Cell - the structural and functional unit of life.

The cell (derived from Latin cella, meaning "small room" ) is the structural, functional, and biological basis of all known organisms. Cells are the smallest part of life, which is why they are often referred to as "living things". Cells are called cell biology, cellular biology, or cytology.

Cells contain a cytoplasm that is trapped inside the membrane, which contains many biomolecules such as proteins and nucleic acids. Most plant and animal cells are visible only under a bright microscope, with a diameter of between 1 and 100 micrometres. Electron microscopy provides a very high resolution showing detailed cell formation. Organisms can be classified as unicellular (consisting of a single cell as bacteria) or multicellular (including plants and animals). Most unicellular substances are considered microorganisms.

The number of cells in plants and animals varies from one species to another; it is estimated that humans contain about 40 billion cells (4 × 1013). [a] [5] The human brain contains 80 billion of these cells. [6]

The cell was discovered by Robert Hooke in 1665, who named it the same as the cells housed by Christian nuns in a convent. [7] [8] Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all living things are composed of one or more cells, that cells are a basic component of the structure and function of all living things, and that all cells originate. [9] Cells appeared on Earth at least 3.5 billion years ago. [10] [11] [12]

Cells are of two types: eukaryotic, nucleus-containing, and prokaryotic, non-eukaryotic. Prokaryotes are single-celled organisms, while eukaryotes can be single-celled or multicellular.