1. Glycolysis
1.1. step 1, two phosphate groups are attached to one glucose molecule, forming a new six carbon compound that has two phosphate groups. The phosphate groups are supplied by two ATP molecules, which are converted into two molecules of ADP in the process.
1.1.1. 2, the six carbon compound formed in step 1 is split into two three-carbon molecules glyceraldehyde 3 phosphate (G3P).
1.1.1.1. step 3, the two molecules of G3P are oxidized and each receives a phosphate group. The product of step 3 is two new three-carbon compound molecules. The oxidation of G3P is accompanied by the reduction of two molecules of nicotinamide adenine dinucleotide (NAD+) to NADH.
1.1.1.1.1. step 4, the phosphate groups which were added in step 1 and step 3 are removed from the three-carbon compounds formed in step 3. This reaction produces two pyruvic acid molecules. Each phosphate group is combined with an ADP molecule to make an ATP molecule. Since a total of four phosphate groups were added in step 1 and step 3, four molecules are ATP are produce in the end.
2. Krebs Cycle
2.1. The acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a molecule of citrate. The acetyl coenzyme A acts only as a transporter of acetic acid from one enzyme to another. After Step 1, the coenzyme is released by hydrolysis so that it may combine with another acetic acid molecule to begin the Krebs cycle again.
2.1.1. The citric acid molecule undergoes an isomerization. A hydroxyl group and a hydrogen molecule are removed from the citrate structure in the form of water. The two carbons form a double bond until the water molecule is added back. Only now, the hydroxyl group and hydrogen molecule are reversed with respect to the original structure of the citrate molecule. Thus, isocitrate is formed
2.1.1.1. In this step, the isocitrate molecule is oxidized by a NAD molecule. The NAD molecule is reduced by the hydrogen atom and the hydroxyl group. The NAD binds with a hydrogen atom and carries off the other hydrogen atom leaving a carbonyl group. This structure is very unstable, so a molecule of CO2 is released creating alpha-ketoglutarate.