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The endoplasmic reticulum (ER) is a system of membranes found in all cells, with two types: smooth and rough. The rough ER has ribosomes attached to it and is responsible for transporting proteins, while the smooth ER is involved in lipid synthesis and is found in certain cells.
Within all cells, there are different organelles, specialized for different functions within the cell. Some organelles occur in all cells, while others occur only in specific cell types. The endoplasmic reticulum is an organelle found in all cells.
The endoplasmic reticulum, or ER, is actually a system of membranes that runs through 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 that surrounds the nucleus, the nuclear membrane. The enclosed flattened sacs are often called a cistern.
There are two different types of endoplasmic reticulum, smooth ER and rough ER. Rough ER is found in all cell types, while smooth ER is found only in parts of some cells. The entire endoplasmic reticulum performs a number of different functions, including providing a large surface area for chemical reactions in a cell, providing a pathway for the transport of materials through the cell, collecting and storing material produced in the cell, and providing structure which maintains the shape of the cell. Both rough and smooth ER also have specialized functions.
Rough ER is so named because it has a rough or grainy appearance. This is due to the fact that there are a number of ribosomes attached to the membrane. Ribosomes are tiny organelles made up of RNA and proteins. They can be found floating in the cytoplasm of the cell and also attached to the ER. Ribosomes are the sites of protein synthesis in the cell.
Most of the endoplasmic reticulum found in cells is rough ER. Due to its affiliation with ribosomes, the general function of the rough ER is to isolate and transport proteins that have been built into ribosomes. Many of the proteins made within a cell are not needed within that cell, but are actually secreted from the cell. Crude ER provides a method to move these proteins to different parts of the cell, both for use and to be secreted from the cell.
The smooth ER is only found in some cells. It is not continuous with the rough ER and looks more like tubular cavities than the flattened ones in the rough ER. The smooth ER has no ribosomes attached to its surface. The function of the smooth ER has to do with lipid synthesis, so it is found in cells that produce and secrete lipids. In particular, the smooth ER is often found in the cells of certain glands and in the liver.
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