0 7 Cellular Respiration and Fermentation Overview: Life Is Work that Requires Energy Living cells require energy from outside sources Some animals, such as the giraffe, obtain energy by eating plants, and some animals feed on other organisms that eat plants ? 2014 Pearson Education, Inc. Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules, which are used as fuel for cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work ? 2014 Pearson Education, Inc. Animation: Carbon Cycle Carbon Cycle Figure 7.2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 ? H2O Cellular respiration in mitochondria Organic molecules ? O2 ATP ATP powers most cellular work Heat