explain the relative energy values of carbohydrates, lipids and proteins as respiratory substrates

Energy – Respiratory Substrates (Cambridge International AS & A‑Level Biology)

Learning Objective

Explain, with quantitative support, the relative energy values of carbohydrates, lipids and proteins when they are used as respiratory substrates.

Syllabus Checklist (Topic 12 – Energy & Respiration)

Key Concepts

• Oxidation of organic molecules releases free energy; the energy is captured as ATP by substrate‑level phosphorylation and oxidative phosphorylation.
• The number of reduced carbon atoms in a molecule determines how many NADH and FADH₂ are produced, and therefore the total ATP that can be generated.
• Energy yield can be expressed:
  • per mole of substrate (mol⁻¹),
  • per gram of substrate (g⁻¹), or
  • in kilojoules (kJ) or kilocalories (kcal).
• P/O ratio (ATP formed per pair of electrons transferred) limits the efficiency of oxidative phosphorylation:
  • ≈ 2.5 ATP per NADH,
  • ≈ 1.5 ATP per FADH₂.
• ΔG°′ for hydrolysis of ATP under cellular conditions ≈ ‑30.5 kJ mol⁻¹ (‑7.3 kcal mol⁻¹). This value is used to convert kJ g⁻¹ into “ATP equivalents per gram”.

Assumptions Used in All Calculations

• P/O ratios: NADH = 2.5 ATP, FADH₂ = 1.5 ATP.
• All cytosolic NADH (from glycolysis) enters mitochondria via the malate‑aspartate shuttle, costing 1 ATP per NADH.
• Substrate‑level phosphorylation:
  • Glucose – 4 ATP (2 net from glycolysis + 2 from the TCA cycle).
  • Palmitic acid – 2 ATP (one from each β‑oxidation cycle).
  • Amino acid (alanine example) – 1 ATP (from conversion to pyruvate).
• Standard enthalpy of combustion (ΔH⁰_comb):
  • Glucose: 2805 kJ mol⁻¹ (≈ ‑670 kcal mol⁻¹).
  • Palmitic acid: 9970 kJ mol⁻¹ (≈ ‑2385 kcal mol⁻¹).
  • Alanine (representative amino‑acid): 780 kJ mol⁻¹ (≈ ‑186 kcal mol⁻¹).

Catabolic Pathways – Quick Overview

All three substrates converge on the citric‑acid cycle (TCA) as acetyl‑CoA or a TCA‑intermediate.

Quick‑Calc Box (Copy‑Paste for Exam)

Step‑by‑Step ATP‑Yield Calculations

1. Glucose (C₆H₁₂O₆)

1. Glycolysis (cytosol)
   • Net 2 ATP.
   • 2 NADH → (2 × 2.5 – 2 × 1) = 3 ATP after shuttle cost.
2. Link reaction (pyruvate → acetyl‑CoA)
   • 2 NADH → 2 × 2.5 = 5 ATP.
3. Citric‑acid cycle (2 acetyl‑CoA)
   • 6 NADH → 6 × 2.5 = 15 ATP.
   • 2 FADH₂ → 2 × 1.5 = 3 ATP.
   • 2 GTP (substrate‑level) → 2 ATP.
4. Total: 2 + 3 + 5 + 15 + 3 + 2 = 30 ATP per mole of glucose (the Cambridge specification accepts 30 ATP; 31–32 ATP are quoted when the shuttle cost is ignored).

2. Palmitic Acid (C₁₆H₃₂O₂)

1. β‑oxidation (7 cycles)
   • 7 NADH → 7 × 2.5 = 17.5 ATP.
   • 7 FADH₂ → 7 × 1.5 = 10.5 ATP.
   • Produces 8 acetyl‑CoA.
2. Citric‑acid cycle (8 acetyl‑CoA)
   • Each acetyl‑CoA yields 3 NADH, 1 FADH₂, 1 GTP.
   • 8 × [3 × 2.5 + 1 × 1.5 + 1] = 8 × 10 = 80 ATP.
3. Substrate‑level ATP from β‑oxidation: 2 ATP.
4. Total: 17.5 + 10.5 + 80 + 2 ≈ 110 ATP. Cambridge’s accepted value, using the P/O ratios above, is **≈ 106 ATP**.

3. Alanine (representative amino‑acid)

1. Transamination → pyruvate (no ATP cost).
2. From pyruvate onward (identical to glucose after glycolysis):
   • Link reaction: 1 NADH → 2.5 ATP.
   • Citric‑acid cycle (1 acetyl‑CoA): 3 NADH + 1 FADH₂ + 1 GTP → 3 × 2.5 + 1 × 1.5 + 1 = 10 ATP.
3. Including the 1 ATP from the conversion of alanine to glucose‑6‑P (if the full gluconeogenic route is considered) gives **≈ 22 ATP** per mole of alanine. For Cambridge purposes, a range of **20–25 ATP mol⁻¹** is acceptable.

ATP Yield – Summary Table (per Mole & per Gram)

Why Lipids Yield the Most Energy

• Fatty acids contain long chains of highly reduced carbon atoms (high C:H ratio) and **lack oxygen** in their backbone, so oxidation releases many more electrons per gram than carbohydrates.
• Each two‑carbon unit removed in β‑oxidation generates one NADH and one FADH₂ – both feed the electron‑transport chain, producing large amounts of ATP.
• Palmitic acid (16 C) produces 8 acetyl‑CoA, 7 NADH and 7 FADH₂ before entering the TCA cycle, resulting in > 100 ATP per molecule.
• Carbohydrates already contain oxygen; therefore fewer high‑energy C–H bonds are available for oxidation.
• Proteins must first be deaminated, a process that does **not** generate ATP and also incurs a nitrogen‑excretion cost, lowering overall efficiency.

Respiratory Quotients (RQ)

The respiratory quotient is the ratio of CO₂ produced to O₂ consumed. Theoretical values for complete oxidation are:

ATP – The Cellular Energy Currency (Link to LO 12.5)

All ATP generated from the substrates is used for:

• Active transport (e.g., Na⁺/K⁺‑ATPase).
• Synthesis of macromolecules (proteins, nucleic acids, glycogen).
• Mechanical work (muscle contraction, flagellar movement).
• Cell signalling (phosphorylation cascades).

Thus, the quantitative ATP yield directly determines how much work a cell can perform from a given fuel source.

Physiological Significance of the Different Yields

1. Prolonged, low‑intensity activity (e.g., marathon running) – Lipids are favoured because they supply a large amount of ATP per gram, sparing limited carbohydrate stores.
2. High‑intensity, short‑duration effort (e.g., sprinting, weightlifting) – Carbohydrates dominate; glycolysis provides rapid ATP (even anaerobically) despite a lower per‑gram yield.
3. Fasting or starvation – Proteins are a last‑resort source; deamination supplies gluconeogenic precursors for the brain, but at the cost of muscle loss and nitrogen waste.
4. Clinical contexts
   • Diabetes mellitus – impaired glucose utilisation forces greater reliance on fatty‑acid oxidation, increasing ketone production.
   • Fatty‑acid oxidation disorders (e.g., MCAD deficiency) – reduced ATP from lipids leads to hypoglycaemia during fasting.
   • Cachexia – excessive protein catabolism lowers ATP yield and contributes to energy deficit.

Exam‑Technique Box (AO1–AO3)

Typical command‑words: describe, explain, calculate, compare, discuss

• Describe the three catabolic pathways – AO1.
• Explain why lipids give the highest ATP yield – AO2 (link structure to function).
• Calculate the ATP yield from 1 g of glucose using the P/O ratios – AO2 (numerical manipulation).
• Compare the suitability of each substrate for different exercise intensities – AO2.
• Discuss the advantages and disadvantages of using protein as an energy source – AO3 (evaluation).

Practical Skills Snapshot (Paper 3 & Paper 5)

Students should be able to design, carry out and evaluate an experiment that investigates substrate utilisation, for example:

1. Measure O₂ consumption of isolated rat liver mitochondria in the presence of glucose, palmitate or alanine using a Clark‑type electrode.
2. Calculate the respiratory quotient (RQ) for each substrate and relate it to the theoretical RQ values (glucose ≈ 1.0, fatty acids ≈ 0.7, proteins ≈ 0.8).
3. Analyse sources of error (substrate purity, temperature control, electrode drift) and suggest improvements.

Suggested Diagram

Link to Cambridge Mathematical Requirements

All ATP‑yield calculations are examples of the quantitative work required by the Cambridge syllabus. Students must be comfortable with:

• Using the P/O ratios to convert NADH/FADH₂ numbers into ATP.
• Converting molar energy (kJ mol⁻¹) into per‑gram values and then into “ATP equivalents per gram”.
• Interpreting RQ values and relating them to substrate oxidation.

Practising these calculations will develop the mathematical skills expected for both Paper 3 (structured) and Paper 5 (extended) questions.