The first step of glucose metabolism is glyclosis wherein glucose is converted to 2 molecules of pyruvate. This process will produce a total of 2 ATP. 2 NADH (important in the oxidative phosphorylation) is also produced in this process. The 2 pyruvate molecules will then be converted to acetyl-CoA and in this process, 2 NADH is produced. Now, we have a total of 2 ATP and 4 NADH.
The second step is the tricarboxylic acid cycle (Kreb's cycle, citric acid cycle). An acetyl-CoA molecule will yield 1 GTP (equivalent to ATP, will use ATP for simplicity), 3 NADH, and 1 FADH2 (this is also important in the oxidative phosphorylation). Since we have 2 acetyl coA molecules, then the yield of this process is doubled at 2 ATP, 6 NADH, and 2 FADH2. Now we have a total of 4 ATP, 10 NADH, and 2 FADH2.
The last step is the oxidative phosphorylation wherein our reducing compounds (NADH and FADH2) is oxidized to NAD and FAD leading to the production of ATP. As a rule, 1 molecule of NADH yields 3 ATP molecules and 1 molecule of FADH2 yields 2 ATP molecules. Since we have 10 NADH molecules, we'll get 30 ATP from that. We also have 2 FADH2 molecules and we'll get 4 ATP from that.
Now we have a total of 34 ATP from oxidative phosphorylation and 4 ATP from glycolysis, conversion of pyruvate to acetyl-CoA, and TCA cycle; ultimately yielding 38 ATP molecules from 1 molecule of glucose.
