Photosynthesis and respiration. Illustration showing the processes of photosynthesis (top) and respiration (bottom) by which plants and animals obtain heat energy, with the ultimate energy source being sunlight. In plants (autotrophs), photosynthesis takes place in chloroplasts (green) with carbon dioxide (CO2) and water (H2O) combined to produce sugars and ATP (adenosine triphosphate), along with oxygen (O2). In animals and plants, mitochondria (orange) are the site of cellular respiration that oxidises fuels to produce energy. For this artwork with labels, see image C023/8803.