Peroxisomes: Eukaryotic Organelles

Peroxisomes are small organelles found in eukaryotic plant and animal cells. Hundreds of these round organelles can be found within a cell. Also known as microbodies, peroxisomes are bound by a single membrane and contain enzymes that produce hydrogen peroxide as a by-product. The enzymes decompose organic molecules through oxidation reactions, producing hydrogen peroxide in the process. Hydrogen peroxide is toxic to the cell, but peroxisomes also contain an enzyme that is capable of converting hydrogen peroxide to water. Peroxisomes are involved in at least 50 different biochemical reactions in the body. Types of organic polymers that are broken down by peroxisomes include amino acids, uric acid, and fatty acids. Peroxisomes in liver cells help to detoxify alcohol and other harmful substances through oxidation.

Peroxisomes Function

In addition to being involved in the oxidation and decomposition of organic molecules, peroxisomes are also involved in synthesizing important molecules. In animal cells, peroxisomes synthesize cholesterol and bile acids (produced in the liver). Certain enzymes in peroxisomes are necessary for the synthesis of a specific type of phospholipid that is necessary for the building of heart and brain white matter tissue. Peroxisome dysfunction can lead to the development of disorders that affect the central nervous system as peroxisomes are involved in producing the lipid covering (myelin sheath) of nerve fibers. The majority of peroxisome disorders are the result of gene mutations that are inherited as autosomal recessive disorders. This means that individuals with the disorder inherit two copies of the abnormal gene, one from each parent.

In plant cells, peroxisomes convert fatty acids to carbohydrates for metabolism in germinating seeds. They are also involved in photorespiration, which occurs when carbon dioxide levels become too low in plant leaves. Photorespiration conserves carbon dioxide by limiting the amount of CO₂ available to be used in photosynthesis.

Peroxisome Production

Peroxisomes reproduce similarly to mitochondria and chloroplasts in that they have the ability to assemble themselves and reproduce by dividing. This process is called peroxisomal biogenesis and involves the building of the peroxisomal membrane, intake of proteins and phospholipids for organelle growth, and new peroxisome formation by division. Unlike mitochondria and chloroplasts, peroxisomes have no DNA and must take in proteins produced by free ribosomes in the cytoplasm. The uptake of proteins and phospholipids increases growth and new peroxisomes are formed as the enlarged peroxisomes divide.

Eukaryotic Cell Structures

In addition to peroxisomes, the following organelles and cell structures can also be found in eukaryotic cells:

• Cell Membrane: The cell membrane protects the integrity of the interior of the cell. It is a semi-permeable membrane that surrounds the cell.
• Centrioles: When cells divide, centrioles help to organize the assembly of microtubules.
• Cilia and Flagella: Both cilia and flagella aid in cellular locomotion and can also help move substances around cells.
• Chloroplasts: Chloroplasts are the sites of photosynthesis in a plant cell. They contain chlorophyll, a green substance that can absorb light energy.
• Chromosomes: Chromosomes are located in the cell's nucleus and carry heredity information in the form of DNA.
• Cytoskeleton: The cytoskeleton is a network of fibers that support the cell. It can be thought of as the cell's infrastructure.
• Nucleus: The cell's nucleus controls cell growth and reproduction. It is surrounded by a nuclear envelope, a double-membrane.
• Ribosomes: Ribosomes are involved in protein synthesis. Most often, individual ribosomes have both a small and large subunit.
• Mitochondria: Mitochondria provide energy for the cell. They are considered the cell's "powerhouse."
• Endoplasmic Reticulum: The endoplasmic reticulum synthesizes carbohydrates and lipids. It also produces proteins and lipids for a number of cell components.
• Golgi Apparatus: The golgi apparatus manufactures, stores, and ships certain cellular products. It can be thought of as the shipping and manufacturing center of the cell.
• Lysosomes: Lysosomes digest cellular macromolecules. They contain a number of hydrolytic enzymes that help to break down cellular components.