Golgi apparatus is an important organelle present in animal and plant cells. To understand what is Golgi apparatus, its structure and function are discussed below.
What is Golgi apparatus?
Golgi apparatus, also known as the Golgi complex is an organelle present in eukaryotic cells only. It was first discovered in 1898 by Italian cytologist Camillo Golgi in nerve cells, so named as Golgi apparatus. In 1956, the Golgi apparatus was called the Golgi complex. It is inter associated stacks of cisternae involved in the secretory pathway. It is involved in the packaging of proteins into membrane-bounded vesicles before they are sent to their destinations.
Structure of Golgi apparatus
Golgi apparatus is a complex structure made up of cisternae and its associated secretory vesicles. Cisternae (singular: Cisterna) are stacks of flattened, membrane-bounded sacs. Each cisterna consists of a sac-like central region and a tubular region at the edge of the central plate, while secretory vesicles are small membrane-bounded packages or sacs that serve as transport vehicles for cell secretions such as hormones. About 40-100 stacks of cisterna are present in a mammalian cell. Cells involved in the synthesis of a large number of proteins have large and many Golgi apparatus.
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| Diagram of Gogli apparatus. Image created in BioRender.com |
Cisterna has two sides, convex and concave. The convex (Cis) side is "forming face" present towards the nucleus, while the concave (Trans) side is towards the cell membrane and known as "maturating face". Secretory vesicles are then formed from the maturing face. Cisternae are formed by the fusion of vesicles derived from the smooth endoplasmic reticulum.
Golgi apparatus resides in the cytoplasm near the endoplasmic reticulum and nucleus to easily perform its function. Studies showed that its shape differs greatly depending on its state of activity in different cell types. It can exist as a compact mass or be distributed as isolated fragments throughout the cytoplasm.
Function of the Golgi apparatus
The functions of Golgi apparatus are:
- The main function of the Golgi apparatus is the transport of cellular products. The proteins formed inside the ribosome are transported to the Golgi apparatus through the endoplasmic reticulum. Inside the Golgi apparatus, proteins are modified or multiple proteins are added and transported towards the plasma membrane by secretory vesicles.
- The proteins or lipids inside the Golgi apparatus are modified by adding carbohydrates to form glycoproteins and glycolipids, respectively.
- The proteins are also tagged inside the Golgi apparatus to direct them towards their final destination.
- It serves the extra purpose of synthesizing the major polysaccharide molecules inside plant cells, which help in the formation of the cell wall.
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Types of vesicles formed by Golgi apparatus
Proteins present in vesicles are sent from the endoplasmic reticulum to the Golgi apparatus where they are fused at forming side of the Golgi apparatus. These proteins are then released in the lumen of the Golgi apparatus, modified, and sorted according to their signal sequences. Signal sequences store the information about the destination of sorted protein.
The vesicles then pass through the network of membrane and reach the maturing face of the Golgi apparatus, from where they are sent to their destinations. There are three types of vesicles formed by the Golgi apparatus depending on their destination.
1. Secretory vesicles
Secretory vesicles after budding off from the Golgi apparatus resided in the cell until the appropriate release signal is received. After which these secretory vesicles fuse with the cell membrane and release their content outside the cell.
2. Exocytotic vesicles
These vesicles after pinching off from the Gogi complex move straight to the cell membrane, fuses with it, and release their content in extracellular space. No signals are required for the release of content.
3. Lysosomal vesicles
These vesicles contain hydrolytic enzymes required by lysosomes for the digestion of material. The content of these vesicles is transferred to lysosomes by an unknown mechanism. A lysosome is a membrane-bounded organelle containing hydrolytic enzymes which are involved in the digestion of substance.
Some questions and answers
Q. Where is Golgi apparatus located?
A. It is located in the cytoplasm near the endoplasmic reticulum and nucleus.
Q. What does the Golgi apparatus look like?
A. It looks like a cracked cup with secretory vesicles budding off from the cup.
Q. Who discovered Golgi apparatus?
A. It was first discovered in 1898 by Italian cytologist Camillo Golgi in nerve cells.
Q. How many Golgi apparatus are present in a cell?
A. The number of Golgi apparatus varies according to its function. In animal cells, about 10-20 Golgi stacks per cell are found, while plant cells might have hundreds.
Q. Why is the Golgi apparatus important?
A. Golgi apparatus is involved in the transportation of cellular secretion inside as well as outside the cell.
Q. What does the Golgi apparatus do?
A. It modifies, tags, and synthesize cellular products and transports them by secretory vesicles.
Q. What are other names of Golgi apparatus?
A. Golgi apparatus is also known as Golgi complex or Golgi bodies.
Q. From which side of the Golgi apparatus secretory vesicles are formed?
A. Secretory vesicles are formed from the maturing face.
Q. Define cisterna.
A. Cisterna is a stack of flattened membrane-bounded sacs present in the Golgi apparatus.
Q. How Golgi apparatus receive proteins for transportation?
A. The proteins formed inside the ribosome are transported to the Golgi apparatus through the endoplasmic reticulum. Inside the Golgi apparatus, proteins are modified or multiple proteins are added and transported towards the plasma membrane by secretory vesicles.
Q. How many types of vesicles are produced by the Golgi apparatus?
A. Secretory vesicles, exocytotic vesicles, and lysosomal vesicles are three types of vesicles pinched off from the Golgi apparatus.
References
- Bentivoglio M. The discovery of the Golgi apparatus. J Hist Neurosci. 1999 Aug;8(2):202-8. doi: 10.1076/jhin.8.2.202.1833. PMID: 11624302.
- Morré D.J., Franke W.W., Deumling B., Nyquist S.E., Ovtracht L. (1971) Golgi Apparatus Function in Membrane Flow and Differentiation: Origin of Plasma Membrane from Endoplasmic Reticulum. In: Manson L.A. (eds) Biomembranes. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-3330-2_
- Beams, H. W., & Kessel, R. G. (1968). The Golgi apparatus: structure and function. International review of cytology (Vol. 23, pp. 209-276). Academic Press.
- Neutra, M., & Leblond, C. (1969). THE GOLGI APPARATUS. Scientific American, 220(2), 100-109. Retrieved November 10, 2020, from http://www.jstor.org/stable/24926286.
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