Free energy, photosynthesis, and cellular respiration

All organisms require energy to survive. How living systems obtain and utilize energy is the focus of study in bioenergetics.

All of the energy in a living system is originally derived from sunlight, and the transformation of energy within living systems is an elegant process in which energy from the sun is stored in the chemical bonds of organic molecules. The stored energy can then either be utilized by the organism that stores it, or it can be consumed and then utilized by another organism. An overview of this transformation of energy is diagramed in Figure 1.

Figure 1: An overview of the transformation of energy in living systems

A student is studying the diagram in Figure 1 and notices that the reactions and cellular intermediates of photosynthesis and cellular respiration differ considerably. However, when the transformation of energy is looked at as a whole, she realizes that photosynthesis and cellular respiration can be thought of as reverse reactions of one another, in that the products of one are the reactants for the other.

She wants to further expand this idea by comparing the change in free energy (∆G) for the two processes by calculating ∆G for photosynthesis and cellular respiration. To do so she sums the bond energies of the reactants and then subtracts the bond energies of the products, and the resultant difference is equal to the ∆G of the reaction. The bond energy values she uses are found in Figure 2. Using this method, she calculates the ∆G of photosynthesis to be +686 kcal/mol.

Table 1: Table of common bond energy values