Cell Structure

"Each of your individual cells is a compact and efficient form of life—self-sufficient, yet interdependent upon the other cells within your body to supply its needs. Independent single-celled organisms must take in nutrients, excrete wastes, detect and respond to their environment, move, breathe, grow, and reproduce. All cells are organized from the atomic level to all their larger forms. Oxygen and hydrogen atoms combine to make the molecule water (H2O). Molecules bond together to make bigger macromolecules. The carbon atom is often referred to as the backbone of life because it can readily bond with four other elements to form long chains and more complex macromolecules. Four macromolecules—carbohydrates, lipids, proteins, and nucleic acids—make up all of the structural and functional units of cells."