Notice how waste products such as Carbon Dioxide and Urea are small fragments of larger molecules such as carbohydrates and proteins. Sunlight is the ultimate source of energy for life on Earth. Note that oxidation can occur in the absence of Oxygen. Prokaryotes don't use oxygen for breakdown. But oxidation occurs nevertheless.
Oxidations release energy, reductions require energy.
Biochemical reactions are aided by enzymes. Enzymes lower the activation energy for chemical reactions. B. Every chemical reaction has an equilibrium constant which represents the ratio of reactant and product concentrations at equilibrium - after a VERY long time . Carbonic Anhydrase catalyzes the reaction of Carbon Dioxide and Water to form Bicarbonate. However, if we wait a very, very long time, the equilibrium concentrations will be the same in the presence or absence of the enzyme.