The fundamental building blocks of all living things are cells. Cells are the basic units of life, responsible for carrying out essential functions such as metabolism, growth, and reproduction. They can be unicellular, like bacteria, or multicellular, like plants and animals. Each cell is surrounded by a membrane and contains various organelles that perform specific tasks, including the nucleus, which houses genetic material (DNA). Cells can vary significantly in size, shape, and function, but they all share these common characteristics, making them the foundational units for all biological organisms.