Question

Explain why energy is harvested over dozens of steps as opposed to one simple release of energy. Relate this to cellular respiration, and give specific examples.

Answer 1

Based on the source of energy, the organisms are broadly classified as autotrophs (from sunlight) and heterotrophs (consumers). On the earth, 95% organisms are heterotrophic in nature.Major contributors of food are carbohydrates followed by fat and protein. They possess covalently bound C-H and C-O. The major energy is obtained from catabolism of carbohydrates through cellular respiration.

A covalent bond is formed by sharing of electrons. These electrons hold the energy through these covalent bonds. Cells harvest energy by using these electrons as carriers to produce ATP. Once a molecule of ATP is formed, the electrons are energy depleted and are donated to some other molecule. In this process, the larger molecules are broken down into smaller molecules in a series of steps (catalyzed by enzymes) by breaking C-H and C-O bonds by which energy is transferred. Other enzymes will fragment the smaller units. That is why a dozen of steps are required for the harvest of energy.

Consider the reaction (cellular respiration)

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (heat or ATP)

The change in free energy in this reaction is –720 kilocalories (–3012 kilojoules) per mole of glucose. The change in free energy results from the breaking of the 6 C—H bonds in the glucose molecule. The negative sign indicates that the products possess less free energy than the reactants. Most of the energy is dissipated as heat and the remaining is harvested as ATP.