# Respiratory substrates

> CIE A-Level Biology · 9700 A2 Energy and Respiration
> Source: https://www.owlsprep.com/study/cie-9700-u13-respiratory-substrates/

This sub-topic covers the different organic molecules used as respiratory substrates, how much ATP each yields, and how to calculate and interpret the respiratory quotient (RQ) to identify which substrate is being used.

**Prerequisites:** [Aerobic respiration stages](https://www.owlsprep.com/study/cie-9700-u13-aerobic-respiration-stages/); [Oxidative phosphorylation and ATP synthesis](https://www.owlsprep.com/study/cie-9700-u13-oxidative-phosphorylation/)

## Learning objectives

- Identify the main types of respiratory substrates and their entry points into respiration pathways
- Compare the ATP yield of different respiratory substrates and explain the difference
- Calculate and interpret respiratory quotient (RQ) values to identify respired substrates

## Types of Respiratory Substrates

Cells use three main classes of organic molecules as respiratory substrates, preferentially using carbohydrates first before switching to other substrates when carbohydrate stores are depleted. Any of these substrates can enter the established respiratory pathway at different points to produce ATP.

**Respiratory substrate** — Any organic molecule broken down during cellular respiration to release energy for ATP synthesis.

*Example:* Glucose is the most common primary respiratory substrate in most human and plant cells.

- **Carbohydrates**: Stored as glycogen (animals) or starch (plants); glucose is the most common carbohydrate substrate.
- **Lipids**: Stored as triglycerides in adipose tissue; broken into fatty acids and glycerol before entering respiration.
- **Proteins**: Only used when carbohydrates and lipids are exhausted; amino acids require deamination before entering pathways.

**Worked example:** Explain why glycerol from digested triglycerides can be used as a respiratory substrate.

1. Glycerol is a soluble product of triglyceride hydrolysis.
2. Glycerol is converted into glyceraldehyde-3-phosphate (GP), which is a native intermediate of glycolysis.
3. GP continues through glycolysis, the link reaction, and Krebs cycle, producing reduced NAD and FAD just like glucose-derived intermediates.
4. Because glycerol can enter the existing respiratory pathway directly, it can be used as a respiratory substrate.

## ATP Yield of Different Substrates

The amount of ATP produced per gram of substrate depends on the number of hydrogen atoms available for oxidation. Energy release in respiration relies on hydrogen atoms to provide electrons for the electron transport chain and protons for chemiosmosis, so more hydrogen per unit mass equals more ATP.

Lipids are highly reduced molecules with far more hydrogen per gram than carbohydrates or proteins, so they yield more than twice as much energy per gram than the other two substrate types. Proteins have a similar energy yield to carbohydrates per gram.

**Worked example:** Calculate how much more energy is released from 10g of lipid than 10g of carbohydrate. Carbohydrate yields 16 kJ g⁻¹ and lipid yields 39 kJ g⁻¹.

1. Calculate total energy from 10g of carbohydrate:
2. $$10 \text{ g} \times 16 \text{ kJ g}^{-1} = 160 \text{ kJ}$$
3. Calculate total energy from 10g of lipid:
4. $$10 \text{ g} \times 39 \text{ kJ g}^{-1} = 390 \text{ kJ}$$
5. Calculate the difference:
6. $$390 \text{ kJ} - 160 \text{ kJ} = 230 \text{ kJ}$$
7. 10g of lipid releases 230 kJ more energy than the same mass of carbohydrate.

> **tip**
>
> Exam questions frequently ask why lipids have a higher energy yield. Always mention more hydrogen per unit mass leading to more ATP, not just more carbon atoms.

## Respiratory Quotient (RQ)

**Respiratory Quotient (RQ)** — A dimensionless ratio used to identify which respiratory substrate is being oxidised, based on the amount of gas exchanged during aerobic respiration.

*Notation:* \text{RQ} = \frac{\text{Volume of CO}_2 \text{ produced}}{\text{Volume of O}_2 \text{ consumed}}

*Example:* Glucose has an RQ of exactly 1.0.

More oxidised substrates (like carbohydrates) require less oxygen for complete breakdown, so they have higher RQ values. Less oxidised substrates (like lipids) require more oxygen, so they have lower RQ values. RQ values for mixed substrates fall between the values of the individual substrates.

**Worked example:** A respirometer measures 25 cm³ of oxygen consumed and 21 cm³ of carbon dioxide produced during respiration by germinating sunflower seeds. Calculate the RQ and identify the main respiratory substrate.

1. Write the RQ formula:
2. $$\text{RQ} = \frac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}}$$
3. Substitute the measured values:
4. $$\text{RQ} = \frac{21}{25} = 0.84$$
5. Standard RQ values are 1.0 for carbohydrate, 0.7 for lipid, and 0.8-0.9 for protein. An RQ of 0.84 matches a mix of stored lipid and protein, which is typical for germinating seeds that rely on endosperm energy stores.

**Exam command terms**

- **Calculate RQ** — Always show your working and write the formula before substituting values *(Never reverse the ratio of CO₂ to O₂)*

- **Interpret an RQ value** — Link the value to the substrate, and explain any out-of-range values *(An RQ > 1 always indicates anaerobic respiration is occurring alongside aerobic respiration)*

## Common pitfalls

- **Wrong:** Calculating RQ as oxygen consumed divided by carbon dioxide produced
  - Why it fails: The formula is flipped, leading to an incorrect value and wrong interpretation
  - Correct: Always remember RQ = (volume of CO₂ produced) / (volume of O₂ consumed)
- **Wrong:** Claiming lipids have higher ATP yield because they have more carbon atoms per gram
  - Why it fails: Energy production relies on hydrogen, not carbon, for the electron transport chain
  - Correct: State that lipids have more hydrogen atoms per unit mass, leading to more reduced coenzymes and more ATP synthesis
- **Wrong:** Assuming proteins are regularly used as a primary respiratory substrate
  - Why it fails: Proteins are only used when carbohydrate and lipid stores are exhausted, such as during prolonged starvation
  - Correct: Recognize that proteins are not a primary substrate, and are only used as a last resort
- **Wrong:** Interpreting an RQ of 0.8 as definitely meaning only protein is being respired
  - Why it fails: RQ values for mixed substrates fall between the values of individual substrates
  - Correct: An RQ between 0.7 and 1.0 almost always indicates a mix of carbohydrate and lipid being respired
- **Wrong:** Forgetting that amino acids need processing before entering respiration
  - Why it fails: The amino group cannot enter the Krebs cycle and is toxic to cells
  - Correct: Remember amino acids undergo deamination to remove the amino group, before the carbon skeleton enters respiratory pathways

## Cheatsheet

| Respiratory Substrate | Typical RQ Value | Approximate Energy Yield |
| --- | --- | --- |
| Carbohydrate (glucose) | 1.0 | ~16 kJ g⁻¹ |
| Lipid (triglyceride) | 0.7 | ~39 kJ g⁻¹ |
| Protein | 0.8 - 0.9 | ~18 kJ g⁻¹ |
| Mixed carbohydrate + lipid | 0.7 - 1.0 | Intermediate between both substrates |
| Anaerobic glucose respiration | > 1.0 | Much lower yield than aerobic |

## What's next

Understanding respiratory substrates builds on your knowledge of aerobic respiration stages and prepares you for investigating respiration experimentally using respirometers, a common practical assessment topic in CIE A-Level Biology. This concept also links to integrated metabolic pathways, such as deamination of amino acids in the liver and metabolic homeostasis, which appear in multiple sections of the syllabus. Being able to calculate and interpret RQ values is a frequent exam question that combines theory and practical skills, so mastering this sub-topic will help you score well in both theory and practical papers.

- [Measurement of respiration rate](https://www.owlsprep.com/study/cie-9700-u13-measurement-of-respiration-rate/)
- [Photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-overview/)
- [Photosynthetic pigments](https://www.owlsprep.com/study/cie-9700-u14-photosynthetic-pigments/)

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