Cell theory

Cell theory states that all living organisms consist of one or more cells, cells are life’s basic structural and functional units, and new cells arise from pre-existing cells.

Cell: The smallest unit of life capable of carrying out the processes needed for living. The theory identifies cells as life’s basic structural and functional units.
Matthias Jakob Schleiden: A German botanist who argued in 1838 that plants are composed of cells. His claim extended cellular organization to the plant kingdom.
Cell biology: The branch of biology that studies cells, their structures, functions, and interactions. It investigates the units that cell theory places at the center of life.
Virus: An infectious agent that replicates inside host cells but lacks independent cellular organization. Viruses challenge simple definitions of life without contradicting the theory’s account of cellular organisms.
Cell membrane: A selectively permeable boundary that separates a cell’s interior from its surroundings. Cell boundaries helped establish the cell as a distinct unit of living structure.
Theodor Schwann: A German physiologist who argued in 1839 that animals are composed of cells. His work extended Schleiden’s proposal to animals, helping form a general theory.
Histology: The study of the microscopic structure of tissues. Tissues can be understood as organized collections of cells, as cell theory proposes.
Endosymbiotic theory: The theory that mitochondria and chloroplasts originated from bacteria living inside ancestral cells. It explains the origins of key cell structures while retaining cells as life’s basic units.
Microscope: An instrument that makes objects too small to see unaided visible at larger scales. Microscopes made cells observable and provided the evidence behind the theory.
Rudolf Virchow: A German physician who popularized the principle that every cell comes from another cell. His formulation supplied the theory’s claim about the origin of new cells.

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