Lesson Summary:
Different molecules like carbohydrates, lipids, and proteins can act as respiratory substrates. Glucose is used first, but lipids provide more energy per gram due to their high hydrogen content. Proteins are used last, after conversion to pyruvate or acetyl-CoA. The energy released during respiration depends on the hydrogen atoms oxidized in the electron transport chain. The Respiratory Quotient (RQ) helps determine which substrate is being used by comparing the volumes of CO₂ produced and O₂ consumed.

Three Things Students Must Learn:

  1. Lipids release the most energy per gram due to more hydrogen atoms.

  2. Proteins are converted before entering respiration and are used only when necessary.

  3. RQ values reveal the type of substrate being used (carbohydrates = 1.0, lipids = 0.7, proteins = 0.9).