Peroxisomes are small, membrane‑bound organelles that are found throughout the cytoplasm of nearly all eukaryotic cells, including plants, animals, protists, and fungi. Unlike most organelles, which are double‑membrane structures, peroxisomes possess a single lipid bilayer. They do not contain their own DNA and therefore rely on proteins imported from the cytoplasm via peroxisomal targeting signals.
Peroxisomes range from 0.1 to 1 µm in diameter, making them one of the smallest organelles. Their single membrane is composed of phospholipids with hydrophilic head groups facing the cytosol and hydrophobic tails directed inward, creating a compact and dynamic boundary that regulates import and export of metabolites.
Each peroxisome houses at least 50 distinct enzymes. A hallmark of the organelle is a crystalline core rich in urate oxidase, which degrades uric acid. Catalase, the most abundant enzyme, neutralizes the hydrogen peroxide (H₂O₂) produced during metabolic reactions by converting it to water and oxygen. This enzymatic repertoire underpins the peroxisome’s role in detoxification and biosynthesis.
Peroxisomes replicate through a fission mechanism similar to mitochondria. When a peroxisome grows beyond a critical size—thanks to the import of additional proteins and lipids—it divides into two daughter organelles, each inheriting a full complement of enzymes. This self‑replication allows peroxisomes to scale with cellular metabolic demand without genomic input.
Peroxisomes are central to several catabolic pathways. In the liver, they oxidize long‑chain fatty acids and detoxify ethanol, preventing toxic lipid accumulation in neural tissues. They also participate in bile acid synthesis, which is essential for fat digestion and vitamin B₁₂ absorption. In the kidneys, peroxisomal enzymes inhibit the formation of calcium‑based kidney stones.
Reactive oxygen species (ROS) are inevitable by‑products of cellular respiration. Peroxisomes balance ROS production with the activity of catalase and other antioxidant enzymes, protecting both the organelle and the cell from oxidative damage while still permitting ROS‑dependent signaling.
Neurons depend on peroxisomes for the synthesis of plasmalogens, a unique class of phospholipids that are critical components of myelin. Myelin integrity is essential for rapid nerve impulse conduction; deficits in peroxisomal function have been linked to demyelinating disorders such as multiple sclerosis and amyotrophic lateral sclerosis.
In plant cells, peroxisomes facilitate photorespiration by converting phosphoglycerate to glycerate, which is then returned to chloroplasts for the Calvin cycle. During seed germination, peroxisomal β‑oxidation breaks down stored lipids into sugars that fuel early growth, underscoring the organelle’s versatility across kingdoms.
