What Are Organelles?

Organelles are specialized subunits within a cell, each carrying out a distinct function, in the way that organs divide labor within a body. The term literally means little organ, and it covers structures ranging from the nucleus, which stores and transcribes the genome, to ribosomes, which assemble proteins from amino acids. Most organelles in eukaryotic cells are enclosed by their own lipid membrane, which creates a chemically separate compartment where reactions can proceed at concentrations, pH values, and redox states that would be incompatible with the rest of the cytoplasm.

Compartmentalization is the defining feature of eukaryotic cell architecture and the main structural difference from bacteria and archaea, which lack membrane-bound organelles though they do contain ribosomes and other non-membranous assemblies. Cell biology traces its origins to Robert Hooke's description of cells in 1665, but the functional dissection of organelles depended on twentieth-century tools: differential centrifugation to separate them by density, electron microscopy to resolve their structure, and fluorescent protein tagging to watch them move in living cells. A summary of the standard inventory appears in the cell structure module maintained by the National Cancer Institute.

Membrane-Bound Organelles

The nucleus holds chromosomal DNA behind a double membrane pierced by nuclear pore complexes that regulate what enters and leaves. The endoplasmic reticulum extends from the nuclear envelope as a network of tubules and sheets: its rough regions, studded with ribosomes, fold and modify proteins destined for secretion or for membranes, while its smooth regions synthesize lipids and store calcium. The Golgi apparatus receives that output, adds and trims carbohydrate groups, and sorts the products to their destinations. Lysosomes maintain an acidic interior that supports hydrolytic enzymes for degrading worn components and imported material, and peroxisomes handle fatty acid oxidation and detoxification of hydrogen peroxide. Plant cells add chloroplasts for photosynthesis and a large central vacuole that maintains turgor pressure.

Mitochondria and Endosymbiotic Origin

Mitochondria generate most of the adenosine triphosphate a cell uses, coupling electron transport across the inner membrane to a proton gradient that drives ATP synthase. Their folded cristae expand the membrane area available for that machinery. Mitochondria retain a small circular genome and their own ribosomes, which differ from cytoplasmic ribosomes and translate a handful of proteins locally, and they divide independently of the cell cycle. These features support the endosymbiotic hypothesis, which holds that mitochondria and chloroplasts descend from free-living bacteria engulfed by an ancestral host cell. Beyond energy production, mitochondria regulate calcium signaling, reactive oxygen species, and programmed cell death, a breadth reviewed in work describing mitochondria as the central organelle of the eukaryotic cell.

Ribosomes and Non-Membranous Structures

Not every organelle has a membrane. Ribosomes are ribonucleoprotein machines built from two subunits: the small subunit reads messenger RNA and pairs each codon with the appropriate transfer RNA, and the large subunit catalyzes formation of the peptide bonds that link amino acids into a chain. Their atomic structures, deposited in the Protein Data Bank beginning in 2000 and recognized with the 2009 Nobel Prize in Chemistry, revealed that the catalytic center is made of RNA rather than protein, as described in the structural account of the ribosome. Other non-membranous organelles include the centrosome, which organizes microtubules during division, the nucleolus, where ribosomal subunits are assembled, and the cytoskeleton of actin filaments, intermediate filaments, and microtubules that gives the cell shape and moves cargo.

Applications

The study of organelles has applications in a range of fields, including:

• Drug development, where mitochondrial toxicity screening is a standard safety test
• Diagnosis and treatment of mitochondrial and lysosomal storage diseases
• Antibiotic design, since bacterial ribosomes are a major drug target
• Synthetic biology, including engineered compartments for metabolic pathways
• Biomanufacturing, where secretory pathway capacity limits protein yield
• Bioimaging and biosensor development for subcellular measurement