12.1.312 · Energy and Respiration
Energy Values of Respiratory Substrates — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 3 of 18 Unit quiz
Lesson summary
12.1.3 Energy Values of Respiratory Substrates — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare the energy values of carbohydrate, lipid and protein and explain the differences.Definition — an organic molecule that can be oxidised in respiration to release en…
- Main substrate
- Other substrates
- Amino acids
- Comparison
- Role of hydrogen
- Carbohydrates
Part 1 of 7Respiratory substrates
Definition
- Definition an organic molecule that can be oxidised in respiration to release energy.
Main substrate
- Main substrate carbohydrate, usually glucose, is the main substrate in most cells.
Other substrates
- Other substrates when glucose is used up, cells use other carbohydrates, then lipids, and in extreme starvation proteins.
Amino acids
- Amino acids they are respired aerobically only when all other substrates are exhausted,
- because they are needed to make structural and functional proteins.
Energy value
- Energy value the energy released when 1 g is completely oxidised.
- It can be measured in a calorimeter by burning a known mass in oxygen and measuring the temperature rise of a known mass of water.
Part 2 of 7Typical energy values
| Substrate | Approximate energy value / kJ per g |
|---|---|
| Carbohydrate | 15.8 |
| Protein | 17.0 |
| Lipid | 39.4 |
Comparison
- Comparison lipids release more than twice as much energy per gram as carbohydrates.
Use
- Use lipids are long-term stores in animals and in seeds such as sunflower and castor bean.
Part 3 of 7Explaining the difference
Role of hydrogen
- Role of hydrogen hydrogen is removed from the substrate by dehydrogenases and passed to NAD and FAD.
- Reduced NAD and FAD carry it to the inner mitochondrial membrane, where it splits into protons and electrons.
- Protons are pumped into the intermembrane space, forming a gradient used in chemiosmosis.
- After flowing back through ATP synthase, protons are oxidised to form water.
Rule
- Rule a substrate with more hydrogen atoms per unit mass gives more reduced coenzyme, a larger proton gradient and more ATP.
Lipids
- Lipids triglycerides have long hydrocarbon chains with many C-H bonds and very little oxygen.
Carbohydrates
- Carbohydrates they have proportionally fewer hydrogen atoms and more oxygen, so they are partly oxidised already (C6H12O6).
Proteins
- Proteins their composition varies, and amino groups must first be removed (deamination) before the carbon skeleton enters respiration.
- The nitrogen is excreted as urea at an energy cost.
Part 4 of 7Entry into respiration
Carbohydrates
- Carbohydrates they are hydrolysed to glucose, which enters glycolysis.
Lipids
- Lipids they are hydrolysed to glycerol and fatty acids.
- Glycerol becomes triose phosphate and enters glycolysis.
- Fatty acids are broken into two-carbon units by beta oxidation in the matrix and enter as acetyl CoA, forming large amounts of reduced NAD and FAD.
Proteins
- Proteins they are hydrolysed to amino acids and deaminated, and the keto acids enter the Krebs cycle or link reaction.
Oxygen demand
- Oxygen demand fatty acid oxidation needs a lot of oxygen, which affects the respiratory quotient.
Part 5 of 7Biological significance
Mass-efficient store
- Mass-efficient store lipids suit migrating birds and hibernating mammals.
Metabolic water
- Metabolic water lipids release more of it on oxidation, an advantage to desert animals such as camels and the kangaroo rat.
Carbohydrates
- Carbohydrates they are available more quickly because glycogen and glucose are soluble and enter glycolysis directly.
- They also allow anaerobic respiration; lipids can be respired only aerobically.
Brain
- Brain it normally relies on glucose because fatty acids do not cross the blood-brain barrier easily.
Part 6 of 7Using energy values
Food energy
(g carbohydrate × 15.8) + (g protein × 17.0) + (g lipid × 39.4).
Fat
- Fat it contributes a large share of the energy despite being a smaller part of the mass.
Calorimetry limits
- Calorimetry limits a bomb calorimeter oxidises everything, but the body does not digest everything and cannot oxidise the nitrogen of protein.
- The in-body energy value for protein is therefore lower.
Simple investigation
- Simple investigation burn a weighed food sample under a boiling tube of water and record the temperature rise.
- Energy mass of water × 4.2 × temperature rise.
- Errors are heat lost and incomplete combustion; use a draught shield, insulation or a closer flame.
- Repeats improve reliability, and results are compared as energy per gram.
Part 7 of 7More from the specification
More from the specification
Calorimetric respiratory substrate analysis
- Calorimetric respiratory substrate analysis burning known masses in bomb calorimeters provides an empirical means to determine energy values of substrates.
Respiratory quotient of metabolic substrates
- The respiratory quotient reflects the substrate respired: approximately 1.0 for carbohydrates, 0.9 for proteins,
- (RQ = volume of CO2 produced / volume of O2 consumed)What the symbols mean: RQ = respiratory quotient (no unit) · about 1.0 for carbohydrate, 0.7 for lipid, 0.9 for protein
- (RQ = volume of CO2 produced / volume of O2 consumed)
- and 0.7 for lipids.
Energy density and hydrogen atom content
- Lipids yield more than twice the energy per gram (~39 kJ/g) compared to carbohydrates (~17 kJ/g) due to their higher ratio of hydrogen-to-carbon atoms,
- producing more reduced coenzymes.
Respirometer measurements
- A respirometer measures oxygen consumption rates by absorbing expired CO2 with potassium hydroxide, recording fluid displacement in a calibrated manometer.
Respiratory quotient equation
- RQ formula equals volume of carbon dioxide produced divided by volume of oxygen consumed during substrate cellular respiration.
ATP synthesis in chemiosmosis
- ATP synthesis in chemiosmosis protons flowing across the inner mitochondrial membrane down an electrochemical concentration gradient power ATP synthase molecular synthesis.
Manometer fluid displacement
- Manometer fluid displacement changes in gas volume inside a closed respirometer move fluid along the manometer tube, indicating respiratory uptake rates.
Substrate-linked phosphorylation
- Direct transfer of a phosphate from a substrate to ADP.
Volume
- The amount of cytoplasm needing supplies and producing waste.
Manometer
- U-tube whose fluid moves as gas volume changes.
Concentration gradient
- The difference in concentration of a substance between two regions.
Key terms
| Term | Meaning |
|---|---|
| Respiratory substrate | An organic molecule oxidised in respiration to release energy. |
| Energy value | The energy released when 1 g of a substance is completely oxidised. |
| Calorimeter | Apparatus that measures energy released by burning a sample. |
| Beta oxidation | Breakdown of fatty acids into two-carbon units that become acetyl CoA. |
| Deamination | Removal of the amino group from an amino acid. |
| Triglyceride | A lipid made of glycerol and three fatty acids. |
Avoid these mistakes
1
Saying carbohydrates give most energy per gram — lipids give the most.
2
Saying lipids contain more oxygen than carbohydrates — they contain far less.
3
Explaining the difference without naming hydrogen and the proton gradient.
4
Saying protein is the main respiratory substrate — it is used only when others are exhausted.
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