What Are Cells (Biology)?

Cells are the fundamental structural and functional units of all living organisms, constituting the smallest entities capable of independent biochemical activity, reproduction, and response to environmental stimuli. All known life on Earth is composed of one or more cells, a principle established by the cell theory formulated by Matthias Schleiden and Theodor Schwann in the 1830s and 1840s. Each cell maintains a boundary with its external environment, carries hereditary information in the form of DNA, and converts chemical energy to sustain its own processes. Biology, biochemistry, biophysics, and biomedical engineering all center on understanding how cells are organized, how they function, and how their collective behavior produces tissues, organs, and organisms.

Cell Structure and Organization

All cells share a set of core structural features. The plasma membrane, a phospholipid bilayer embedded with proteins, encloses the cell contents and controls the passage of molecules into and out of the cell. Inside the membrane, the cytoplasm provides the aqueous medium in which biochemical reactions occur. Ribosomes, found in all cell types, translate messenger RNA into proteins. Genetic material is always present as DNA, either dispersed throughout the cytoplasm in prokaryotes or enclosed in a membrane-bound nucleus in eukaryotes. The NCBI Bookshelf chapter on the origin and evolution of cells documents the conserved molecular machinery shared across cell types and the structural organization that has been maintained across billions of years of evolution.

Prokaryotic and Eukaryotic Cells

The primary classification of cells distinguishes between prokaryotes, which lack a membrane-bound nucleus, and eukaryotes, which possess one. Prokaryotic cells, which include bacteria and archaea, are typically 1 to 10 micrometers in diameter and carry their DNA as a single circular chromosome in the cytoplasm alongside smaller circular plasmids. Eukaryotic cells, found in animals, plants, fungi, and protists, are generally 10 to 100 micrometers in diameter and compartmentalize biochemical processes within membrane-bound organelles. The mitochondrion generates ATP through oxidative phosphorylation; the endoplasmic reticulum synthesizes lipids and processes proteins; the Golgi apparatus packages and sorts proteins for secretion or delivery to other organelles. Plant cells additionally contain chloroplasts, which carry out photosynthesis, and a rigid cell wall external to the plasma membrane. As the Nature Scitable eukaryotic cells resource explains, eukaryotic compartmentalization allows the cell to run incompatible chemical reactions in parallel by separating them spatially.

Cell Division and Reproduction

Cells reproduce by division, with the genetic material duplicated and partitioned between daughter cells before the cytoplasm divides. Prokaryotes divide by binary fission, a relatively simple process in which the circular chromosome replicates and the cell splits into two genetically identical daughters. Eukaryotic cell division follows mitosis or meiosis. Mitosis produces two genetically identical daughter cells and is the basis of growth and tissue repair in multicellular organisms. Meiosis generates four genetically distinct haploid cells and is the mechanism by which sexually reproducing organisms produce gametes. Accurate chromosome segregation during division depends on the spindle apparatus, a structure built from microtubules that attach to chromosomes at kinetochores and pull them to opposite poles of the cell. Errors in this process cause aneuploidy, which is implicated in many cancers. The Technology Networks overview of eukaryotic cell structure provides a detailed survey of these processes in the context of biomedical research.

Applications

Cells (biology) have applications in a range of fields, including:

• Biomedical research, where cultured cells serve as model systems for studying disease mechanisms
• Tissue engineering and regenerative medicine, using stem cells to repair or replace damaged tissues
• Pharmaceutical development, employing cell-based assays to screen drug candidates for efficacy and toxicity
• Synthetic biology, engineering cells to produce therapeutic proteins, biofuels, or biosensor outputs
• Biophysics, studying the mechanical and electrical properties of cells to understand force transduction and signaling