The smallest unit of life is the cell. While there are other building blocks smaller than a cell, such as organelles and quarks, a cell is the smallest structure that functions independently. It can also replicate and perform all the necessary functions of life.
A common term for a cell is that it is a "building block of life." Cells are made up of membranes that surround a sac of fluid, and the study of cells is called cell biology. The cell was discovered in 1665 by Robert Hooke. He named the structure after the cells that were inhabited by monks in a monastery.
Millions of cells make up the organs and tissues found in humans, as well as animals and plants. The organs and tissues work together to make up a body, or an individual person. Various races of humans make up a population, and different populations live together to form a community. The "population" and "community" designation apply to animals and plants as well.
Communities of non-living and living things interact with each other, forming an ecosystem. Other building blocks, such as air, earth and water, work together to form the ecosystem. Many different ecosystems combine together to form the biosphere, or the zone of life on Earth.