Endoplasmic reticulum and Golgi apparatus are two membrane-bound organelles found in eukaryotes. Both these organelles are closely associated and functionally related. Both endoplasmic reticulum (ER) and Golgi apparatus are two components of the endomembrane system of a cell. Lysosomes and vesicles are the other components of the endomembrane system. Two types of ER are found in the eukaryotic cell, rough ER and smooth ER. Ribosomes are bound with rough ER, transporting synthesized polypeptide chains into the rough ER. Post translational modifications of the proteins are taken place inside the ER, maturing the proteins. These matured proteins are transported into the Golgi apparatus in order to be transported to their eventual destination, either into lysosomes, plasma membrane or secreted out from the cell into the extracellular environment. The secretion into the extracellular environment occurs by exocytosis.
This article explains,
1. What is Endoplasmic Reticulum
– Structure, Functions
2. What is Golgi Apparatus
– Structure, Functions
3. Relationship Between Endoplasmic Reticulum and Golgi Apparatus
– Similarities Between Endoplasmic Reticulum and Golgi Apparatus
– Difference Between Endoplasmic Reticulum and Golgi apparatus
4. How are Proteins Transported out of Endoplasmic Reticulum to Golgi Apparatus
What is Endoplasmic Reticulum
Endoplasmic reticulum (ER), which is an organelle found in eukaryotes, contains flattened membranous sacs, interconnected with each other. These sacs are tube-like structures, which are called cisternae. Cisternae are held together by the cytoskeleton of the cell. Two types of ER is found: smooth ER and rough ER. Only rough ER contains bound ribosomes to the membrane of the ER. Smooth ER is involved in the lipid metabolism. Rough ER provides sites for protein synthesis.
What is Golgi Apparatus
Golgi apparatus is another organelle found in eukaryotic cells. It is made up of fluid-filled, four to six cisternae. Golgi apparatus provides a site for syntheses for carbohydrates like pectin and hemicellulose. Glycosaminoglycans, which are found in the extracellular matrix of the animal cells are also synthesized in the Golgi apparatus. Two faces can be identified in Golgi: cis face and trans face.
Relationship Between Endoplasmic Reticulum and Golgi Apparatus
Similarities Between Endoplasmic Reticulum and Golgi Apparatus
Both ER and Golgi apparatus are involved in the formation of endomembrane system of the cell. They are made up of flattened, membranous, fluid-filled sacs called cisternae. Cisternae are held together by the cytoskeleton of the cell.
Rough ER provides sites for the protein synthesis in the cell. Ribosomes are bound to the membrane of rough ER. The translated proteins are exported into the ER for maturation. These proteins are again transported into Golgi apparatus for further maturation and sorting out for their final destination. Therefore, both ER and Golgi apparatus are involved in protein maturation. Newly synthesized polypeptide chains interact with chaperone proteins in the ER lumen. The proteins, which are to be secreted and destined to the cell surface achieve their 3D structure by forming disulfide bonds between cysteine residues in the polypeptide chain. The formation of disulfide bonds between cysteine residues is facilitated by protein disulfide isomerase found in the ER. Once proteins achieve their proper 3D structure, they are released from the chaperone proteins. Glycosylation, which is the addition of polysaccharide chains into the protein, occurs in the ER as well. Usually, membrane proteins and secretion proteins are glycosylated. Some glycosylations occur in the ER and the others occur in the Golgi apparatus.
Both ER and Golgi apparatus are capable of forming transport vesicles. Proteins destined to the lysosomes, plasma membrane or secretion, are transported from ER to Golgi apparatus by small transport vesicles called COPII-coated transported vesicles. Golgi apparatus also forms secretory vesicles in order to transport sorted proteins to their final destinations. Endomembrane system of the cell is shown in figure 1.
Figure 1: Endomembrane system of the cell
Difference Between Endoplasmic Reticulum and Golgi Apparatus
Cisternae in ER are interconnected with each other, facilitating the transport of macromolecules throughout the cell. In contrast, the cisternae in Golgi apparatus contains four to six small cisternae. They are not interconnected with each other. But two faces can be identified in Golgi as cis face and trans face. A directional flow of material from the cis cisternae to trans cisternae is observed in Golgi. The secretory vesicles enter Golgi from its cis face and mature and dissociate from the trans face. The network of tubular and cisternal structures found on either side of the Golgi are called cis Golgi network (CGN) and trans Golgi network (TGN). The transportation of proteins from CGN to TGN is called vascular transport. The structure of the Golgi apparatus is shown in figure 2.
ER and Golgi apparatus are involved in other functions in the cell as well. Smooth ER is involved in the lipid metabolism. In contrast, Golgi apparatus provides a site for the carbohydrate synthesis like pectin and hemicellulose. Glycosaminoglycans, which are found in the extracellular matrix of the animal cells are also synthesized in the Golgi apparatus.
Figure 2: Golgi apparatus
How are Proteins Transported out of Endoplasmic Reticulum to Golgi Apparatus
Most proteins which are translated are destined to ER, Golgi apparatus, lysosomes or plasma membrane. The pathway which secretes proteins from ER to Golgi apparatus, transporting into other destinations is called biosynthetic-secretory pathway. These proteins are synthesized by ribosomes, which are bound to the rough ER. The translated polypeptide chains are transported into the ER. Protein folding and processing is taken place inside the ER. Golgi apparatus is the factory which receives proteins from the ER. It is found in the exit root of the ER. From the ER, mature proteins are transported into the Golgi apparatus. This transportation occurs by small vesicles called COPII-coated transported vesicles, which exit from ER exit sites.
COPII-coated transport vesicles enter the Golgi apparatus from the cis face of the organelle by fusing with the membrane of cis cisternae. The proteins then enter the CGN and are sequentially transported into the TGN, while matured further and prepared for their final destinations. Proteins in the Golgi apparatus can be are destined to lysosomes, plasma membrane or secreted out into the extracellular environment. From the TGN, mature proteins exit the Golgi by secretory vesicles.
Conclusion
ER, Golgi apparatus, lysosomes, and secretory vesicles are collectively called the endomembrane system of the eukaryotic cell. ER contains rough and smooth surfaces on it. Smooth ER is involved in the lipid metabolism. Rough ER is involved in the protein synthesis by binding ribosomes into its membrane. The proteins synthesized in the ribosomes are transported into rough ER. Inside the ER, these proteins undergo maturation by post translational modifications. Oligosaccharide-tagged proteins are transported from ER to Golgi apparatus through small transport vesicles called, COPII-coated vesicles. These proteins enter the Golgi by CGN and transported to TGN, while sorted out in order to be transported to their eventual destination. The transportation of proteins from CGN to TGN is called vascular transport. During vascular transport, proteins still undergo modifications like glycosylation. The sorted out proteins are transported into either lysosomes, plasma membrane or secreted out to the extracellular environment. The transportation of translated proteins from ribosomes to Golgi apparatus through the endoplasmic reticulum is called biosynthetic-secretory pathway.
Reference:
1. Cooper, Geoffrey M. “The Endoplasmic Reticulum.” The Cell: A Molecular Approach. 2nd edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 24 Apr. 2017.
2. Cooper, Geoffrey M. “The Golgi Apparatus.” The Cell: A Molecular Approach. 2nd edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 24 Apr. 2017.
3. Alberts, Bruce. “Transport from the ER through the Golgi Apparatus.” Molecular Biology of the Cell. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 24 Apr. 2017.
Image Courtesy:
1. “Endomembrane system diagram en” By Mariana Ruiz LadyofHats – (Public Domain) via Commons Wikimedia
2. “Golgi apparatus (borderless version)-en” By Kelvinsong – Own work (CC BY 3.0) via Commons Wikimedia
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