The smallest unit of life is the cell. Cells are the fundamental building blocks of all living organisms, both unicellular and multicellular. They perform essential functions required for life, including metabolism, energy conversion, and reproduction. Each cell is composed of various organelles, such as the nucleus, mitochondria, and endoplasmic reticulum, which carry out specific tasks. In multicellular organisms, cells differentiate to specialize in various roles, forming tissues, organs, and systems. The study of cells, or cell biology, is crucial for understanding life processes, disease mechanisms, and the development of medical treatments.