Biology  5/e   Raven/Johnson
	Student   Online Learning Center

Life as we know it on the earth today depends on photosynthesis. When organisms first evolved the ability to photosynthesize, they evolved the ability to convert solar energy into chemical energy. The first photosynthesizers evolved more than 3 billion years ago and were anaerobic. They used sunlight as a source of energy to synthesize ATP. More than 2 billion years ago, oxygen-forming photosynthesizers evolved. They used solar energy to produce not only ATP, but also a reduced compound called NADPH. Oxygen gas, O₂, was produced as a by- product of this process. The two energy-rich compounds, ATP and NADPH, provided the energy to convert carbon dioxide to glucose in another series of reactions. Glucose, you remember, is the beginning compound of cellular respiration. It is oxidized to provide the energy to synthesize the ATP that keeps organisms alive. Essentially then, photosynthetic organisms capture solar energy and convert it to chemical energy. This chemical energy is then utilized by the photosynthesizers and by all other organisms to fuel their life processes. Almost every single food chain on earth can be traced back to green plants or other photosynthetic organisms. Of equal importance to life as we know it is the production of O₂ during aerobic photosynthesis. It is the sole source of the oxygen that we and all other aerobic organisms need to stay alive. Without aerobic photosynthesis there would be no O₂ in the atmosphere or in bodies of water, there would be no aerobic respiration, and there would be no us or any other aerobic organism.

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.