Plants and animals evolve to survive and reproduce in their environments. Both their internal and external structures will be well suited to support survival, growth, behavior and reproduction.

All animals have a mouth. Its function is to take in food, without which it could not survive to reproduce. The type of mouth, however, will vary between animals, depending on what they have evolved to eat. Internal organs of the digestive system will likewise vary depending on an animal’s diet. All parts of the system for eating and digesting food must work together.

Plants also have internal systems for processing food and water. Although most of the “food” used by plants is produced in the leaves from air and light, many essential nutrients, like nitrogen, are brought in with water through the roots. A system of tubes, called the xylem, carry water from the roots up through the other structures of the plant, and finally to the leaves. A second system of tubes, called the phloem, carry sugars produced in the leaves down to all the other parts of the plant.