Cell (biology)

Cells are bounded systems that carry out the chemical activities required for life. A plasma membrane separates the interior from the environment and regulates exchange, while the cytoplasm contains water, ions, macromolecules, and molecular machinery. Cells also contain genetic material, normally DNA, and ribosomes that translate genetic instructions into proteins.¹

Prokaryotic cells, found in Bacteria and Archaea, lack a membrane-enclosed nucleus; their DNA occupies a nucleoid region. Eukaryotic cells have a nucleus and specialized membrane-bound organelles, including mitochondria, endoplasmic reticulum, and Golgi apparatus. This division describes cellular organization rather than a simple ladder of complexity: some prokaryotes have elaborate internal membranes, and some eukaryotic cells are structurally reduced.

Cellular life depends on coordinated information flow, chemical transformation, and compartmentalization. DNA is replicated and expressed through RNA and protein synthesis, while regulatory networks control which genes are active. Membrane proteins mediate transport and signaling, and the cytoskeleton helps organize the interior, move materials, and change cell shape.²

Mitochondria generate much of the usable ATP in most animal, plant, and fungal cells; chloroplasts perform photosynthesis in plants and algae. Both organelles contain their own genomes and ribosomes, evidence consistent with endosymbiotic theory, which proposes that they descended from once-independent bacteria.³ Cells obtain nutrients by transport, ingestion, or synthesis, and maintain internal conditions through homeostasis.

New cells arise from pre-existing cells through regulated growth and division. In bacteria and archaea, binary fission generally produces two daughter cells; eukaryotic somatic cells divide by mitosis, preserving chromosome number, whereas meiosis produces reproductive cells with reduced chromosome number and reshuffled genetic combinations.⁴

Multicellular organisms depend on cooperation among specialized cells. Differentiation changes patterns of gene expression rather than usually changing the genome itself, producing neurons, muscle cells, epithelial cells, immune cells, and many other forms. Cell signaling coordinates development, tissue repair, metabolism, and programmed cell death. Errors in division or signaling can contribute to cancer, while controlled cell death is essential for normal development and tissue maintenance.

Cells do not always fit the textbook picture of a single nucleus enclosed in a compact unit. Mature mammalian red blood cells discard their nuclei to maximize space for hemoglobin, while skeletal muscle fibers form large multinucleate syncytia through the fusion of precursor cells. Some fungi and algae also grow as networks in which cytoplasm extends across many compartments.

The boundaries of cellularity are especially important in biology. Viruses contain genetic material and evolve, but lack the independent metabolism and ribosome-based protein-production machinery normally associated with cells; they therefore depend on host cells for replication and are generally not classified as living cells.⁵ Cell theory likewise has a historical qualification: mitochondria and chloroplasts retain partial genetic independence, and no known cell survives indefinitely without exchanging matter and energy with its environment.

Plasma membrane

A selectively permeable lipid bilayer enclosing the cell and regulating communication and transport.

Prokaryote

An organism whose cells lack a membrane-bound nucleus, including Bacteria and Archaea.

Eukaryote

An organism whose cells contain a nucleus and other membrane-bound organelles.

Organelle

A specialized structure within a cell that performs a particular function.

Endosymbiotic theory

The explanation that mitochondria and chloroplasts originated from symbiotic bacteria living inside an ancestral eukaryotic cell.