C1.2.17 Differences between lipids and carbohydrates as respiratory substrates (HL) Notes

Lipids Yield More Energy Per Gram

1. Lipids yield ~37 kJ/g, while carbohydrates yield ~17 kJ/g.
2. Lipids contain more carbon-hydrogen (C-H) bonds and less oxygen compared to carbohydrates.
3. C-H bonds are highly oxidizable, they release substantial energy when broken down during cellular respiration.
4. Carbohydrates already contain more oxygen, meaning they are partially oxidized and release less energy per gram.

Only Carbohydrates Can Be Used Anaerobically

1. Carbohydrates enter via glycolysis:
   1. Glucose is broken down into pyruvate during glycolysis (occurs in the cytoplasm).
   2. Glycolysis produces small amounts of ATP and reduced NAD (NADH).
   3. This process does not require oxygen.
2. If oxygen is unavailable (anaerobic respiration):
   1. Pyruvate is converted to lactate (in animals) or ethanol + CO₂ (in yeast).
   2. This regenerates NAD⁺, allowing glycolysis to continue producing ATP.
3. If oxygen is available (aerobic respiration):
   1. Pyruvate enters the mitochondria and is fully oxidized via the Krebs cycle and electron transport chain.
   2. This produces significantly more ATP.

Lipids Require Oxygen and Cannot Be Used Anaerobically

1. Lipids Enter via Beta-Oxidation:
   1. Fatty acids are broken down through beta-oxidation in the mitochondrial matrix.
   2. The fatty acid chain is repeatedly cleaved, removing 2-carbon fragments.
   3. Each fragment combines with coenzyme A to form acetyl-CoA.
   4. This repeats until the entire fatty acid is converted into multiple acetyl-CoA molecules.
2. Acetyl-CoA Only Works Aerobically
   1. Acetyl-CoA enters the Krebs cycle, producing NADH and FADH₂.
   2. NADH and FADH₂ donate electrons to the electron transport chain, which generates ATP.
   3. The ETC requires oxygen as the terminal electron acceptor.

Key Differences Summarized