The smallest functional unit of life is the cell. Cells are often referred to as the building blocks of all living organisms, ranging from single-celled microorganisms to complex multicellular beings like humans. Each cell performs essential functions that sustain life, such as metabolism, growth, and reproduction. Eukaryotic cells, which contain a nucleus and organelles, contrast with prokaryotic cells, which do not have a defined nucleus. Understanding cells is fundamental to the fields of biology and medicine.