Edexcel A Level (IAL) Biology -7.8 Core Practical 16: Respirometer to Calculate RQ- Study Notes- New Syllabus

CORE PRACTICAL 16

Using a Respirometer to Measure Respiration and RQ

🌱 Introduction

A respirometer measures the rate of respiration by detecting the uptake of oxygen and/or release of carbon dioxide.
The respiratory quotient (RQ) can be calculated using:

RQ = \(\dfrac{CO_2 \ produced}{O_2 \ consumed}\)

This helps identify which substrate (carbohydrate, fat, protein) the organism is respiring.

Principle

• Germinating seeds or small invertebrates consume oxygen during respiration.
• Oxygen uptake causes a drop in volume or movement of liquid in a manometer/respirometer.
• CO₂ can be absorbed by KOH to measure O₂ uptake only.
• Rate of oxygen uptake = respiration rate.
• Measuring both O₂ consumed and CO₂ produced allows calculation of RQ.

Materials

• Germinating seeds (e.g., beans, peas) or small invertebrates
• Respirometer setup: test tube / respiratory chamber, capillary tubing with scale, syringe/manometer
• Potassium hydroxide (KOH) to absorb CO₂
• Oil to reduce evaporation
• Cotton wool plug
• Timer / stopwatch
• Water bath (optional)

Method

• Place respiring material in a test tube or respirometer chamber.
• Add KOH if measuring only oxygen uptake.
• Seal with cotton plug and connect to capillary tube with scale/liquid indicator.
• Set up a control tube with dead material or no respiring organism.
• Place in constant temperature environment.
• Measure liquid movement over time and record readings.
• Calculate rate of oxygen consumption and RQ using measurements.

🟡 Variables

Independent variable: Type of substrate, temperature, or organism used

Dependent variable: Rate of oxygen consumption (respiration rate) and RQ value

Controlled variables: Temperature, volume of respiring material, type of respirometer/chemical, measurement intervals

📋 Example Data Table

🟠 Calculations

• Rate of respiration: \( \text{Rate} = \dfrac{\text{Change in O₂ volume}}{\text{Time}} \)
• Respiratory Quotient (RQ): \( \text{RQ} = \dfrac{CO_2 \ produced}{O_2 \ consumed} \)
• RQ values indicate substrate:
  • Carbohydrate ≈ 1.0
  • Fat ≈ 0.7
  • Protein ≈ 0.8–0.9

🦺 Safety & Ethical Considerations

• Use small, non-harmful organisms.
• Avoid overheating or stressing animals.
• Handle chemicals like KOH carefully.
• Wash hands after handling seeds, animals, or chemicals.