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

What is Energy?

In biology, energy is the ability to do work. Our bodies get energy from food, which is broken down into molecules that can be used by cells. Think of it like a car that needs fuel to move.

Carbohydrates 🏎️

Carbohydrates are the quickest source of energy. They are like the gasoline that powers a sports car – easy to burn and gives a rapid burst of power.

• Primary molecule: glucose ($C_6H_{12}O_6$)
• Energy per gram: ~4 kcal
• ATP yield from one glucose: ~30–32 ATP molecules
• Used in glycolysis → pyruvate → Krebs cycle → oxidative phosphorylation

Lipids 🛢️

Lipids are the high‑energy, long‑lasting fuel. They’re like diesel – they give more power per gram but take longer to start.

• Primary molecules: fatty acids (e.g., palmitic acid $C_{16}H_{32}O_2$)
• Energy per gram: ~9 kcal (more than double that of carbs)
• ATP yield per fatty acid: up to ~120 ATP (depends on chain length)
• Metabolism: β‑oxidation → acetyl‑CoA → Krebs cycle → oxidative phosphorylation

Proteins ⚙️

Proteins are the backup generator. They’re not primarily for energy but can be used when carbs and fats are low.

• Primary molecules: amino acids (e.g., alanine, leucine)
• Energy per gram: ~4 kcal (similar to carbs)
• ATP yield: variable, usually < 30 ATP per amino acid
• Metabolism: deamination → entry into Krebs cycle or gluconeogenesis

1. Remember the key energy values: 4 kcal g⁻¹ for carbs and proteins, 9 kcal g⁻¹ for lipids.
2. Use the analogy of fuel types to explain why lipids give more energy per gram.
3. When asked about ATP yield, state the approximate range for glucose and note that fatty acids can yield up to ~120 ATP.
4. Highlight that proteins are not the main energy source but can be used when other substrates are scarce.
5. Include a quick diagram in your answer (draw a simple flow: glycolysis → Krebs → oxidative phosphorylation) to show the metabolic pathway.