# Respiratory Quotient (RQ) ## What is the Respiratory Quotient? - The **respiratory quotient (RQ)** is the ratio of carbon dioxide molecules produced to oxygen molecules consumed during respiration - It's calculated using a simple formula

$\text{RQ} = \dfrac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ consumed}}$

- The RQ value tells us what type of respiratory substrate is being used by an organism - Different biological molecules (carbohydrates, lipids, proteins) have characteristic RQ values ## RQ Values for Different Substrates - Carbohydrates, lipids and proteins have different typical RQ values - This is because the number of carbon-hydrogen bonds differs in each type of biological molecule ### Why the differences matter - More carbon-hydrogen bonds = more hydrogen atoms available to create a proton gradient - More hydrogens = more ATP molecules can be produced - More oxygen is therefore required to break down the molecule (in the final step of oxidative phosphorylation to form water) ### Standard RQ Values Table | Respiratory substrate | RQ value | |----------------------|----------| | Carbohydrate | 1.0 | | Lipid | 0.7 | | Protein | 0.9 | The differences in RQ values between substrates are easier to visualize in a comparison:

Compare experimental RQ values to these standards to determine which substrate an organism is respiring

## Glucose as an Example - When glucose is aerobically respired, equal amounts of carbon dioxide are produced to oxygen taken in - This gives glucose an RQ value of 1.0 $$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$$ - The 6:6 ratio means: $\text{RQ} = \dfrac{6}{6} = 1.0$ ## Calculating RQ from Respiration Equations - The respiratory quotient is calculated from balanced respiration equations - It involves comparing the ratios of carbon dioxide given out to oxygen taken in ### Step-by-step process 1. **Create the respiration equation** for your substrate 2. **Balance the equation** by ensuring equal numbers of each atom on both sides 3. **Apply the RQ formula** using the coefficients from the balanced equation

In a balanced equation, the number before the chemical formula represents molecules/moles of that compound

### Worked Example: Palmitic Acid A common fatty acid in olive oil is palmitic acid with molecular formula $\text{C}_{16}\text{H}_{32}\text{O}_2$. Calculate its RQ value. **Step 1: Create respiration equation** $$\text{C}_{16}\text{H}_{32}\text{O}_2 + \text{O}_2 \longrightarrow \text{CO}_2 + \text{H}_2\text{O}$$ **Step 2: Balance the equation** - C × 16 → C × 1 (multiply CO₂ by 16) - H × 32 → H × 2 (multiply H₂O by 16) - O × 4 → O × 3 (adjust O₂) Counting oxygen atoms: - Left side needs: $(16 \times 2) + (16 \times 1) = 48$ oxygen atoms - Right side has: $2$ oxygen atoms (from palmitic acid) - Additional needed: $48 - 2 = 46$ oxygen atoms = $23$ O₂ molecules $$\text{C}_{16}\text{H}_{32}\text{O}_2 + 23\text{O}_2 \longrightarrow 16\text{CO}_2 + 16\text{H}_2\text{O}$$ **Step 3: Calculate RQ** $$\text{RQ} = \dfrac{16}{23} = 0.70$$ This matches the typical RQ for lipids! ## Why Lipids Have Lower RQ Values - Fatty acids in lipids have long hydrocarbon chains packed with hydrogen atoms - These hydrogen atoms are released when the lipid is broken down - During respiration, hydrogen atoms play a vital role: - Substrate molecules break down and hydrogen atoms become available - Coenzymes NAD and FAD collect them (become reduced) - Hydrogens are transferred to the inner mitochondrial membrane - They split into protons and electrons - Protons are pumped into intermembrane space, forming a gradient - This gradient drives ATP synthesis via chemiosmosis - Finally, protons combine with oxygen to form water

A molecule with higher hydrogen content produces a greater proton gradient, allowing formation of more ATP

- Because lipids release so many hydrogens during breakdown, they need proportionally more oxygen to accept all those protons and electrons at the end - This means: **high CO₂ produced** but **even higher O₂ consumed** - Result: RQ value **less than 1.0** (typically 0.7) ## RQ for Anaerobic Respiration - Anaerobic respiration occurs without oxygen but still produces small amounts of ATP - Different organisms use different anaerobic pathways: - **Mammalian muscle cells**: lactate fermentation - **Plant tissue and yeast**: ethanol fermentation ### Lactate Fermentation in Mammals - The RQ **cannot be calculated** for anaerobic respiration in muscle cells - Why? Because **no oxygen is used** and **no carbon dioxide is produced** during lactate fermentation $$\text{Glucose} \longrightarrow \text{Lactic acid} + \text{Energy}$$ ### Ethanol Fermentation in Yeast - For yeast cells, the RQ tends towards **infinity** - Why? Because **no oxygen is used** while **carbon dioxide is still being produced** #### Worked Example: Fermentation in Bread Dough Yeast in bread dough produces bubbles of CO₂ through fermentation: $$\text{glucose} \longrightarrow \text{ethanol} + \text{carbon dioxide} + \text{energy}$$ **Step 1: Create the respiration equation** $$\text{C}_6\text{H}_{12}\text{O}_6 \longrightarrow \text{C}_2\text{H}_5\text{OH} + \text{CO}_2 + \text{energy}$$ **Step 2: Balance the equation** $$\text{C}_6\text{H}_{12}\text{O}_6 \longrightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 + \text{energy}$$ **Step 3: Calculate the RQ value** $$\text{RQ} = \dfrac{\text{CO}_2}{\text{O}_2} = \dfrac{2}{0} = \infty \text{ (infinity)}$$

Ethanol fermentation is a two-step process producing both CO₂ and ethanol waste (unlike lactate fermentation which is one-step)

## Investigating RQ Using Respirometers - Respirometers measure oxygen consumption during respiration in organisms - They can also be used to calculate respiratory quotients - Experiments typically use organisms like germinating seeds or invertebrates (e.g., woodlice, maggots) ### Equipment Setup - The manometer fluid moves according to how much oxygen has been consumed and carbon dioxide produced - **With soda-lime present**: CO₂ is absorbed, so only O₂ consumption affects the fluid - **Without soda-lime**: both O₂ consumption and CO₂ production affect the fluid ### Calculating Gas Volume Change The volume of oxygen consumed ($\text{cm}^3 \text{ min}^{-1}$) can be calculated using: - **r** = radius of the capillary tube (cm) - **h** = distance moved by manometer fluid (cm) in one minute $$\text{Volume} = \pi r^2 h$$ ### Method **1. Measure oxygen consumption** - Set up respirometer with soda-lime present in both tubes - Run the experiment - Use the manometer reading to calculate gas volume change in a given time: **x** cm³ min⁻¹ - This is the **oxygen consumption** **2. Reset the apparatus** - Allow air to re-enter tubes via screw cap - Reset manometer fluid using syringe **3. Run experiment again** - Remove soda-lime from both tubes - The manometer reading now shows both oxygen consumed AND carbon dioxide produced: **y** cm³ min⁻¹ - Note: Less CO₂ is often given off than O₂ consumed (because some carbon may end up in other compounds) - So pressure drops and manometer moves toward the organism **4. Calculate the difference** - Difference between reading 1 and reading 2 gives carbon dioxide produced - **Carbon dioxide produced** = x - y cm³ min⁻¹

When x and y volumes are equal, the manometer fluid level won't change, giving an RQ of 1

### RQ Calculation from Respirometer Data The measurements can be used to calculate RQ: - **x** = volume of oxygen consumed by respiration in a given time - **y** = volume showing both O₂ consumed and CO₂ produced in a given time $$\text{RQ} = \dfrac{x - y}{x}$$ Or alternatively: $$\text{RQ} = \dfrac{\text{CO}_2}{\text{O}_2}$$ ### Worked Example: Respirometer Calculation In a respirometer experiment with germinating beans: - x = 3.2 cm³ min⁻¹ - y = 0.9 cm³ min⁻¹ Calculate the RQ. **Answer:** $$\text{RQ} = \dfrac{x - y}{x} = \dfrac{3.2 - 0.9}{3.2} = \dfrac{2.3}{3.2} = 0.72$$ This suggests the beans are respiring **lipids** (as the RQ is close to 0.7) ## Interpreting RQ Values ### Mixed Substrate Respiration - Cells may use a **mixture of substrates** during respiration - For example, an RQ value of **0.85** suggests both carbohydrates and lipids are being used simultaneously - This makes sense because: - Glucose has RQ = 1.0 - Lipids have RQ = 0.7 - A value between them (0.85) indicates both are being respired ### Normal Substrate Order - Under normal cell conditions, substrates are used in this order: 1. **Carbohydrates** first 2. **Lipids** second 3. **Proteins** last (only when other substrates exhausted) ### Nutritional Indicators The RQ can indicate feeding status: - **RQ > 1.0**: suggests excessive carbohydrate/calorie intake - Excess glucose is being converted to fats for storage - This releases more CO₂ than O₂ consumed - **RQ < 0.7**: suggests underfeeding/starvation - Body is breaking down proteins for energy - Last resort when carbohydrate and lipid stores depleted

An RQ changing over time means the substrate being respired has changed

## Experimental Investigations - Respirometers can investigate how different factors affect RQ over time - Example factors: - **Temperature**: using a series of water baths - **Substrate availability**: providing different food sources - **Exercise intensity**: comparing resting vs. active organisms ### Controlling Variables Important controls for reliable results: - Use a **controlled water bath** to keep temperature constant - Include a **control tube** with equal volume of inert material (e.g., glass beads) to compensate for atmospheric pressure changes - **Repeat experiments** multiple times and calculate an average - Ensure organisms are similar size/mass for fair comparison ### Analysis: Substrate Switching During Exercise - When an RQ value changes during an experiment, it indicates a **substrate switch** - For example, a marathon runner might show substrate switching as glycogen stores are depleted:

The chart above illustrates a typical pattern: - **Early race** (0-60 min): RQ ≈ 1.0 (using glycogen stores - carbohydrate) - **Mid race** (60-120 min): RQ ≈ 0.85 (using mix of glycogen and fat stores) - **Late race** (120+ min): RQ ≈ 0.7 (primarily using fat stores when glycogen depleted)

Always ensure your respiration equation is fully balanced before calculating RQ values

## Common Mistakes to Avoid - **Not balancing equations properly** before calculating RQ - Count all atoms carefully on both sides - **Forgetting that gases of equal molecules occupy equal volumes** - This is why we can use the coefficients directly from balanced equations - **Assuming RQ can be calculated for lactate fermentation** - Remember: no O₂ consumed and no CO₂ produced = cannot calculate - **Not accounting for soda-lime in respirometer experiments** - With soda-lime: measures only O₂ consumption - Without soda-lime: measures net effect of both gases