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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.
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.