Plants and animals are components of the living world. They vary greatly in size. They are multicellular organism whos bodies are organized at several levels. Cell, the basic unit of organisms, are organized into tissues. A tissue is composed of many cells, usually similar in both structure and function, bound together by intercellular material. Tissues make up an organ. An organ is composed of various tissues grouped together into a structural and functional unit. A group of interacting organs that cooperate as a functional complex in the life of an organism is a system. Each part in the system performs aparticular function that is coordinated with the function of the rest of the body parts. Such an organized body is necessary for proper growth, development, and survival of the organism.