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Biology LibreTexts

17.6: The Flow of Energy in Nature

Skills to Develop

  • Describe the relationship between photosynthesis and aerobic respiration and relate this to the first law of thermodynamics.

For the vast majority of life on earth, the flow of energy begins with sunlight and involves a cycle involving photoautotrophs and chemoorganoheterotrophs. Photoautotrophs use sunlight as a source of energy and through the process of photosynthesis, reduce carbon dioxide to form carbohydrates such as glucose. The radient energy is converted to the chemical bond energy within glucose.

The overall reaction for photosynthesis (in the presence of light and chlorophyll) is as follows:

\[6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\]

Note that carbon dioxide (CO2) is reduced to produce glucose (C6H12O6 ) and water (H2O) is oxidized to produce oxygen (O2).

Both chemoorganoheterotrophs and photoautotrophs then convert the chemical bond energy of glucose to the chemical bond energy of ATP, the form of energy required to do most cellular work. This is done through the process called aerobic respiration.

The overall reaction for aerobic respiration is:

C6H12O6 + 6O2 yields 6CO2 + 6H2O + energy (as ATP)

Note that glucose (C6H12O6 ) is oxidized to produce carbon dioxide (CO2) and oxygen (O2) is reduced to produce water (H2O).

As can be seen, the end products for aerobic respiration, carbon dioxide and water, are the reactants for photosynthesis while the end products of photosynthesis, glucose and oxygen, are the reactants for aerobic respiration. In other words, the nutrients are continuously recycled between the two processes. Energy, however, is converted from one form to another: from radiant energy to the chemical bond energy of glucose to the chemical bond energy of ATP, the first law of thermodynamics.

Summary

  1. The overall reaction for photosynthesis is 6CO2 + 6H2O in the presence of light and chlorophyll yields C6H12O6 + 6O2.
  2. The overall reaction for aerobic respiration is C6H12O6 + 6O2 yields 6CO2 + 6H2O + energy (as ATP).
  3. As can be seen, the end products for aerobic respiration, carbon dioxide and water, are the reactants for photosynthesis while the end products of photosynthesis, glucose and oxygen, are the reactants for aerobic respiration. In other words, the nutrients are continuously recycled between the two processes.
  4. Energy, however, is not recycled but rather is converted from one form to another: from radiant energy to the chemical bond energy of glucose to the chemical bond energy of ATP.

Questions

Study the material in this section and then write out the answers to these question. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. 6CO2 + 6H2O are the reactants for ____________________ and the products of ___________________.
    1. aerobic respiration / photosynthesis (ans)
    2. photosynthesis / aerobic respiration (ans)
  2. C6H12O6 + 6O2 are the reactants for ____________________ and the products of ___________________.
    1. aerobic respiration / photosynthesis (ans)
    2. photosynthesis / aerobic respiration (ans)

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)