Organelle

An organelle is a specialized subunit within a cell that carries out a defined function. In eukaryotic cells, organelles are typically membrane-bound compartments that segregate chemical reactions from the cytosol. Some cell components, such as ribosomes, are not surrounded by membranes but are commonly treated as organelles in broader usage.

Major examples include the nucleus, which houses the genome and coordinates gene expression; mitochondria, the main source of cellular energy; chloroplasts in plants and algae, which perform photosynthesis; the endoplasmic reticulum and Golgi apparatus, which synthesize, fold, and sort proteins and lipids; lysosomes and peroxisomes, which carry out digestion and detoxification; and vacuoles, which store water and other substances.

Organelles are generally enveloped by lipid bilayers, with selective transport systems that regulate material passage. The nucleus is enclosed by the double membrane called the nuclear envelope. Mitochondria and chloroplasts commonly contain their own circular DNA and ribosomes, reflecting their endosymbiotic origins. Many organelles work together in pathways such as protein trafficking and energy metabolism.

Understanding organelles highlights the cellular organization that underpins metabolism, growth, and response to the environment. Variations in organelle number and size across cell types contribute to specialization.