What is an Endoplasmic Reticulum?

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The synthesis of lipids in smooth endoplasmic reticulum

The ER can be seen as a series of membrane-enclosed sacs and tubules that collectively modify proteins and synthesize lipids. If the ER has attached ribosomes it is called rough endoplasmic reticulum (Rough ER) and if it does not it is smooth endoplasmic reticulum (Smooth ER).

All proteins destined for other organelles such as Golgi apparatus, lysosomes, or plasma membrane must be first made by the ER. The proteins must fold, oligomerize, and have disulfide bonds added to make them stable. They also have to be properly glycosylated, a process known as N-linked glycosylation.

Lipids synthesis in the ER is a complex process that requires specific enzymes to be located near each other so they can react simultaneously. Interestingly, lipid-synthesis enzymes in the ER are enriched at the contact sites of the organelles they are targeting. This allows lipids to reach their target organelle 10 times faster than they would via the vesicle-dependent transfer route.

The synthesis of hormones in smooth endoplasmic reticulum

Smooth endoplasmic reticulum (sER) is part of, or a region in, the endoplasmic reticulum found in eukaryotic cells. It is tubular and lacks the ribosomes that give rough endoplasmic reticulum its “ruffled” appearance under an electron microscope. It is associated with lipid metabolism and synthesis, carbohydrate metabolism, regulation of calcium concentration, and drug detoxification. It is also responsible for the production of phospholipids for cell membranes. Proteins produced in the sER are transported to the Golgi complex by transport vesicles that bud from its membrane.

The sER contains enzymes that can be used to synthesize various hormones. These include steroid hormones that are secreted by the adrenal gland and other endocrine cells. The sER can also be used to store and release calcium ions, an important function in neurons and muscle cells that use $Ca_2$ + signaling for excitation and contraction. It is also present in liver cells that handle toxic substances and in hepatocytes that produce cytochrome P450 (CYP) enzymes that eliminate foreign compounds from the body.

The synthesis of steroids in smooth endoplasmic reticulum

The smooth endoplasmic reticulum is an organelle found in the cells of most eukaryotic organisms. It is a tube-like structure and differs from the rough endoplasmic reticulum (RER) in that it does not contain ribosomes. It is associated with lipid metabolism and synthesis, carbohydrate metabolism, regulation of calcium concentration, drug detoxification, and the secretion of steroid hormones. Cells that secrete these products, such as the Leydig cells in the testes and follicular cells in the ovary and sebaceous glands, typically have abundant sER.

The smooth endoplasmic reticulum also transports phospholipids to other cellular organelles, especially the Golgi apparatus, and participates in carbohydrate metabolism by catalyzing the reaction of glycogenolysis. It is particularly prominent in liver cells that process harmful chemicals, endocrine system cells such as those in the adrenals that manufacture steroid hormones, and excitable cells such as neurons and muscle cells that use Ca2+ signaling. In skeletal muscle cells, a specialized sER known as the sarcoplasmic reticulum stores and releases the calcium ions necessary for the coordinated contraction of muscle fibers.