The cell is the fundamental and structural unit of life, and cells are often called the "building blocks of life". Cell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. The human body is composed of trillions of cells. Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell. Trees in a forest, fish in a river, horseflies on a farm, lemurs in the jungle, reeds in a pond, worms in the soil - all these plants and animals are made of the building blocks we call cells. Like these examples, many living things consist of vast numbers of cells working in concert with one another. Organisms that are made up of more than one cell are called multicellular organisms.