What Are Biological Cells?

Biological cells are the fundamental structural and functional units of all living organisms, from the simplest bacteria to complex multicellular animals and plants. First described systematically by Robert Hooke in 1665 and later formalized in cell theory during the 1830s, the cell provides the physical boundary within which life's chemical processes occur. Every cell coordinates metabolism, responds to environmental signals, maintains internal conditions, and replicates genetic information. These functions are carried out through a highly organized system of molecular machinery distributed across the cell's interior.

Cells are broadly classified into two categories: prokaryotic cells, which lack a membrane-bound nucleus and include bacteria and archaea, and eukaryotic cells, which possess a nucleus and a range of specialized organelles and make up fungi, plants, animals, and protists. Despite this structural diversity, all cells share core features: a plasma membrane enclosing the cytoplasm, genetic material encoding heritable information, and ribosomes for protein synthesis.

Biochemistry of the Cell

The internal chemistry of a cell involves thousands of simultaneous biochemical reactions organized into metabolic pathways. Carbohydrates, lipids, proteins, and nucleic acids are the four major classes of biological macromolecules, each synthesized through enzyme-catalyzed reactions and each serving distinct structural or informational roles. Energy currency in the form of adenosine triphosphate (ATP) is generated primarily through cellular respiration in mitochondria, or through photosynthesis in the chloroplasts of plant cells. The NCBI Bookshelf's Molecular Biology of the Cell covers these biochemical systems in detail, including enzyme kinetics, signal transduction cascades, and the regulatory mechanisms that coordinate cellular metabolism.

Biomembranes

The plasma membrane and the internal membranes of organelles are composed of phospholipid bilayers embedded with proteins, a model established definitively by Singer and Nicolson's fluid mosaic model in 1972. Biomembranes perform selective permeability, controlling the passage of ions, nutrients, and signaling molecules between compartments. Membrane proteins serve as receptors, channels, pumps, and structural anchors, and their dysfunction underlies many inherited and acquired diseases. Research published in PMC on biological membranes details the biophysical properties of lipid bilayers and how membrane composition affects cellular signaling and mechanical integrity. In bacteria and archaea, the plasma membrane also functions as the site of electron transport and energy generation, roles performed by organelle membranes in eukaryotes.

DNA and Genetic Information

Every cell contains a copy of the organism's genome, stored as a double-helical sequence of deoxyribonucleic acid. The NCBI account of DNA structure and function describes how the four-nucleotide code carried in DNA is transcribed into messenger RNA and translated into proteins by the ribosome, a process conserved across all domains of life. In eukaryotes, DNA is organized into chromosomes within the nucleus and tightly wrapped around histone proteins in a structure called chromatin. Gene expression is controlled by regulatory sequences, transcription factors, and epigenetic modifications that determine which proteins are produced in a given cell type or physiological state. The fidelity of DNA replication and repair is essential for cell viability and for preventing mutations that can lead to cancer.

Applications

Biological cells have applications in a wide range of scientific and engineering fields, including:

• Biotechnology and recombinant protein production using engineered cell lines
• Cellular therapies, including stem cell transplantation and CAR-T immunotherapy
• Tissue engineering and the construction of in vitro organ models
• Drug screening using cell-based assays
• Study of microorganisms in fermentation, probiotics, and industrial bioprocessing