Cells

"The cell (from Latin cella, meaning "small room") is that the basic structural, functional, and biological unit of all known organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is named cell biology, cellular biology, or cytology.

Cells contain cytoplasm enclosed within a membrane, which contains many biomolecules like proteins and nucleic acids. Most plant and animal cells are only visible under a microscope, with dimensions between 1 and 100 micrometers. Organisms are often classified as unicellular (consisting of one cell like bacteria) or multicellular (including plants and animals). Most unicellular organisms are classed as microorganisms.

The number of cells in plants and animals varies from species to species; it's been estimated that humans contain somewhere around 40 trillion (4×1013) cells. The human brain accounts for around 80 billion of those cells."