Endoplasmic Reticulum and Centrosome: Structure, Function, and Roles
Endoplasmic Reticulum (ER)
The endoplasmic reticulum (ER) is an intracellular membranous system located between the plasma membrane and the nuclear envelope. Its membrane is continuous with the nuclear membranes. This divides the cytosol into two compartments: the luminal space (inside the ER) and the cytosolic space (outside the ER).
Rough Endoplasmic Reticulum (RER)
The RER has ribosomes attached to the cytosolic side of its flattened membranes. It consists of interconnected sacs or cisternae and vesicles. The lumen of the RER is filled with a dense material. The RER is more developed in cells involved in protein synthesis, such as pancreatic cells or cells lining the digestive tract. It is present in all cells except red blood cells of prokaryotes and mammals.
Roles of the RER
- Protein Synthesis and Storage: The RER membrane contains enzymes involved in protein synthesis. These enzymes are asymmetrically distributed, with different enzymes on the cytosolic and luminal faces. Proteins are synthesized on ribosomes attached to the RER membrane.
- Glycosylation of Proteins: Most proteins synthesized and stored in the RER undergo glycosylation to become glycoproteins.
Smooth Endoplasmic Reticulum (REL)
The smooth endoplasmic reticulum (REL) is a network of interconnected tubules. Its membranes are continuous with the RER but lack attached ribosomes. The REL is scarce in most cells but abundant in striated muscle cells, interstitial cells of the ovary and testis, Leydig cells, cells of the adrenal cortex, and hepatocytes.
Functions of the REL
- Lipid Synthesis: The REL is the primary site for the synthesis of phospholipids, cholesterol, and most other lipids.
- Muscle Contraction: The REL plays a crucial role in muscle contraction by releasing calcium ions.
- Detoxification: The REL detoxifies harmful substances through oxidation processes involving cytochromes. Cells involved in detoxification include those in the skin, intestines, lungs, liver, and kidneys.
- Glucose Release: The REL is involved in the release of glucose from glycogen granules in hepatocytes. Glycogen stores are found in granules attached to the REL.
Centrosome
The centrosome is a non-membrane-bound structure present in all dividing animal cells. It is generally absent in plant cells.
Structure and Composition
The centrosome consists of a central body composed of two centrioles, surrounded by a dense, amorphous pericentriolar material. This entire structure is called the microtubule organizing center. The two centrioles are arranged perpendicular to each other. Each centriole is a cylindrical structure with walls formed by nine groups of three microtubules (9+0 structure). The three microtubules in each triplet are associated. These microtubules are named A (innermost), B (middle), and C (outer). The triplets are linked by a protein called nexin. Microtubule A is complete, while B and C share protofilaments with the adjacent microtubules. The centriole has a proximal end near the cell nucleus and a distal end near the periphery. This structure is known as the “cartwheel”.
Origin and Function
A centriole develops from a cylinder-shaped procentriole, which consists of nine microtubules A. The procentriole grows longitudinally, forming microtubules B and C. The centrosome is the organizing center for microtubules. Structures derived from microtubules include the mitotic spindle.