Energy Flow in Nature

The Flow of Energy through Nature

Learning Objectives for this Section

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 is as follows:

6CO₂ + 6H₂O in the presence of light and chlorophyll yields C₆H₁₂O₆ + 6O₂

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 radient energy to the chemical bond energy of glucose to the chemical bond energy of ATP.

(See the tutorials on aerobic respiration and photosynthesis.)

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