Inside all cells, there are number of different organelles, which are specialized for different functions within the cell. Some organelles occur in all cells, while others only occur in specific types of cells. The endoplasmic reticulum is an organelle that occurs in all cells.
The endoplasmic reticulum, or ER, is actually a system of membranes that runs throughout the inside of all cells. When a cell is stained and viewed using an electron microscope, the ER often appears as a system of flattened cavities. The cavities run parallel to each other and are continuous with the membrane surrounding the nucleus, the nuclear membrane. The enclosed flattened sacs are often referred to as cisterna.
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There are two different types of endoplasmic reticulum, smooth ER and rough ER. Rough ER is found within all cell types, while smooth ER is only seen in certain parts of some cells. All endoplasmic reticulum performs a number of different functions, which includes providing a large surface area for chemical reactions in a cell, providing a pathway for transporting materials throughout the cell, collecting and storing material produced in cell and providing the structure that maintains the shape of the cell. Both rough and smooth ER also have specialized functions.
Rough ER is so named as it has a rough or granular appearance. This is due to the fact that there are a number of ribosomes attached to the membrane. Ribosomes are tiny organelles that are made of RNA and protein. They can be found free-floating in the cytoplasm of the cell and also attached to the ER. Ribosomes are the sites of protein synthesis in the cell.
Most endoplasmic reticulum found in cells is rough ER. Due to its affiliation with the ribosomes, the general function of rough ER is to isolate and transport the proteins that have been constructed at the ribosomes. Many of the proteins that are made within a cell are not needed within that cell, but are actually secreted out of the cell. The rough ER provides a method of moving these proteins to different parts of the cell, whether for use or to be secreted out of the cell.
Smooth ER is only found in some cells. It is not continuous with the rough ER and appears more like tubular cavities compared to the flattened ones of the rough ER. Smooth ER does not have ribosomes attached to its surface. The function of smooth ER has to do with lipid synthesis, so it is found in cells that produce and secrete lipids. In particular, smooth ER is often found in the cells of certain glands and the liver.