Organelle

An organelle is a specialized structure within a cell that performs a particular function. Many organelles are enclosed by membranes, though some, such as ribosomes, are not.

Cell: The smallest unit of life that can function independently. Organelles are specialized parts within cells, not independent living units.
Compartmentalization: The division of a system into distinct spaces with different conditions or functions. Organelles compartmentalize cell chemistry so incompatible reactions can occur separately.
Mitochondrion: A double-membraned organelle that generates much of a eukaryotic cell's ATP through cellular respiration. Mitochondria show how an organelle's compartments support energy production.
Ribosome: A molecular complex of RNA and proteins that translates messenger RNA into proteins. Ribosomes are organelles by some definitions, despite lacking a surrounding membrane.
Endosymbiotic theory: The theory that some eukaryotic organelles originated as free-living bacteria engulfed by ancestral cells. It explains the bacterial ancestry of mitochondria and chloroplasts.
Cell membrane: A selectively permeable boundary that separates a cell from its surroundings. The cell membrane encloses the cell, while internal membranes divide many organelles.
Endomembrane system: A network of eukaryotic membranes that includes the endoplasmic reticulum, Golgi apparatus, and vesicles. Its connected compartments process and transport many organelle-bound materials.
Chloroplast: A photosynthetic organelle in plants and algae that converts light energy into chemical energy. Chloroplasts specialize in photosynthesis within plant and algal cells.
Nucleus: A membrane-enclosed compartment in eukaryotic cells that contains most of their genetic material. The nucleus is a prominent organelle, but some definitions reserve the term for smaller structures.
Lynn Margulis: An American biologist who developed and championed the modern endosymbiotic theory. Margulis helped establish an evolutionary account of organelle origins.

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