The basic unit of structure and function in living organisms is the cell. Cells are the fundamental building blocks of life and carry out essential processes necessary for an organism's survival. They can be categorized into two main types: prokaryotic and eukaryotic. Prokaryotic cells, such as bacteria, lack a defined nucleus, while eukaryotic cells, found in plants and animals, have membrane-bound organelles. The study of cells, known as cell biology, provides insights into their functions, interactions, and the processes that maintain life.