The endoplasmic reticulum is found in both plant and animal cells. It comprises two distinct components: rough endoplasmic reticulum (rough ER or RER) and smooth endoplasmic reticulum (smooth ER or SER). The two types of endoplasmic reticulum have different structures, but they are two parts of the same organelle. They have distinct functions but also work together to process and distribute molecules to other organelles within the cell and to export molecules outside the cell.
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The two types of endoplasmic reticulum in cells are rough ER and smooth ER. They have separate functions but work together to process protein molecules in the cell.
Endoplasmic Reticulum Structure
Rough endoplasmic reticulum is made of a long, folded membrane that forms a series of narrow pockets. The pockets run parallel to each other and are formed from one continuous membrane. The space between the rows of pockets is called the lumen. The “rough” texture of rough ER comes from the ribosomes attached to its folds, giving the membrane a nubby surface.
Smooth endoplasmic reticulum consists of a set of interconnecting narrow tubes that are connected to the outer fold of the rough ER. The tubes are open at one end. The network of the smooth ER takes up less volume in the cell than the rough ER. As its name implies, it has a smooth surface because it is not covered in ribosomes.
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Role in Protein Synthesis and Processing
Protein synthesis occurs in the ribosomes attached to the rough ER. Messenger RNA (mRNA) molecules in the nucleus contain the code for making proteins. The membrane of the rough ER is connected to the nuclear membrane and acts as a conduit for mRNA between the nucleus and the ribosomes.
The main rough ER functions are to process the newly synthesized proteins and package them so they can be carried in vesicles to other organelles or transported to the cell membrane where they will be excreted outside the cell. Many of the proteins are conveyed in vesicles produced by the smooth ER.
Proteins must be folded to be used effectively by the organelles. Before they are transported out of the ER, proteins receive a quality check in the lumen. Unfit molecules are broken down into their components and stored in the lumen until they can be recycled.
Fat Synthesis, Metabolism and Detoxification
The chief function of smooth ER is the production of fats. Two types of fat molecules made in the smooth ER are steroids and phospholipids. Steroids are made in the cells of the adrenal and endocrine glands.
Smooth endoplasmic reticulum has varied roles depending on the type of cells it is found in. In brain and muscle cells, it plays a role in carbohydrate metabolism. Calcium ions necessary for muscle contraction are released from the smooth ER in muscle cells.
In liver cells, it aids in processing toxins such as poisonous substances and drugs by breaking down the chemicals into water-soluble molecules. The smooth ER can expand to temporarily increase its surface area when necessary to process large loads of toxins more efficiently.
The Golgi complex, or Golgi apparatus, is another cell organelle that works in conjunction with the ER and ribosomes in the production of proteins. It is often located in proximity to the endoplasmic reticulum, which allows molecules to be transported easily between the two organelles. After the endoplasmic reticulum processes and packages proteins, the molecules move to the Golgi complex for finalization where they are further modified to be ready for use within or outside the cell.