Oxygen: friend and foe!

Food molecules such as carbohydrates are first oxidized to the 3-carbon compound pyruvate via the glycolytic process in the cytosol; however, only about 4% of the available energy is extracted. Pyruvate then enters the mitochondria where it is oxidized all the way to CO2 in the Krebs cycle (Figure 1). e electrons extracted from the carbon substrates, in the form of NADH and avin adenine dinucleotide (FADH2), are sent through the respiratory chain in the inner membrane, where four electrons end up reducing O2 to water. The respiratory chain comprises four multisubunit protein complexes in which a series of redox reactions are coupled to the pumping of protons from the mitochondrial matrix across the inner mitochondrial membrane and into the intermembrane space, creating a transmembrane electrochemical proton gradient. The energy conserved in this gradient is Respiration uses oxygen to maximize energy conservation