A cell is the fundamental structural and functional unit of cellular life. All known cellular organisms are composed of one or more cells, and cells contain the molecular machinery necessary to maintain biological processes. A cell is surrounded by a selectively permeable cell membrane that separates its internal environment from its surroundings and regulates movement of substances.
Inside the cell are aqueous and molecular environments in which chemical reactions occur, together with genetic material and structures that carry out specific functions. Cells range from tiny bacterial cells to much larger specialised eukaryotic cells, and their shapes and internal organisations vary enormously. Despite this diversity, cells share fundamental features such as genetic information, membranes, ribosomes and metabolic processes. Cell theory provides the foundational framework for understanding this organisation and states in its modern form that living organisms consist of cells, that the cell is a basic unit of life and that new cells arise from pre existing cells.
Cells contain genetic information that provides instructions and regulatory information for biological processes. In most cellular organisms, DNA is the primary hereditary molecule. In prokaryotic cells, the main chromosome is located in a region called the nucleoid rather than enclosed within a membrane bound nucleus. In eukaryotic cells, most DNA is contained inside a nucleus, although mitochondria and chloroplasts also contain their own DNA.
Genes are expressed through processes involving transcription and translation, allowing information encoded in DNA to contribute to the production of proteins and functional RNA molecules. Proteins perform enormous numbers of cellular functions, including catalysis, structural support, transport, signalling and regulation. The relationship between DNA, RNA and proteins is therefore central to cell biology, although modern molecular biology recognises many regulatory and non protein coding roles for RNA and complex mechanisms controlling gene expression.
Cells require energy to maintain their organisation and perform work. ATP is a major short term energy transfer molecule in cells, although other molecules and electrochemical gradients are also essential for energy storage and transfer. Cellular respiration converts energy stored in nutrients into forms that cells can use, while photosynthesis captures light energy in plants, algae and some microorganisms. Metabolism includes many interconnected pathways, such as glycolysis, the citric acid cycle and oxidative phosphorylation in aerobic respiration.
The exact organisation differs among organisms. Prokaryotes lack mitochondria and perform respiratory processes using structures associated with their plasma membranes, whereas eukaryotic cells possess mitochondria that provide major sites for aerobic energy metabolism. Energy is required for active transport, biosynthesis, movement, signalling, cell division, repair and maintenance of concentration gradients. Without continuous energy management, cellular organisation would eventually break down.
Cells maintain boundaries and internal conditions through membranes and transport mechanisms. The plasma membrane is primarily a phospholipid bilayer containing proteins, cholesterol in many eukaryotic membranes and various carbohydrates attached to lipids or proteins. Its selective permeability allows some substances to cross readily while restricting others. Small non polar molecules can often diffuse through the lipid bilayer, whereas ions and many polar molecules require transport proteins.
Passive transport occurs without direct metabolic energy expenditure and includes diffusion, facilitated diffusion and osmosis. Active transport moves substances against electrochemical gradients and requires energy, directly or indirectly. Endocytosis and exocytosis allow eukaryotic cells to move large particles or bulk material across membranes. These processes enable cells to regulate internal composition while exchanging matter with their surroundings.
Cell structure differs according to biological function. Animal cells can specialise into neurons, muscle cells, red blood cells and many other forms. Plant cells can specialise into root cells, guard cells, xylem related cells and photosynthetic cells. Fungal cells have their own characteristic organisation, while bacterial and archaeal cells have prokaryotic organisation. Specialisation often results from selective gene expression rather than different basic genetic material. Cells can communicate using chemical signals, electrical signals, direct membrane interactions and extracellular structures. In multicellular organisms, specialised cells cooperate through tissues and organs, creating levels of organisation that cannot be achieved by isolated cells. Cell differentiation is therefore fundamental to development and multicellular life.
Cell division allows organisms to grow, reproduce or replace damaged cells. Prokaryotic cells commonly divide by binary fission, in which DNA replication is followed by separation into daughter cells. Eukaryotic cells divide through processes such as mitosis and meiosis. Mitosis generally produces genetically similar daughter cells and is important for growth, tissue maintenance and asexual reproduction in some organisms. Meiosis produces reproductive cells with reduced chromosome numbers and introduces genetic variation through mechanisms including crossing over and independent assortment. Cell cycle regulation is critical because uncontrolled division can contribute to cancer. Cells also possess mechanisms for repairing DNA, removing damaged components and, when necessary, initiating programmed cell death. These processes maintain tissue integrity and genomic stability.
The history of cell biology began with microscopy. Robert Hooke used a microscope to examine cork and published observations in the seventeenth century, using the term cell because the small compartments reminded him of small rooms. Antonie van Leeuwenhoek subsequently observed living microscopic organisms and cells with greatly improved lenses. In the nineteenth century, Matthias Schleiden and Theodor Schwann helped formulate cell theory, while Rudolf Virchow contributed the principle that cells arise from pre existing cells. Modern microscopy has expanded far beyond the capabilities of early optical instruments. Electron microscopy reveals ultrastructural details at much smaller scales, while fluorescence microscopy can locate specific molecules and structures. Modern cell biology combines microscopy with molecular genetics, biochemistry, electrophysiology, imaging and computational analysis.
Cells should not be treated as miniature versions of entire organisms because cellular organisation varies substantially among different forms of life. A bacterium can function as a complete organism despite lacking a nucleus and many membrane bound organelles found in eukaryotes. A human red blood cell loses its nucleus during maturation and has a highly specialised structure suited to oxygen transport. Some cells contain extensive internal membrane systems, while others have relatively simple organisation.
Cell size is constrained by factors including surface area to volume ratio, diffusion distances and metabolic requirements, although specialised structures and multicellular organisation can overcome some limitations. The cell concept therefore provides a unifying framework while allowing enormous diversity in size, structure, metabolism, communication and function across life.
