At the end of cellular respiration, two hydrogen atoms are removed by NAD forming two NADH. This event takes place at glycolysis. Glucose (C6) is broken down during this stage. Glycolysis does not require oxygen and occurs in the cytoplasm. It involves 10 steps, each catalyzed by a specific enzyme.
Cellular respiration is the process in which energy in glucose is transferred to ATP. The glucose energy is used to produce ATP. This activity allows organisms to use (or release) energy stored in the chemical bonds of glucose. During respiration, glucose is oxidized to produce energy. Reduction of oxygen releases water. Carbon dioxide is released from the carbon atoms of the sugar molecules.