What are the Organelles in a Cell
Cell organelles :
Small membrane bound structures, which perform a lot of chemical activities to support the function & structure of a cell, called cell organelles –
Some important cell organelles are following –
Endoplasmic Reticulum :
The endoplasmic reticulum (ER) is noticeable only with an electron microscope.
Structure :
The ER is an extensive network of intracellular membrane-bound tubes and that occupies most of the cytoplasm in almost all eukaryotic cells. The membranes of this system are lipoproteinic in nature similar in structure to the plasma membrane. The ER is more prominent in young and dividing cells as compared to older cells. It is absent in prokaryotic cells.
Types.
The ER is of two types :
- Rough endoplasmic reticulum (RER)
- Smooth endoplasmic reticulum (SER)
1. Rough Endoplasmic reticulum (RER). These appear rough under a microscope because of the presence of a large number of grain-like ribosomes over their cytoplasmic surface. The ribosomes are the sites of protein synthesis. Thus, RER is engaged in the synthesis and transport of proteins. Generally, RER is more abundant in the deeper part of cytoplasm near the nucleus where it is connected with the outer membrane of the nuclear envelope. RER is well developed in the cells that synthesize and secrete proteins.
2. Smooth Endoplasmic Reticulum (SER). It consists mainly of tubules and vesicles. It is free of ribosomes and is more abundant near the peripheral part of the cytoplasm where it may be attached to the plasma membrane. The SER helps in the synthesis of fat or lipid molecules. It is, therefore, well developed in the cells that secrete lipids.
Functions :
- Support. The ER acts as supporting skeletal
framework of the cell and also maintains its form. - Transport of materials. The ER facilitates transport of materials from one part of the cell to another.
- Exchange of materials. The ER helps in the exchange of materials between the cytoplasm and the nucleus.
- Localization of organelles. It keeps the cell organelles properly stationed and distributed in relation synthesis.
- Surface for protein synthesis. The RER offers extensive surface on which ribosomes carry protein synthesis.
- Surface for synthesis of other substances. The SER provides surface for the synthesis of lipids including phospholipids, cholesterol and steroid hormones.
- Packaging. The proteins formed on ribosomes pass into ER lumen where they are modified. Then, the modified proteins move into the transitional area where the ER buds off transport vesicles carrying the proteins to the Golgi apparatus. Here, they are further processed and packaged into secretory vesicle for export by exocytosis at the plasma membrane. Examples of secretory proteins include, mucus, digestive enzymes and hormones.
- Detoxification. The SER brings about detoxification in the liver, i.e., it converts harmful materials (drugs, insecticides, pollutants and poisons) into harmless substances for excretion by the cell.
- Formation of organelles. The SER produces Golgi apparatus, lysosomes and vacuoles.
- Membrane formation. Plasma membrane and other cellular membranes are formed by ER.