How it fits together · lesson 2 of 11
Energy — how a cell makes ATP
Glycolysis, the Krebs cycle, and the electron transport chain — and the cofactors they run on.
7 min read · reviewed October 2026
Your cells don't run on glucose directly — they run on ATP, a tiny rechargeable battery. Food's job is to recharge ATP, and that happens on a three-stage line that ends inside your mitochondria, the cell's power plants.
When people say they feel 'tired' despite sleeping enough, the story is sometimes not in the bedroom — it's on this line, where a missing cofactor is throttling the recharge.
The three stages of energy production
Happens in the cell's cytoplasm. One glucose is split into two pyruvate, netting a small amount of ATP fast and without oxygen. The quick-but-low-yield opening act. Cofactors: needs B3 (as NAD⁺) and magnesium-bound ATP.
Inside the mitochondria. Pyruvate is fed in and spun through a circular series of reactions that don't make much ATP directly — instead they load up electron carriers (NADH, FADH₂). The middle stage that preps the real payoff. Cofactors: B1, B2, B3, B5, plus magnesium.
The mitochondrial inner membrane. The loaded carriers drop their electrons down a chain, pumping protons that drive a turbine-like enzyme to mass-produce ATP — using oxygen as the final electron catcher. This is where the vast majority of your ATP is made. Cofactors: B2, CoQ10, iron, copper.
From ONE glucose molecule fully burned with oxygen, roughly how many ATP does a cell actually net?
Somewhere between 2 and 50 ATP.
Reveal the answer →
~30–32 (older texts say ~36–38)
Modern estimates put the realistic net yield at about 30–32 ATP per glucose, once you account for the cost of shuttling electrons into the mitochondria and proton leak. The classic ~36–38 figure you may have memorized in school is the theoretical maximum and is now considered an over-count. Either way the point holds: oxygen-based respiration yields ~15× more ATP than glycolysis alone (which nets just 2).
How much ATP (by mass) does your whole body recycle in a single day?
Somewhere between 5 and 120 kg.
Reveal the answer →
~your body weight
Roughly your entire body weight in ATP each day — commonly cited as ~50–75 kg for an adult. That doesn't mean you contain that much: you hold only a few grams of ATP at any moment and recharge each molecule thousands of times a day. It's a turnover figure, not a stockpile — which is exactly why a throttled recharge line is felt as fatigue so quickly.
Why 'tired' can be a cofactor story
Every stage of the energy line leans on B-vitamins, plus iron, magnesium, and CoQ10. Run low on any of them and the line makes less ATP per unit of food — you eat and sleep normally but still feel flat. Fixing a real deficiency can restore energy; piling extra on top of an already-adequate level does little.
How it works →
NAD⁺/NADH (from B3) and FAD/FADH₂ (from B2) are the electron-carrier currency of the Krebs cycle and ETC. Thiamine (B1) is a cofactor for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase — gateway steps into and within the cycle. Iron sits in the ETC's cytochromes and iron–sulfur clusters; CoQ10 ferries electrons between complexes; magnesium is required for ATP to be biologically active (ATP works as Mg·ATP). A shortfall anywhere lowers the effective P/O ratio — ATP made per oxygen consumed.
What the studies show →
Frank deficiencies clearly impair energy metabolism — iron-deficiency fatigue and thiamine deficiency (beriberi) are unambiguous and reversible with repletion. The weaker, more marketed claim is that CoQ10 or B-complex supplements boost energy in already-replete healthy people; here the evidence is thin and effects are small to absent. CoQ10 has more credible (still modest) support in statin-associated muscle symptoms and certain mitochondrial conditions, not as a general energy booster.