Because rough ER is covered with ribosomes, it has a bumpy appearance when viewed with an electron microscope. Rough ER is primarily involved in the production of proteins that will be exported from the cell to help with other functions of building the plant. Such proteins include antibodies, digestive enzymes, and certain hormones. Amino acid chains are assembled into proteins by the ribo-somes. The protein units needed outside the manu facturing cell are transported into the rough ER for further processing. Once inside, they are shaped into the correct three-dimensional configuration that will be useful outside the cell. Necessary chemicals, such as carbohydrates or sugars, are then added to complete the proteins. The ER then transports these proteins to other areas of the ER, called transitional Er, where they are packed in vesicles to be sent to the Golgi complex for export, or secretion, to other parts of the plant. Occasionally, some of the completed proteins are transported to areas of the manufacturing cell where they are needed.
Because the smooth ER does not have attached ribosomes, it appears relatively smooth when viewed through an electron microscope. It also appears to bud off from vesicles that contain material from the lumen of the ER. Using the many different enzymes that are anchored to its walls, the smooth ER is involved with the synthesis, secretion, and storage of lipids as well as the manufacture of new membranes and the metabolism of carbohydrates.
Lipids are a group of fatty substances needed for building membranes and storing energy in plants. Among the more important lipids are phospho-lipids, which make up major components of the cell membrane. When a plant has excess energy available from photosynthesis, it sometimes stores that energy in the form of lipids known as triglycerides. When the plant is in need of more energy, the triglycerides can be broken down to produce it. Waxes are other important lipids stored in the smooth ER. They form protective coatings on the leaves of plants. Research indicates that smooth ER is also involved in the formation of cellulose for the cell wall.
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