Plants make energy in a process called photosynthesis.  In this process the plant absorbs water(H2o),sunlight, and carbon dioxide(co2). In the leaf organelles called chloroplasts use these matierals to make gluclose and oxygen(O2).  Gluclose is a sugar that the plant uses to for energy. Oxygen is the air we need to live.  In a way animals and plants can't live without photosyntesis.

For more details visit:

http://en.wikipedia.org/wiki/Photosynthesis