In eukaryotic cells, peroxisomes and mitochondria are the two major sites of oxygen utilization. Peroxisomes are single membrane-bound organelles. They contain up to 50 different enzymes, which allow for biosynthetic and degradative functions, such as oxidizing long-chain fatty acids and producing hydrogen peroxide as a by-product. Because hydrogen peroxide is cytotoxic, catalase — a peroxisomal enzyme, rapidly converts it into water and oxygen. Catalase can also break down alcohol using hydrogen peroxide into acetaldehyde and water. The mitochondrion is a double membrane-bound organelle found in nearly all eukaryotic cells except red blood cells. Its inner membrane is folded many times to form cristae, which contain the electron transport chain— protein complexes responsible for producing ATP through oxidative phosphorylation. Because mitochondria are the major sites of ATP synthesis, they are present in greater numbers in tissues with high energy requirements, such as muscle and liver tissues. Mitochondria are also sites for the oxidation of most dietary fatty acids producing large amounts of acetyl coenzyme A, which can be used to generate ATP.