Cell biology

Lore & Background

Cells were first seen in 17th-century Europe with the invention of the compound microscope. These observations preceded the cell theory, which states that all living things are made up of cells and that cells are organisms' functional and structural units. Nineteen years later, Rudolf Virchow added that all cells come from the division of pre-existing cells.

Reader's Guide

Cell biology research uses techniques such as microscopy, cell culture, and cell fractionation to study how cells function, providing insight into larger organisms. Knowing the components of cells and how they work is fundamental to all biological sciences and essential for research in biomedical fields such as cancer and other diseases. Research in cell biology is interconnected with genetics, molecular genetics, molecular biology, medical microbiology, immunology, and cytochemistry. The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology, commonly used to investigate diseases such as cancer and infectious diseases, for example, the Pap smear for cervical cancer. Viruses are not considered in cell biology as they lack the characteristics of a living cell and are studied in virology.

Did You Know?

• Rudolf Virchow added that all cells come from the division of pre-existing cells.

The Discovery of the Cell and the Birth of Cell Theory

He borrowed the Latin word cellula, meaning 'small room,' to name these structures, and thus the term 'cell' entered scientific vocabulary. For nearly two centuries the concept remained descriptive rather than foundational. Their framework rested on three pillars: every living organism is built from one or more cells, the cell serves as the fundamental unit of both structure and function across all life, and every new cell arises from a pre-existing one. This was a radical unification, placing everything from the simplest microbe to the most complex animal under a single organizational principle. The timeline stretches even further back: cells first appeared on Earth roughly four billion years ago, making them among the oldest continuous biological entities in our planet's history. Most individual cells remain invisible to the unaided eye, requiring magnification to reveal their presence.

The Great Divide: Prokaryotes and Eukaryotes

Every organism on Earth falls into one of two broad categories based on its cellular architecture. Prokaryotes, which encompass all bacteria and archaea, are exclusively single-celled and lack a membrane-bound nucleus; instead, their genetic material sits in a region called the nucleoid, in direct contact with the cytoplasm. Eukaryotes, by contrast, house their DNA inside a nucleus enclosed by a nuclear membrane, and they can exist as either single-celled organisms—such as many protists, certain yeasts, and some algae—or as multicellular forms like all animals, all plants, most fungi, and some algal species. In multicellular eukaryotes, the body's diploid cells are termed somatic cells, distinct from the haploid gametes involved in reproduction. Eukaryotic cells also harbor additional membrane-bound organelles: mitochondria that fuel cellular processes with energy, and, in plant cells, chloroplasts that manufacture sugars through photosynthesis. Ribosomes, however, appear in both groups, though their structures differ. Prokaryotic cells are generally simpler and smaller, and their organelles tend to be membrane-less and less complex than their eukaryotic counterparts.

Bacterial Architecture: Envelopes, Genomes, and Microcompartments

A typical bacterium is wrapped in a layered cell envelope. The innermost layer is the plasma membrane, often reinforced by a peptidoglycan cell wall that prevents the cell from bursting under osmotic pressure. Some species add a third, gelatinous capsule—composed of polysaccharides in pneumococci and meningococci, polypeptides in Bacillus anthracis, or hyaluronic acid in streptococci. Mycoplasma, an exception, possesses only the membrane. Inside, a single circular chromosome typically occupies the nucleoid, though some bacteria carry multiple or even linear chromosomes. Extrachromosomal plasmids, usually circular, can encode traits like antibiotic resistance; notably, linear plasmids have been found in spirochete species including Borrelia, the Lyme disease agent. Bacteria also build protein-based microcompartments such as gas vesicles, carboxysomes, and encapsulin nanocompartments, and certain species harbor membrane-bound organelles like the magnetosome in magnetotactic bacteria or the anammoxosome in anammox bacteria. Surface appendages—flagella for motility, fimbriae for attachment, and conjugative pili for cell-to-cell communication—extend through the envelope. Most prokaryotes measure between 0.1 and 5.0 micrometers, though Thiomargarita magnifica reaches an average length of one centimeter and can stretch to two, making it visible without magnification.

Archaea: A Distinct Prokaryotic Domain

Although archaea share the prokaryotic blueprint of being single-celled and lacking a true nucleus, they carry several structural and metabolic features that set them apart from bacteria. Their cell envelopes generally consist of a plasma membrane plus a cell wall, but Thermoplasma is a notable exception, possessing only the membrane. The membrane lipids themselves are distinctive: rather than the ester-linked lipids common in bacteria and eukaryotes, archaeal membranes are built from ether-linked lipids, a chemical difference that likely confers stability under extreme conditions. The archaeal cytoskeleton includes homologues of the eukaryotic proteins actin and tubulin, hinting at deep evolutionary connections. Metabolically, methanogenesis—a process of producing methane—is a uniquely archaeal pathway. Where bacteria use flagella for movement, archaea deploy the archaellum, a structurally different appendage. Their DNA is organized into circular chromosomes, and like their bacterial relatives, they secrete substances from their membranes. Archaea, together with bacteria, represent two of the three domains of life and were likely the first forms of life to emerge on Earth, roughly four billion years ago.