Smooth endoplasmic reticulum (SER) possesses mixed-function oxidases that detoxify phenobarbital and other drugs.

Explanation:
The smooth endoplasmic reticulum (SER) is a membrane-bound organelle that lacks ribosomes on its surface, giving it a smooth appearance under a microscope. It plays a crucial role in various cellular functions, including lipid metabolism, detoxification, and drug metabolism.

The SER contains a group of enzymes called mixed-function oxidases, also known as cytochrome P450 enzymes. These enzymes are responsible for metabolizing a wide range of substances, including drugs, toxins, and hormones. They achieve this by catalyzing various oxidation reactions, converting these substances into more water-soluble forms that can be easily excreted from the body.

Phenobarbital, a barbiturate drug used as a sedative and anticonvulsant, is primarily metabolized by the liver. When phenobarbital enters the liver cells, it is taken up by the smooth endoplasmic reticulum. The mixed-function oxidases present in the SER then oxidize phenobarbital, converting it into a more water-soluble metabolite.

These mixed-function oxidases are highly versatile enzymes and can metabolize a wide range of drugs and foreign compounds, including other barbiturates, steroids, and environmental toxins. By detoxifying these substances, the smooth endoplasmic reticulum plays a crucial role in protecting the body from the harmful effects of these compounds.

In conclusion, the smooth endoplasmic reticulum possesses mixed-function oxidases that detoxify phenobarbital and other drugs. These enzymes catalyze oxidation reactions, converting drugs and toxins into more water-soluble forms that can be easily eliminated from the body.