eukaryotic cell
/ yoo-KAIR-ee-OT-ik SEL /
Now picture a large house with many rooms — a kitchen, a bedroom, a workshop, a furnace room — each walled off so that very different activities can happen at once without interfering. A eukaryotic cell is organized like that house. Its interior is divided by membranes into specialized compartments, and the most important of these is the nucleus, a membrane-bound chamber that holds the cell's DNA. 'Eukaryote' is Greek for 'true kernel (nucleus).'
Besides the nucleus, a eukaryotic cell contains many membrane-bound organelles, each doing a job: mitochondria that release energy, an endoplasmic reticulum and Golgi apparatus that build and ship proteins, and (in plants and algae) chloroplasts that capture sunlight. This compartmentalization lets eukaryotic cells be larger and more internally complex than prokaryotes — typically 10 to 100 micrometers across, often 10 or more times the diameter of a bacterium. Every animal, plant, fungus, and protist is built from eukaryotic cells.
Because their parts are separated, eukaryotic cells can specialize and cooperate, which is what made large multicellular bodies like ours possible. A widely accepted idea, the endosymbiotic theory, holds that mitochondria and chloroplasts descend from ancient free-living bacteria that were engulfed and kept — which is why those organelles still carry their own small genomes. So a eukaryotic cell is, in a real sense, a community that became a single being.
A single cell from the lining of your cheek is eukaryotic: it has a clearly visible nucleus, energy-producing mitochondria, and a cytoskeleton — a whole tiny factory with separate workrooms, all in something far too small to see without a microscope.
A human cheek cell: nucleus, mitochondria, and membrane compartments mark it as eukaryotic.
Eukaryotic does not mean multicellular: yeasts and amoebas are single eukaryotic cells. The defining feature is the membrane-bound nucleus, not body size.