Energy

http://www.nature.com/scitable/topicpage/cell-energy-and-cell-functions-14024533

https://www.trueorigin.org/atp.php

http://www.ck12.org/book/Biology/section/4.1/

http://www.kidsbiology.com/biology_basics/needs_living_things/energy2.php

http://eschooltoday.com/science/characteristics-of-living-organisms/living-things-obtain-and-use-energy.html