Plants are not on the MCAT, so we can use plant biology as a pressure free zone to practice our comprehension of fundamental principles that are on the test. Photosynthesis utilizes solar energy to drive an oxidation-reduction reaction uphill against positive free energy, the transformation of carbon, oxygen and hydrogen from an initial state composed of carbon dioxide and water into a final state composed of carbohydrate and molecular oxygen. Looking at photosynthesis in terms of bond energy, the conceptual framework underlying oxidation-reduction, the final state in photosynthesis is higher energy than the initial state because the bonds in molecular oxygen and carbohydrate are weaker than the bonds in carbon dioxide and water. Why are the initial state bonds in carbon dioxide and water stronger? When oxygen binds to lower electronegativity elements such as carbon and hydrogen, the total internal energy of the system decreases because oxygen has seized the bonding electrons and has drawn them in towards its strong nucleus. This is why more energy tends to be released when polar covalent bonds are formed than when nonpolar bonds are formed, so it requires more energy to break polar covalent bonds than nonpolar bonds. The bond energy perspective is the underlying coherence of oxidation-reduction: polar bonds are stronger (in redox terms, when polar bonds form, a high reduction potential element is reduced). Summarizing this point, photosynthesis breaks strong bonds and forms weak bonds, fixing energy in the process. Another way of saying this in terms of oxidation-reduction and electrochemistry, photosynthesis transforms light into reduction potential, removing electron control from oxygen and placing it with carbon.
In aerobic respiration, the oxygen gains back electron control, a process with positive cell potential (negative free energy change) that can be coupled with the nonspontaneous phosphorylation of ATP.