Fatty Acid Oxidation, Ketone Bodies and Choosing a Fuel
Fatty acids are shortened two carbons at a time, feeding acetyl groups into the cycle and reduced carriers into the chain, which is why fat yields more ATP per carbon than sugar. When carbohydrate runs low the liver converts some of that acetyl into ketone bodies the brain can use.
What a learner can do afterwards
- Calculates the ATP yield of a stated fatty acid from the rounds it takes to break down.
- Explains why fat cannot be converted to glucose in humans.
- Links ketone body production to a shortage of cycle intermediates rather than to fat excess.
1 · Read
Fatty acids are shortened two carbons at a time by beta oxidation. Each round produces one acetyl CoA plus one NADH and one FADH2, with the chain two carbons shorter. Those carriers feed the electron transport chain and the acetyl groups feed the cycle, which is why fat yields more ATP per carbon than sugar.
Count the rounds from the chain length: rounds equal carbons divided by two, minus one, while acetyl units equal carbons divided by two. An 8-carbon acid therefore takes 3 rounds and gives 4 acetyl CoA. Add the carriers plus the cycle output per acetyl to price the full yield.
Acetyl CoA from fat cannot become net glucose in humans, because the pyruvate step is irreversible and the cycle releases two carbons for every two that enter. Fat is superb fuel but no sugar source, so the fasting brain runs on ketone bodies. Ketones appear when oxaloacetate runs short and stranded acetyl piles up, which signals scarce cycle capacity, not simply abundant fat.
Read ketones as an oxaloacetate report: fasting diverts it to glucose making, so acetyl piles up and ketones rise. Most tissues including the fasting brain burn ketones, but red blood cells cannot, since they lack mitochondria.
Fat is chopped two carbons per round for a rich carrier harvest, yet its acetyl cannot become glucose, so fasting livers park stranded acetyl as ketones.
2 · Watch
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Where it sits
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Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.