By Kevin Beck • Updated Aug 30, 2022
Cells are the fundamental units of life, possessing the essential characteristics that define living systems—metabolism, growth, and reproduction. Just as atoms underpin chemistry, cells underlie biology. The human body houses more than 30 trillion cells, each a complex, self‑contained system.
The idea that matter is composed of discrete units dates back to Democritus (5th–4th B.C.). However, it was not until the 17th century, with the advent of the microscope, that scientists could actually observe these units. Robert Hooke coined the term “cell” in 1665 while studying cork; two decades later, Anton van Leeuwenhoek discovered bacteria. In 1855, Rudolph Virchow correctly asserted that living cells arise only from other living cells, a principle that would later underpin cell theory.
Prokaryotes—Bacteria and Archaea—have existed for roughly 3.5 billion years, constituting about three‑quarters of Earth’s age. They are typically unicellular. Eukaryotes, encompassing animals, plants, and fungi, emerged through an endosymbiotic event nearly 3 billion years ago, when a primitive cell engulfed an aerobic bacterium that evolved into the mitochondrion. This partnership enabled the evolution of complex multicellular life.
Read more about the similarities and differences of prokaryotic and eukaryotic cells.
Despite vast diversity, all cells share core components: a plasma membrane, cytoplasm, DNA, and ribosomes. Prokaryotes also possess a rigid cell wall, as do plant cells. In eukaryotes, DNA is enclosed within a nucleus surrounded by a nuclear envelope.
The plasma membrane is a phospholipid bilayer composed of hydrophilic heads and hydrophobic tails. This arrangement confers semi‑permeability, allowing small molecules like glucose and CO₂ to diffuse freely while larger or charged molecules require active transport via ATP‑powered pumps.
Read more about the structure and function of the plasma membrane.
The nucleus, surrounded by the nuclear envelope, houses chromosomes—segments of DNA that differ in number across species (humans have 23 chromosome types, 46 in total). During cell division, DNA replication precedes mitosis, ensuring accurate genetic inheritance.
Ribosomes, located in the cytoplasm or bound to the endoplasmic reticulum, translate messenger RNA (mRNA) into proteins. The process begins with transcription of DNA to mRNA in the nucleus, followed by translation of the amino acid sequence on ribosomes.
Mitochondria are essential for eukaryotic respiration. After glycolysis generates ATP, the pyruvate enters mitochondria, where it undergoes the Krebs cycle in the matrix and the electron transport chain on the inner membrane—producing the majority of cellular ATP.
In bacteria, binary fission copies DNA and divides the cell in two. Eukaryotic division involves mitosis (prophase, metaphase, anaphase, telophase—plus prometaphase in many accounts) followed by cytokinesis. Errors in DNA replication or repair can lead to uncontrolled cell proliferation, the hallmark of cancer.
