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Eukaryotic cells have multiple, membrane-bound structures called peroxisomes containing digestive enzymes that help oxidize organic molecules.

The shape, number,  and size of peroxisomes vary according to cell type and environmental signals. For example, fat cells have numerous small peroxisomes, while kidney cells have fewer and larger ones.

New peroxisomes can be formed from the growth and fission of existing peroxisomes. Or, they can be synthesized in the ER, where peroxisomal membrane proteins are packaged into vesicles that fuse to form peroxisomes. The enzymes are imported from the cytosol into the matrix through membrane-bound translocons.

These enzymes use molecular oxygen to oxidize substrates such as fatty acids and produce hydrogen peroxide as a by-product.

Because hydrogen peroxide is cytotoxic, Catalase, a peroxisomal enzyme, rapidly converts it into water and oxygen.

Besides lipid metabolism, peroxisomes are also involved in synthesizing lipids such as dolichol and cholesterol and contain enzymes needed for cholesterol-derived bile acid synthesis.