3.2.9Inquiry Process
Respiratory Substrates — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 115 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Lipids as Respiratory Substrates
Cells can respire different organic molecules, and lipids are an especially rich source of energy.
Energy content
- Energy content Per gram, lipids release more than twice as much energy as carbohydrates when oxidised.
Reason
- Reason Lipid molecules contain less oxygen, so there are more carbon-hydrogen bonds that can be oxidised during respiration.
Storage advantage
- Storage advantage Lipids are insoluble, so they do not lower the water potential of the cell and cause water to enter by osmosis.
Examples
- Examples Animals store fat to survive unfavourable periods, and many plants store oils in seeds and fruits.
Metabolic water
- Metabolic water Oxidation of lipids produces more water than the same mass of carbohydrate, which helps animals in dry habitats.
Part 2 of 5Entering the Pathway
1
Lipids are first hydrolysed into glycerol and fatty acids by lipase enzymes.
2
Fatty acids are broken down in steps into two-carbon acetyl groups.
3
The acetyl groups combine with coenzyme A to form acetyl CoA.
4
Acetyl CoA enters the Krebs cycle, and the hydrogen it yields is used in oxidative phosphorylation.
5
Glycolysis and anaerobic respiration can use only carbohydrate, so fatty acids can be respired only aerobically.
Part 3 of 5Comparing the Two Substrates
Comparing the Two Substrates
Speed of release
- Speed of release Carbohydrates are hydrolysed more easily, so energy is transferred more quickly than from lipids.
Solubility
- Solubility Glucose is soluble and affects the osmotic properties of cells, so it is stored as insoluble glycogen or starch instead.
Anaerobic use
- Anaerobic use Only carbohydrate can be respired anaerobically, which is why intense exercise relies on glucose.
Oxygen needed
- Oxygen needed Because lipids have more hydrogen to oxidise, they require more oxygen per mole.
| Feature | Lipids | Carbohydrates |
|---|---|---|
| Energy per gram | Higher | Lower |
| Metabolic water | More | Less |
| Solubility | Insoluble | Soluble |
| Speed of breakdown | Slower | Faster |
| Anaerobic use | No | Yes |
Part 4 of 5Worked Example
Question
- A lipid releases 38 kJ per gram and a carbohydrate releases 17 kJ per gram when oxidised.
- Compare the energy released from 50 g of each, and calculate how many times larger the lipid value is.
- Lipid50 x 38 = 1900 kJ.
- Carbohydrate50 x 17 = 850 kJ.
- Ratio = 1900 / 850 = 2.2.
Answer The lipid releases 1900 kJ and the carbohydrate 850 kJ, so the lipid gives about 2.2 times more energy, consistent with more than double.
Part 5 of 5Exam Checklist & Tips
3.2.9 – Key Points for the Exam
- Respiratory substrate A molecule that is oxidised in respiration to release energy.
- Metabolic water Water produced as a product of respiration.
- Acetyl CoA A two-carbon compound that enters the Krebs cycle.
- Lipase An enzyme that hydrolyses lipids.
Common Mistakes
- Saying lipids contain more oxygen than carbohydrates, when they contain less.
- Writing that lipids can be used in anaerobic respiration through glycolysis.
- Claiming that camels store water in the hump; it stores fat that gives metabolic water on oxidation.
- Forgetting that lipids enter through acetyl CoA, not as pyruvate.
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