entire dictyosome of the Golgi
apparatus takes on a bow-like appearance.
The cisternae at the convex end of the dictyosome comprise proximal, forming or cis-face,
The convex side is called forming face and cisternae at the concave end of the dictyosome
comprise the distal, maturing or transface, The concave side is called maturing face.

B. Tubules
A complex array of associated vesicles, branched and anastomosing tubules
(interconnected) (30 to 50 nm diameter) surround the dictyosome and radiate from it. In fact,
the peripheral area of the dictyosome is fenestrated (lace-like) in structure.

C. Vesicles
The vesicles (60 nm in diameter) are of three types:
(i) Transitional vesicles are small membrane limited vesicles which are thought to form
as blebs (bubbles) from the transitional ER to migrate and converge to cis face of Golgi, where
they coalesce (combine) to form new cisternae. The transitional vesicles or transport vesicles
pinched off from the rough ER and fuse with the cis face of the Golgi to transport proteins
and lipids between the cisternae and the tubule.

(ii) Secretory vesicles are varied-sized membrane-limited vesicles that discharge from
margins of cisternae of Golgi. They, often, occur between the maturing face of Golgi and
the plasma membrane. The secretary vesicles that derive from TGN carry glycoproteins,
glycolipids and polysaccharides to different destinations in the cell or outside the cell.

(iii) Clathrin-coated vesicles are spherical protuberances, about 50 μm in diameter and
with a rough surface. They are found at the periphery of the organelle, usually at the ends of
single tubules, and are morphologically quite distinct from the secretory vesicles. The
clathrin-coated vesicles are known to play a role in intracellular traffic of membranes
and of secretory products, i.e., between ER and Golgi, as well as, between the GELR
region and the endosomal and lysosomal compartments.

4. Function of Golgi Apparatus
1. Golgi vesicles are often, referred to as the “traffic police” of the cell. They play a key role
in sorting many of the cell’s proteins and membrane constituents, and in directing them to their
proper destinations.
To perform this function, the Golgi vesicles contain different sets of enzymes in different
types of vesicles— cis, middle and trans cisternae—that react with and modify secretory
proteins passing through the Golgi lumen or membrane proteins and glycoproteins that are
transiently in the Golgi membranes as they are en route to their final destinations.
The Golgi apparatus hence acts as the assembly factory of the cell where the raw materials
are directed to the Golgi apparatus before being passed out from the cell.

2. In animals, the Golgi apparatus is involved in the packaging and exocytosis of the
following materials:
Zymogen of exocrine pancreatic cells;
Mucus (=a glycoprotein) secretion by goblet cells of the intestine;
Lactoprotein (casein) secretion by mammary gland cells (Merocrine secretion);
Secretion of compounds (thyroglobulins) of thyroxine hormone by thyroid cells;
Secretion of tropocollagen and collagen;
Formation of melanin granules and other pigments; and
Formation of yolk and vitelline membrane of growing primary oocytes.

3. It is also involved in the formation of certain cellular organelles such as plasma
membrane, lysosomes, acrosome of spermatozoa and cortical granules of a variety of
oocytes.

4. They are also involved in the transport of lipid molecules around the cell.

5. The Golgi complex also plays an important role in the production of proteoglycans. The
proteoglycans are molecules that are present in the extracellular matrix of the animal cells.

6. It is also a major site of synthesis of carbohydrates. These carbohydrates include the
synthesis of glycosaminoglycans, Golgi attaches to these polysaccharides which then attaches
to a protein produced in the endoplasmic reticulum to form proteoglycans.

7. The Golgi involves in the sulfation process of certain molecules (addition of sulphur).

8. The process of phosphorylation (addition of phosphate groups) of molecules by the Golgi
requires the import of ATP into the lumen of the Golgi.

9. In plants, Golgi apparatus is mainly involved in the secretion of materials of primary and
secondary cell walls (e.g., formation and export of glycoproteins, lipids, pectins and monomers
for hemicellulose, cellulose, lignin, etc.)

Posttranslational modification: It facilitates glycosylation (addition of carbohydrates) of
proteins synthesized in the rough endoplasmic reticulum (RER).
• Modification of molecules: It facilitates liposylation (formation of lipoprotein), sulphation
(addition of sulphates) and phosphorylation (addition of phophates).
• Secretion: It is the major function of Golgi apparatus, which help in collection, storage,
condensation, modification and packaging of various materials into secretory vesicles.

5. The secretory vesicles release the contents to the exterior through exocytosis, e.g., secretion of
mucilage by root cap cells, secretion of hormones, gum, wax, cell wall
material, ground matrix of connective tissue, etc.
• Membrane transformation: It helps in the transformation of one type of membrane (e.g.that
of ER) into other types (e.g., selectively permeable plasma membrane, differentiated membrane
of lysosome, etc.).
• Structural maintenance: It helps in the formation of cell plate, cell wall and plasma
membrane during cell division.
• Lysosomes: It helps in the formation of primary lysosomes, sperm acrosome, nematocysts in
coelenterates and root hairs.
• Hormone synthesis: It is the site of production of hormones in the secretory cells of
endocrine glands,
Synthesis of complex carbohydrates: Most of the complex carbohydrates except glycogen
and starch, such as pectic compounds, mucopolysaccharides, hyaluronic acid, chondroitin
sulphate, hemicellulose, etc., are synthesized.
• Synthesis of special simple carbohydrates: Special simple carbohydrates, such as sialic acid
and galactose are formed in the Golgi complex.
• Matrix: Matrix of connective tissue is formed by Golgi complex of its cells.
• Fat transport: Fatty acids and glycerol absorbed by intestinal epithelium are transferred as
fat to lacteal through Golgi complex.
• Pigment synthesis: It is believed to synthesize retinal pigments in Chick embryo.
• Vitellogenesis: In oocytes of animals, Golgi apparatus functions as the centre around which
yolk is deposited – a process that is known as vitellogenesis.
• Acrosome formation: Acrosome is formed by Golgi complex with the help of its vesicles.
• Root hair: Root hair is formed form their mother (root hair stem) cells by the Golgi complex.
• Hypnotoxin: Nematoblasts hypnotoxin is formd by Golgi apparatus.
• Proteolysis: Cleavage of some precursor proteins, e.g., prohormones.








References
1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology,
Evolution & Ecology (1 ed.). S .Chand and company Ltd.
2. Stephen R. Bolsover, Elizabeth A. Shephard, Hugh A. White, Jeremy S. Hyams (2011).
Cell Biology: A short Course (3 ed.).Hoboken,NJ: John Wiley and Sons.
3. Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.

6. Clathrin-coated vesicles:
Clathrin-coated vesicles, transporting products from the Golgi apparatus to lysosomes and
carrying products from the exterior of the cell to lysosomes

Clathrin-coated vesicles start as small pits on the cell surface. When the vesicle is fully
intracellular it loses its clathrin coat and becomes an endosome, which fuses with
primary lysosomes that have a high content of acid hydrolases and other proteases. These lead
to degradation of the ingested material, and further processing depending on the cell type.
Certain cell surface receptors are recycled to the cell membrane during this process to engage
further extracellular ligand.