Cells were named for monks' rooms, and one bacterium is visible unaided
When Robert Hooke peered through a microscope in 1665, the tiny compartments he saw reminded him of monks' rooms in a monastery, so he called them cells, from the Latin for small room. Most cells are far too small to see without help, but one bacterium averages a centimetre long.
Life divides into two broad camps. Prokaryotes, the bacteria and archaea, are always single-celled and keep their DNA loose in a region called the nucleoid rather than inside a nucleus. They were probably the first living things, and cells of some kind appeared on Earth about four billion years ago. Most prokaryotes measure between 0.1 and 5 micrometres across, yet Thiomargarita magnifica can stretch to 2 centimetres. Eukaryotes, whose name means true kernel after the nucleus, include animals, plants and most fungi, and a eukaryotic cell can be up to a hundred times wider than a typical prokaryote.
In 1839 Matthias Schleiden and Theodor Schwann set out cell theory: every organism consists of one or more cells, the cell is the basic unit of structure and function, and each cell comes from an earlier cell. Every multicellular body starts as a single zygote that differentiates into hundreds of cell types, steered by signals from neighbouring cells and by molecules shared unevenly when cells divide.
A bacterium usually carries one circular chromosome, plus optional loops of DNA called plasmids that can hold genes such as those for antibiotic resistance. Some bacteria even build compartments; magnetotactic species make magnetosomes that let them align with magnetic fields. Archaea look similar from outside but have membranes of unusual ether-linked lipids, a distinctive propeller called the archaellum, and a talent for extreme heat, cold, acid or salt. No archaean is known to cause disease.
Inside a eukaryotic cell, the nucleus is the largest enclosed structure, joined by mitochondria that supply energy and, in plants, chloroplasts that make sugar from sunlight. The outer membrane is a double layer of fatty molecules that lets some substances through freely, others only partly and some not at all. The gel-like cytosol fills 30 to 50 percent of the cell's volume and is where most proteins are built.
Source: Cell (biology)
