What is Golgi Apparatus
Golgi apparatus, alternatively known as Golgi complex, or Golgi body, is a membrane-bound organelle found in all animal and plant cells. It is a series of flattened, stacked pouches called cisternae lying close to the nucleus. It plays a key role in the transportation, modification, and packaging of proteins and lipids into vesicles for delivery to their respective targeted destinations.
Golgi is analogous to the post office. It receives molecules, makes changes to them, and then categorizes and addresses them for transport to different cell parts, just like a post office does with letters and packages.
Who Discovered Golgi Apparatus
In 1897, Italian scientist Camillo Golgi first identified the Golgi apparatus. Later, in 1898 it was named after him.
Where is the Golgi Apparatus Located
The location of Golgi varies, depending on the cell type.
In mammals, it is generally located in the cell cytoplasm, near the nucleus. As it is a part of the endomembrane system, it is functionally associated with the endoplasmic reticulum (ER). So, particularly it lies next to the ER.
In plants, it remains dispersed throughout the cytoplasm.
The Golgi apparatus is structurally very similar in both plant and animal cells. However, it appears compact and limited in some cells, while in others, it spreads out and looks net-like or reticular.
The electron microscope reveals the ultra-structure of Golgi apparatus. Under the microscope, the Golgi is seen to be composed of an array of interconnecting cisternae, tubules, vesicles, and Golgian vacuoles.
Composition: What is the Golgi Apparatus Made of
- They are flattened, sac-like closed compartments held in parallel bundles or stacks. Each sac contains a fluid called a matrix.
- In animals, the Golgi comprises nearly four to eight cisternae. However, in plants, it may contain as many as 20 or even more.
- The margins of each cisterna are curved, giving a bow-like appearance.
- Each cisterna is enclosed by a smooth unit membrane having a lumen, with a thickness of 7.5 nm.
- There is a gap of 20 to 30 nm in between each cisterna. This gap is called intercisternal space, having a thin layer of cytoplasm containing parallel fibrils.
- They are divided into three compartments: cis, medial and trans. The convex side facing the ER is called cis-face or forming face; the center part is medial, and the concave side facing the plasma membrane is trans-face or maturing face.
- At the cis-face, cisternae receive vesicles from the smooth or rough endoplasmic reticulum (SER or RER). In the medial part, the incoming molecules are passed through the chambers of Golgi via coated vesicles and intercisternal connectives for processing. At the trans-face, the vesicles bud off as Golgian vesicles and reach their respective destinations.
- They are short and branched, with a diameter of 30 to 50 nm.
- They form the internal connections between various cisternae, helping to establish a communication channel between them, thus, synchronizing their functions.
- They form a lace-like network towards the periphery of the apparatus.
- They are made up of microtubules. So, they provide structural support to the whole apparatus.
- These are small sacs of 20-80 nm diameters that bud off from tubules.
- They are of two types: coated and smooth.
i. Coated vesicles
- These are spherical structures, having a diameter of about 50 μm.
- They have a rough surface, as fine bristle-like outgrowths cover them.
- They bud off from the ends of peripheral tubules, usually at the ends of single tubules.
- They move to the cell membrane, helping to transport substances inside the cell (endocytosis).
ii. Smooth vesicles
- Another type of vesicle that comes in varied sizes.
- They have a smooth surface.
- They bud off from the tubules within the network.
- As they contain secretory substances, they are also called secretory vesicles.
- On being pinched off, they move towards the cell membrane and help to secrete substances out of the cell (exocytosis).
D. Golgian vacuoles
- These are round vesicles or sacs, developing from the maturing face of cisternae, which gets modified to vacuoles.
- These vacuoles bud off from the swollen edge of a cisterna and help transfer the modified proteins of Golgi from one cisterna to the adjacent one. In the end, they bud off to form vesicles containing the final modified product. Through these vesicles, the modified products reach their targeted destinations.
- They may also contain hydrolytic enzymes. In that case, they are referred to as lysosomes.
Functions: What Does the Golgi Apparatus Do
Golgi complex is referred to as the ‘traffic police’ of the cell, as it sorts the cell’s proteins and membrane constituents and directs them to their proper destinations.
Some of its characteristic functions are listed below:
1. Processing or packaging of protein: Golgi body processes or packs the proteins received from rough endoplasmic reticulum (RER) before sending them to their respective destinations. First, the proteins enter from the cis-face via transport vesicles. Then they move through the internal cisternal chambers of Golgi, containing various protein modifying enzymes. There these incoming proteins get modified. In the end, they are sorted for transport to their eventual destinations, such as lysosomes, the plasma membrane, or secretion from the trans-face.
2. Formation of secretory vesicles and secretion: The principal function of Golgi is secretion. In several types of cells, products from the RER are transferred to Golgi for processing. Then they are liberated from the cell through cell membrane via exocytosis.
3. Role in post-translational modification: Post-translational modification and enzymatic processing, such as phosphorylation and glycosylation, happen near the membrane surface in Golgi bodies. In glycosylation, carbohydrate is added to proteins and lipids, forming glycoproteins and glycolipids, respectively. On the other hand, in phosphorylation, a phosphate group gets attached to an amino acid.
4. Synthesize lipids: Golgi apparatus is the site for synthesizing various glycolipids and sphingomyelin.
5. Helps in enzyme formation: Golgi apparatus functions as a great intracellular center of enzyme formation. For instance, the production of follicular fluid from granulosa cells of ovary.
6. Formation of lysosome: It helps in the formation of primary lysosomes.
7. Production of hormones: Golgi body in endocrine cells helps in the secretion of hormones.
8. Storage of protein: Vacuoles and vesicles of Golgi complex help to store proteins.
9. Formation of acrosome: It forms the acrosome of sperm during sperm maturation.
10. Formation of intracellular crystals: In Limnaria ligmorum, Golgi complex forms several proteins and iron-containing crystals present in their midglands.
11. Formation of plant cell wall: During mitotic cell division, Golgi bodies form a cell plate at the center of spindle. The cell plate gradually enlarges and gets thickened by the deposition of pectin, hemicellulose, and cellulose secreted by Golgi complex.
12. Glycoproteins secretion: Golgi complex forms glycoproteins by attaching the carbohydrate to the protein products of the endoplasmic reticulum.
Ans. Golgi bodies prepare proteins for secretion.
Ans. Both lysosomes and Golgi bodies are cell organelles.
Ans. No, prokaryotes do not have Golgi apparatus.
Ans. Yes, plant cells have Golgi apparatus.
Ans. Yes, animal cells have Golgi apparatus.
Ans. No, bacteria do not have Golgi apparatus.
Article was last reviewed on Monday, October 11, 2021