Home Science Biology Cells, Organs & Tissues Alternate titles: SER, smooth ER Show smooth endoplasmic reticulum (SER), meshwork of fine disklike tubular membrane vesicles, part of a continuous membrane organelle within the
cytoplasm of eukaryotic cells, that is involved in the synthesis and storage of lipids, including
cholesterol and phospholipids, which are used in the production of new cellular membrane. The smooth endoplasmic reticulum (SER) is distinguished from the rough endoplasmic reticulum (RER), the other basic type of
endoplasmic reticulum, by its lack of ribosomes, which are protein-synthesizing particles that can be found attached to the outer surface of the RER to give the membrane its “rough” appearance. SER occurs both in
animal and in plant cells. The function of the SER can vary, depending on cell type. In some cells, such as those of the adrenal gland and certain other
endocrine glands, it plays a key role in the synthesis of steroid hormones from cholesterol. In the liver, enzymes in the SER
catalyze reactions that render drugs, metabolic wastes, and harmful chemicals water-soluble, thereby contributing to their detoxification, or removal, from the body. The SER also plays a role in the conversion of
glycogen to glucose, with glucose-6-phosphatase, an enzyme present in SER, catalyzing the final step in glucose production in the liver. Read More on This Topic cell: The smooth endoplasmic reticulum The functions of the SER, a meshwork of fine tubular membrane vesicles, vary considerably from cell to cell. One important role is the synthesis... In skeletal muscle cells, SER occurs as a specialized membrane structure known as the sarcoplasmic reticulum. The sarcoplasmic reticulum is a critical storage site for calcium ions, taking up the ions from the cytoplasm. It also releases calcium ions when the muscle cell is triggered by nerve stimuli, resulting in muscle contraction. In this way, the sarcoplasmic reticulum helps regulate calcium ion concentrations in the cytoplasm of skeletal muscle cells. The sarcoplasmic reticulum is also found in smooth muscle cells, though in a more loosely organized form than in skeletal muscle. Kara Rogers The endoplasmic reticulum can either be smooth or rough, and in general its function is to produce proteins for the rest of the cell to function. The rough endoplasmic reticulum has on it ribosomes, which are small, round organelles whose function it is to make those proteins. Sometimes, when those proteins are made improperly, the proteins stay within the endoplasmic reticulum. They're retained and the endoplasmic reticulum becomes engorged because it seems to be constipated, in a way, and the proteins don't get out where they're suppose to go. Then there's the smooth endoplasmic reticulum, which doesn't have those ribosomes on it. And that smooth endoplasmic reticulum produces other substances needed by the cell. So the endoplasmic reticulum is an organelle that's really a workhorse in producing proteins and substances needed by the rest of the cell. Quick look Rough ER (RER) is involved in some protein production, protein folding, quality control and despatch. It is called ‘rough’ because it is studded with ribosomes Smooth E R (SER) is associated with the production and metabolism of fats and steroid hormones. It is ‘smooth’ because it is not studded with ribosomes and is associated with smooth slippery fats. To view a micrograph of ER interpreted using the Gridpoint cross-hairs device, click here. CELLS NEED THE ROUGH AND THE SMOOTH Think of a cell as a “multitude of membranes” we said in an earlier section. This statement certainly applies to the endoplasmic reticulum an organelle found in eukaryotic cells.
There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). Both types are present in plant and animal cells. The two types of ER often appear as if separate, but they are sub-compartments of the same organelle. Cells specialising in the production of proteins will tend to have a larger amount of rough ER whilst cells producing lipids (fats) and steroid hormones will have a greater amount of smooth ER. Part of the ER is contiguous with the nuclear envelope. The Golgi apparatus is also closely associated with the ER and recent observations suggest that parts of the two organelles, i.e. the ER and the Golgi complex, are so close that some chemical products probably pass directly between them instead of being packaged into vesicles (droplets enclosed within a membrane) and transported to them through the cytoplasm ROUGH ENDOPLASMIC RETICULUM This is an extensive organelle composed of greatly convoluted but flattish sealed sacs, which are contiguous with the nuclear membrane. It is called ‘rough’ endoplasmic reticulum because it is studded on its outer surface (the surface in contact with the cytosol) with ribosomes. These are called membrane bound ribosomes and are firmly attached to the outer cytosolic side of the ER About 13 million ribosomes are present on the RER in the average liver cell. Rough ER is found throughout the cell but the density is higher near the nucleus and the Golgi apparatus. Ribosomes on the rough endoplasmic reticulum are called ‘membrane bound’ and are responsible for the assembly of many proteins. This process is called translation. Certain cells of the pancreas and digestive tract produce a high volume of protein as enzymes. Many of the proteins are produced in quantity in the cells of the pancreas and the digestive tract and function as digestive enzymes. The rough ER working with membrane bound ribosomes takes polypeptides and amino acids from the cytosol and continues protein assembly including, at an early stage, recognising a ‘destination label’ attached to each of them. Proteins are produced for the plasma membrane, Golgi apparatus, secretory vesicles, plant vacuoles, lysosomes, endosomes and the endoplasmic reticulum itself. Some of the proteins are delivered into the lumen or space inside the ER whilst others are processed within the ER membrane itself. In the lumen some proteins have sugar groups added to them to form glycoproteins. Some have metal groups added to them. It is in the rough ER for example that four polypeptide chains are brought together to form haemoglobin. Protein folding unit Protein quality control section Rigorous quality control plays a part in cystic fibrosis From Rough ER to Golgi SMOOTH ENDOPLASMIC RETICULUM Smooth ER is more tubular than rough ER and forms an interconnecting network sub-compartment of ER. It is found fairly evenly distributed throughout the cytoplasm. Smooth ER – the detox stop The contraction of muscle cells is triggered by the orderly release of calcium ions. These ions are released from the smooth endoplasmic reticulum. SUMMARY
Amended 19.11.15 DA. What ER is the site of protein synthesis?Ribosomes are the sites in a cell in which protein synthesis takes place. Cells have many ribosomes, and the exact number depends on how active a particular cell is in synthesizing proteins. For example, rapidly growing cells usually have a large number of ribosomes (Figure 5).
Is protein synthesis smooth ER or rough ER?Cells specialising in the production of proteins will tend to have a larger amount of rough ER whilst cells producing lipids (fats) and steroid hormones will have a greater amount of smooth ER.
Is the smooth ER involved in protein production?As discussed below, there are two distinct types of ER that perform different functions within the cell. The rough ER, which is covered by ribosomes on its outer surface, functions in protein processing. The smooth ER is not associated with ribosomes and is involved in lipid, rather than protein, metabolism.
Which endoplasmic reticulum synthesizes proteins?The ribosome that is synthesizing the protein is directly attached to the ER membrane. These membrane-bound ribosomes coat the surface of the ER, creating regions termed rough endoplasmic reticulum, or rough ER (Figure 12-36A).
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