Stages of the Catabolism Process Cellular respiration takes place in the mitochondria through the process of glycolysis, followed by the process of oxidative decarboxylation and then the Krebs cycle, where at each level of this process energy is produced in the form of ATP (Adenosine Tri Phosphate) and hydrogen. The energized hydrogen combines with a hydrogen acceptor to be carried to electron transfer, the energy is released and the hydrogen is accepted by O2 to become H2 O.In the process of cellular respiration, the fuel is hexose sugar.This combustion requires free oxygen, so the overall reaction can be written as follows. C6 H12O6 + 6 O2 6 CO2 + 6 H2 O + 675 Cal In respiration, hexose sugar undergoes disassembly in a very long process. Firstly, glucose as fuel undergoes a phosphorylation process, namely the process of adding phosphate to glucose molecules to become fructose - 1.6 diphosphate. In phosphorylation, ATP and ADP play an important role as phosphate fillers.The conversion of fructose - 1.6 diphosphate to finally become CO2 and H2 O can be divided into 4 stages, namely glycolysis (oxidative decarboxylation), the Krebs cycle and electron transfer. a. Glycolysis Glycolysis is a series of reactions changing glucose moleculesinto pyruvic acid by producing NADH and ATP.The characteristics of glycolysis are: 1) It takes place anaerobically. 2) In glycolysis there are enzymatic activities and ATP and ADP(Adenosine Diphosphate). 3) ADP and ATP play a role in transferring phosphate from molecule oneto another molecule. b. Oxidative decarboxylation (transition reaction or intermediate reaction) Each pyruvic acid resulting from glycolysis will react with Nicotinamide Adenine Dinucleotide (NAD+ ) and coenzyme A (Co-A) to form Acetyl Co-A in a reaction that takes place in the mitochondria. This will result in a reduction of one C atom in the form of CO2 .Pyruvate will only continue into the Kerbs cycle if there is enough oxygen in the cells. c. Citric acid cycle (Krebs cycle) It is known as the TCA (tricarboxylic) cycle because citric acid is an intermediate compound consisting of 3 carboxylic acid groups. d. Electron transport chain Electron transport chain scheme: NAD FMN Fe.s Q Cyt.b Cyt.a Cyt.a3 O2 ATP ATP ATP H2 O The scheme above is the sequence of electron carrier molecules in the respiratory chain. NAD = Nicotinamide Adenine Dinucleotide FMN= Flavin mononucleotide Fe.s = Proteins that contain Fe and s in their prosthetic groups Q = Ubiquinone (coenzyme-Q) Cyt.b = Cytochrome b Cyt.c = Cytochrome c Cyt a = Cytochrome a Cyt a3 = Cytochrome a3