CJ

18.8 respiratory substrates

Use of Lipids + Proteins as Respiratory Substrates

Alternative substrates: substances other than glucose ex. lipids + proteins can be oxidized for energy by cells, instead of glucose.

Lipids as Respiratory Substrates

  • Hydrolyzed into glycerol + fatty acids.

  • Glycerol converted to triose phosphate → enters glycolysis.

  • Fatty acids broken into 2-carbon fragments → converted into acetyl CoA → enters Krebs cycle.

Proteins as Respiratory Substrates

  • Hydrolyzed into amino acids.

  • Amino group removed (deamination).

  • Three-carbon compounds converted → pyruvate; four- + five-carbon compounds → Krebs cycle intermediates.

Amino acids used for energy mainly as a last resort due to other vital functions.

Comparing Energy in Respiratory Substrates

  • Different substrates release varying ATP amounts.

Respiratory Substrate

Energy Yield

Description

Lipids

Highest yield of ATP

Over double the ATP per gram vs. carbs due to high C-H bond content. Ideal for long-term energy storage.

Proteins

Intermediate yield

Moderate energy, diverse functions. Used when other sources are limited.

Carbohydrates

Least energy per gram

Primary choice for quick energy due to easy metabolic access.

Respiratory Quotients (RQ)

respiratory quotient (RQ): ratio of volume of carbon dioxide produced to volume of oxygen used during respiration.

  • RQ = volume of CO2 produced / volume of O2 used.

  • Calculated using a respirometer

  • Is a quotient so has no units

Interpreting RQ Values

  • Indicates metabolic conditions + substrates being respired.

  • Lipids + proteins need more oxygen for oxidation, leading to lower RQ values.

Respiratory Substrate

RQ

Carbohydrate

1.0

Lipid

0.7

Protein

0.9

  • RQ > 1 indicates anaerobic respiration.

  • In plants, low RQ may suggest photosynthesis as CO2 from respiration is used.

explain why lipids have a higher yield of ATP per gram than carbohydrates

  • Lipids (fatty acids) have higher hydrogen to carbon ratio than carbs.

  • Beta oxidation of fatty acids produces reduced NAD + FAD.

  • More acetate produced

  • More turns of Krebs cycle

  • More reduced NAD + FAD produced.

  • More electrons flow along ETC in oxidative phosphorylation.

  • More protons pumped → intermembrane space from matrix so steeper proton gradient

  • More ATP by chemiosmosis.

  • Also ATP from substrate level phosphorylation in Krebs.