Living organisms can be made of a single cell, such as bacteria and protists, or they can be multicellular, like plants, animals, and fungi. Unicellular organisms, like bacteria, are able to perform all life functions within one single cell. They can transport molecules, metabolize nutrients, and reproduce within this one cell. As organisms evolved & diversified into larger more complex multicellular organisms they needed many different types of cells to carry out the same life processes. Each of these special types of cells has a different structure that helps it perform a specific function. Groups of specialized cells cooperate to form a tissue, such as a muscle. Different tissues are in turn grouped together to form larger functional units, called organs. Each type of cell, tissue, and organ has a distinct structure and set of functions that serve the organism as a whole.
For example, humans have many different types of cells with different jobs, such as blood cells that carry oxygen and nerve cells that transmit signals to all parts of the body.
