4.11 : Peroxisomes and Mitochondria

Peroxisomes and mitochondria are two important oxygen-utilizing organelles in eukaryotic cells. Mitochondria carry out cellular respiration—the process that converts energy from food into ATP. Peroxisomes carry out a variety of functions, primarily breaking down different substances, such as fatty acids.

The peroxisome is a single membrane-bound cellular organelle that can perform several different functions, including lipid metabolism and chemical detoxification. The enzymes within peroxisomes serve to transfer hydrogen atoms from various molecules to oxygen, producing hydrogen peroxide (H₂O₂). In this way, peroxisomes can neutralize poisons such as alcohol.

Peroxisomes oversee reactions that neutralize free radicals. Peroxisomes produce large amounts of toxic H₂O₂ in the process but contain enzymes that convert H₂O₂ into water and oxygen, releasing them safely into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so that they do not wreak havoc in the cells. The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body, and liver cells contain an exceptionally high number of peroxisomes. The kidneys also contain a high number of peroxisomes for blood detoxification.

Mitochondria are surrounded by a double membrane— a smooth outer membrane and an inner membrane that has many folds called cristae. The space within the inner membrane is called the matrix. During cellular respiration, pyruvate from the breakdown of glucose in the cytoplasm travels into the matrix, where it enters the citric acid cycle. Then, oxidative phosphorylation through the electron transport chain occurs in the inner membrane of the mitochondria, producing a significant amount of ATP. The cristae increase the surface area of the inner membrane, providing more regions for ATP production.

Both peroxisomes and mitochondria are self-replicating, but mitochondria additionally have their DNA and ribosomes, enabling them to produce their proteins. Mitochondria and peroxisomes are both highly concentrated in cells where they are needed the most. For example, liver cells—which break down toxic substances in the blood—have high numbers of peroxisomes, and muscle cells—which have large energy requirements—are rich in mitochondria.