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 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.

• 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.