Investigating respiration (Edexcel GCSE Biology): Revision Notes

Aim: To investigate the rate of respiration in living organisms by measuring the oxygen uptake using a respirometer and to explore how changes in temperature affect the rate of respiration.

Risks and Safety Considerations:

• Sodium hydroxide is corrosive and can cause skin burns. Handle with care, wear gloves, and avoid skin contact.
• Ensure that living organisms are handled gently and returned to their habitat after the experiment.
• Avoid trapping fingers in the respirometer apparatus or other equipment.
• Use thermometers and water baths carefully to avoid burns or spills.

Equipment:

Respirometer (with a manometer or coloured liquid)

Living organisms (e.g., insects, maggots, woodlice, or germinating peas)

Glass beads (for control)

Sodium hydroxide (or another alkali to absorb carbon dioxide)

Stopwatch

Gauze

Philtre paper

Clamp stand

Thermometer

Water bath (for temperature control)

Method:

1. Setup:

• The respirometer consists of two tubes:
• One tube contains the living organisms.
• The other tube contains glass beads of the same mass as the organisms, acting as a control.
• Both tubes also contain sodium hydroxide, which absorbs any carbon dioxide produced during respiration, ensuring that any volume changes in the respirometer are due to oxygen uptake only.
• The tubes are connected by a manometer (or coloured liquid in a U-shaped tube), which measures the change in gas volume.

2. Procedure:

• Once the setup is complete, the tap on the apparatus is closed to isolate the system.
• As the organisms respire, they will take up oxygen, which causes a reduction in the gas volume in the tube.
• The reduction in volume causes the coloured liquid in the manometer to move towards the tube containing the organisms.
• The distance moved by the coloured liquid is measured using a ruler. This distance represents the volume of oxygen taken up by the organisms.

3. Measurements:

• The volume of oxygen taken up is calculated using the formula for the volume of a cylinder:

where R is the radius of the manometer tube, and the distance moved is measured in cm.

• The unit for the rate of respiration is cm³/min.

4. Control:

• The glass beads in the control tube ensure that any movement of the liquid is due to the respiration of the organisms and not due to other factors like temperature changes or pressure differences.

5. Investigating Temperature:

• To explore the effect of temperature on the rate of respiration, the experiment is repeated at different temperatures (e.g., 10°C, 15°C, 20°C, 25°C, and 30°C).
• Place the respirometer in a water bath to control the temperature and repeat the experiment at each temperature.
• Measure the movement of the coloured liquid at each temperature and calculate the rate of respiration for comparison.

Evaluation:

• Limitations: Ensure that the apparatus is airtight and that there is no gas leakage. Variations in the size and activity of the organisms used could affect results.
• Improvements: Repeat the experiment multiple times to ensure reliable data. Using a digital manometer could increase precision. This practical demonstrates how respiration can be measured by the uptake of oxygen and how external factors like temperature affect metabolic processes in living organisms.