Cellular Energy

Energy molecules
ATP is the body's main energy molecule. Energy is stored in forms of glycogen or triglyceride, which can be broken down to regenerate ATP. Some high-energy molecules include:

1. ATP, UTP, GTP: donators of phosphoryl group.
2. NADH, FADH₂: donators of electrons or hydrogen.
3. Biotin: donator of carboxyl group.
4. Acetyl CoA: donator of acyl group.
5. Others: THF-C, Thpp, S-Adenosylmethionine, UDP-Glucose.

A note about enzymatic biochemistry reactions
Enzymes and substrates of a reverse reaction are usually found in a different area of the body, so forward and reverse reactions do not compete. If a two-way reaction is found in the same area, then negative (or positive) feedback of enzymes prevent a "futile cycle" (competition within two-way reactions).

ATP production
Glycolysis occurs in all organ cells, producing two pyruvates per glucose. Pyruvate is converted to acetyl CoA, which can enter the Krebs cycle. Besides glycolysis, acetyl CoA can also be formed through fatty acid degradation or by transformation of certain amino acids.

Glycolysis (cytosol)
Produces 2 pyruvate, net 2 ATP and 2 NADH per glucose.

Anaerobic glycolysis (cytosol)
The Krebs cycle needs O₂ to run, otherwise anaerobic glycolysis will occur. It is like glycolysis with an extra step: cycle the 2 NADH to replenish 2 NAD⁺ via transformation of 2 pyruvate to 2 lactate. Net 2 ATP.

Pyruvate decarboxylation (mitochondria)
Produces 2 acetyl CoA and 2 NADH per glucose.

Krebs cycle and Oxidative phosphorylation (mitochondria)
Including pyruvate decarboxylation, the Krebs cycle produces 1 ATP, 1 GTP, 8 NADH, and 2 FADH₂ per glucose. In the electron transport chain, phosphorylation (ATP production) is coupled with oxidation (requiring O₂ at the end).

Based on newer sources, 2.5 ATP are generated per NADH in the mitochondria. 1.5 ATP are generated per NADH in the cytosol, and per FADH₂. This is a total of 30-32 ATP per glucose in aerobic respiration. 

Note that the presence of ADP stimulates oxidation, and the lack of ADP (and abundance of ATP) slows the rate of oxidation.

Cell Respiration by RegisFrey

Connections to other processes
Glyceraldehyde 3-P (halves of glucose) can be directed back towards gluconeogenesis, or towards acetyl CoA and the Krebs cycle. Amino acids can be transformed into acetyl CoA or other molecules in the Krebs cycle, and vice versa. Acetyl CoA is also used to generate lipids (lipogenesis). Succinyl CoA can be used to produce heme molecules, while other molecules from the Krebs cycle can be used to make purines and pyrimidines for DNA synthesis.