Comparison of Photosynthesis & Cellular Respiration Activity

Only plants can use the energy from the sun for direct metabolic benefit. However, both plants and animals use respiration to acquire energy.

Carbon as a Life Force

Carbon is one of the most important elements for the life of organisms. All organisms are composed mostly of carbon, and an endless series of reactions reduce and oxidize the element, creating energy in all living things.

Photosynthesis

Photosynthesis is the process of reducing the carbon atom, using light energy. Plants, algae and bacteria use this process to create carbohydrates for energy. They use that energy for growth, maintenance and reproduction.

Respiration

The energy that is not used by the plant for growth, maintenance or reproduction is released through respiration. All organisms must conduct respiration. Through this process the carbon that was reduced during photosynthesis is oxidized, releasing the energy stored in the bonds of the carbohydrate. Organisms make use of this energy much the same way as plants use the energy from carbohydrates. They use it to grow, maintain and reproduce. Any energy not used for these purposes is released as heat.

Respiration and Energy Production

During respiration oxygen is reduced and combines with hydrogen to form a water molecule. Carbon, on the other hand, is oxidized, forming a CO2 molecule. The energy required to reduce oxygen during respiration is less than is needed to reduce carbon. This means that the oxidization of carbon during respiration results in excess energy that can be used by an organism.

Complementary Reactions

Photosynthesis and respiration have opposite effects on carbon. While one reduces the element into a carbohydrate, the other oxidizes it, producing carbon dioxide. These two processes are complementary, as can be seen in the equation used to represent them:

Photosynthesis: energy + 6CO2 + 6H20 ' C6H12O6 + 6O2

Respiration: 6O2 + C6H12O6 ' 6H20 + 6CO2 + energy

References

• The Economy of Nature, 5th Ed.; Robert E. Ricklefs; 2007