These rejects are stored in the lumen or sent for recycling for eventual breakdown to amino acids. Proteins are subjected to a quality control check and any that are found to be incorrectly formed or incorrectly folded are rejected. It is also in the lumen that an amazing process of quality control checking is carried out. It is in the lumen of the rough ER that proteins are folded to produce the highly important biochemical architecture which will provide ‘lock and key’ and other recognition and linking sites. It is in the rough ER for example that four polypeptide chains are brought together to form haemoglobin. In the lumen some proteins have sugar groups added to them to form glycoproteins. Some of the proteins are delivered into the lumen or space inside the ER whilst others are processed within the ER membrane itself. Proteins are produced for the plasma membrane, Golgi apparatus, secretory vesicles, plant vacuoles, lysosomes, endosomes and the endoplasmic reticulum itself. 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. Many of the proteins are produced in quantity in the cells of the pancreas and the digestive tract and function as digestive enzymes. Certain cells of the pancreas and digestive tract produce a high volume of protein as enzymes. Ribosomes on the rough endoplasmic reticulum are called ‘membrane bound’ and are responsible for the assembly of many proteins. Rough ER is found throughout the cell but the density is higher near the nucleus and the Golgi apparatus. 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. It is called ‘rough’ endoplasmic reticulum because it is studded on its outer surface (the surface in contact with the cytosol) with ribosomes. This is an extensive organelle composed of greatly convoluted but flattish sealed sacs, which are contiguous with the nuclear membrane. 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 The Golgi apparatus is also closely associated with the ER and recent observations suggest that parts of the two organelles, i.e. Part of the ER is contiguous with the nuclear envelope. 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. The two types of ER often appear as if separate, but they are sub-compartments of the same organelle. Both types are present in plant and animal cells. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). (courtesy of Chris Hawes, The Research School of Biology & Molecular Sciences, Oxford Brookes University, Oxford, UK) This is an electron microscope image showing part of the rough endoplasmic reticulum in a plant root cell from maize. Many of these products are made for and exported to other organelles. The organelle called ‘endoplasmic reticulum’ occurs in both plants and animals and is a very important manufacturing site for lipids (fats) and many proteins. This statement certainly applies to the endoplasmic reticulum an organelle found in eukaryotic cells.Ībout 50% of the total membrane surface in an animal cell is provided by endoplasmic reticulum (ER). Think of a cell as a “multitude of membranes” we said in an earlier section. To view a micrograph of ER interpreted using the Gridpoint cross-hairs device, click here. It is ‘smooth’ because it is not studded with ribosomes and is associated with smooth slippery fats. Smooth E R (SER) is associated with the production and metabolism of fats and steroid hormones. It is called ‘rough’ because it is studded with ribosomes Rough ER (RER) is involved in some protein production, protein folding, quality control and despatch.
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