Photosynthesis uses the energy of sunlight to produce sugars and other organic molecules. These molecules in turn serve as food for other organisms. Many of these organisms carry out respiration, a process that uses O₂ to form CO₂ from the same carbon atoms that had been taken up as CO₂ and converted into sugars by photosynthesis. In the process, the organisms that respire obtain the chemical bond energy that they need to survive. The first cells on the earth are thought to have been capable of neither photosynthesis nor respiration (see Chapter 14). However, photosynthesis must have preceded respiration on the earth, since there is strong evidence that billions of years of photosynthesis were required before O₂ had been released in sufficient quantity to create an atmosphere rich in this gas to support respiration. (The earth's atmosphere presently contains 20% O₂.)  Fair Use and Copyright info