The basic building block of all living organisms is the cell. Cells are the smallest unit of life that can replicate independently and perform all essential life functions. They come in various types, including prokaryotic cells (like bacteria, which lack a nucleus) and eukaryotic cells (like those found in plants, animals, and fungi, which have a nucleus). Cells contain organelles that carry out specific functions, such as mitochondria for energy production and ribosomes for protein synthesis. The study of cells, cellular biology, or cytology, is fundamental in understanding life sciences, including genetics, microbiology, and physiology.