Question

Biological compounds such as carbohydrates, fats, and proteins are thermodynamically unstable in the presence of O2, since, upon combustion, they will undergo exothermic reactions with O2 to form CO2, H2O and other simple and stable compounds. On the other hand, aerobic life forms require O2 for generating energy, via oxidation reactions (respiration), needed for myriad metabolic activities. Explain how the special electronic structure of O2 allowed aerobic life on the planet to evolve in the presence of an oxidizing atmosphere, and how these life forms are able to harvest energy from oxidation reactions under controlled conditions within cells. Hint: Most aerobic organisms have enzymes containing transition metal elements; these enzymes mediate and control the rates of the respiration/oxidation processes

Answer 1

The presence of free molecular oxygen allowed ammonium to be oxidized to nitrate and subsequently denitrified. The interaction between the oxygen and nitrogen cycles in particular led to a negative feedback, in which increased production of oxygen led to decreased fixed inorganic nitrogen in the oceans. This feedback, which is supported by isotopic analyses of fixed nitrogen in sedimentary rocks from the Late Archaean, continues to the present. However, once sufficient oxygen accumulated in Earth's atmosphere to allow nitrification to out-compete denitrification, a new stable electron ‘market’ emerged in which oxygenic photosynthesis and aerobic respiration ultimately spread via endosymbiotic events and massive lateral gene transfer to eukaryotic host cells, allowing the evolution of complex (i.e. animal) life forms. The resulting network of electron transfers led a gas composition of Earth's atmosphere that is far from thermodynamic equilibrium (i.e. it is an emergent property).   

Almost all organisms use organic molecules from their surroundings to obtain energy. Organisms capable of photosynthesis produce food molecules like carbohydrates. There are many forms of cellular respiration. Some organisms require oxygen to carry out respiration known as aerobic cellular respiration and the respiration that does not require presence of oxygen is called anaerobic cellular respiration.