Tiny particle, the site of protein synthesis, that is present in large numbers in living cells. They occur both as free particles within cells and, in eukaryotes, as particles attached to the membranes of the endoplasmic reticulum. They are 40percnt protein and 60percnt RNA. Ribosomes account for a large proportion of the total RNA of a cell. Proteins newly formed on ribosomes detach and migrate to other parts of the cell to be used.

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In cell biology, a microsome is a small vesicle that is derived from fragmented smooth endoplasmic reticulum (SER) produced when tissues such as liver are mechanically broken (homogenized). Microsomes contain the cell's cytochrome P450 (CYP) enzymes, involved in oxidative metabolism.

Microsomes can be concentrated and separated from other cellular organelles by using a centrifuge to produce differential centrifugation. Unbroken cells, nuclei, and mitochondria sediment out at 10,000g, whereas soluble enzyme and fragmented ER, which contains the P450s, remain in solution (g is the earth's gravitational acceleration). At 100,000g, achieved by faster centrifuge rotation, ER sediments out of solution as a pellet but the soluble enzymes remain in the supernatant. In this way, P450s in microsomes are concentrated and isolated. Microsomes have a reddish brown color, due to the presence of the iron-containing co-factor, heme (haem), in the P450s. P450s are highly abundant in livers of rats, mice and humans.

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