STRUCTURE OF ENDOPLASMIC RETICULUM
The endoplasmic reticulum is often the biggest membrane in eukaryotic cells, forming a complex network of interconnecting membrane sacs or channels. It makes up 30 to 60 percent of a cell’s entire membrane. Ribosomes may or may not be linked to the endoplasmic reticulum’s outer membrane. As a result, they are categorised as either rough (RER) or smooth endoplasmic reticulum (SER). Ribosomes with a diameter of about 150 nm and a high protein and RNA content are found in the rough endoplasmic reticulum. Ribosomes are not present in smooth endoplasmic reticulum. It is made up of three different sorts of components: cisternae, tubules, and vesicles.
- Cisternae are flattened, unbranched, sac-like structures that have a diameter of 40–50 m. They are stacked (piled) next to one another but connected to one another. Cytosolic gaps separate them from one another. The ribosomes, which are tiny granular structures, may or may not be found on the surface of cisternae.
- Tubules – These irregular, branching elements, combined with other elements, form a network. They have a diameter of between 50 and 100 m and frequently lack ribosomes.
- Vesicles are oval, vacuole-like structures that range in size from 25 to 500 m. In the cytoplasmic matrix, they frequently exist in isolation. Ribosomes are not present either. The lumen of the ER contains a liquid known as the endoplasmic matrix. Every component of ER is available.
Smooth and Rough Endoplasmic Reticulum
The smooth surface of this form of endoplasmic reticulum (SER) results from the absence of connected ribosomes. Cells that are actively involved in steroid synthesis, glucose metabolism, pigment generation, etc. contain smooth ER, while Ribosomes are adhered to the Rough Endoplasmic Reticulum’s (RER) surface all throughout. These are seen in cells that are actively synthesizing proteins.
Ultra structure of Endoplasmic Reticulum
- Endoplasmic Reticulum consists of cisternae, tubules, and vacuoles which are bounded by membrane which is same as the membrane of cell.
- Similar to other cell membranes, the endoplasmic reticulum’s membranes are made up of two layers of phospholipid molecules sandwiched between two layers of protein molecules.
- The ratio of proteins to lipids in the ER membrane is relatively high. Along with the Golgi membranes and the outer membrane of the nuclear envelope, it is continuous. Some cisternae have openings in the cell membrane that allow them to open.
- Palade noticed secretary granules in the endoplasmic reticulum’s lumen. The secretary products travel through the lumen.
- The ER is connected to about 30–40 distinct enzymes for the diverse synthesis activities. These may be found on the luminal surface, cytoplasmic surface, or both.
Two types of ER are present in cells depending on whether ribosomes are present or not.
Smooth Endoplasmic Reticulum (SER)
Endoplasmic reticulum (ER) called smooth or agranular because there are no ribosomes on its walls, which gives the ER its smooth appearance. Most often, it takes tubular forms. The tubules are between 500 and 1000 in diameter and form erratic lattices. According to Christensen and Fawcett (1961), smooth ER is frequently seen in cells that synthesize lipids or steroids, i.e., non-protein types of synthesis, including the interstitial cells of the gonads, sebaceous glands, and adrenal glands. There is also higher SER in some cells that are involved in carbohydrate metabolism (such as liver cells), impulse conduction (such as muscle cells), pigment formation (such as retinal pigment cells), and electrolyte excretion (such as chloride cells of fish gills).
Rough Endoplasmic Reticulum (RER)
It is sometimes referred to as granular ER because of the ribosomes that can be found on the surface of the reticulum. It takes the shape of 400–500 mm wide flattened cisternae. RER primarily occurs in cells that are actively producing proteins, such as enzymes (such as pancreatic, plasma, and liver cells) or mucus (goblet cells). RER is made up of reticular sheets and fenestrated cisternae in the basal region of pancreatic exocrine cells. These cisternae have groups that are 400–1000 nm in diameter and a length of 5–10 microns. Granular reticulum appears as vesicles in the apical area of the cells. The membranes of granular and agranular ER are continuous in