Cells are the building blocks of all living things. Cells are so small that they cannot be seen with the unaided eye. The diameter of a cell is about 10–100 times smaller than the width of a strand of hair. An organism can be made up of one or many cells. Because an individual cell is alive, it grows and develops, can reproduce, responds to its surroundings, and uses energy. The human body is made of structures at different scales. Different types of cells make up tissues and organs. Various combinations of organs and tissues make up systems that perform functions important for sustaining life. The human body has several different systems, each with a specific function. Each system works with other systems to ensure that the body works properly.
