Core practical 7: Rate of respiration

Aim

To use a respirometer to measure the rate of respiration of germinating seeds at different temperatures, by measuring how far a drop of coloured liquid moves as oxygen is used up.

Background

Living organisms release energy by respiration. In aerobic respiration, glucose reacts with oxygen to produce carbon dioxide and water: glucose + oxygen → carbon dioxide + water. Germinating seeds respire quickly because they are growing and need a lot of energy.

A respirometer measures how fast organisms use up oxygen. The carbon dioxide they produce is absorbed by soda lime, so the total volume of gas in the apparatus falls. This pulls a drop of coloured liquid in a capillary tube towards the organisms. The further it moves in a set time, the faster the rate of respiration.

Respiration is controlled by enzymes, so temperature affects its rate. A rise in temperature gives particles more energy, so enzymes and substrate collide more often and the rate increases. At high temperatures the enzymes denature and the rate falls.

The volume of oxygen used can be calculated from the distance moved by the liquid: volume = πr²h, where r is the radius of the capillary tube and h is the distance moved.

Hypothesis

As the temperature increases from 10 °C to 40 °C, the rate of respiration of the seeds will increase, because enzymes work faster at higher temperatures. At a much higher temperature the rate will fall, because the enzymes denature.

Variables

Equipment

• Germinating seeds (for example peas or mung beans soaked for 24 hours), about 10 g for each tube
• Respirometer tubes: boiling tube with a bung connected to a short length of capillary tube and a scale in mm
• Soda lime granules (corrosive)
• Cotton wool or gauze to separate the seeds from the soda lime
• Coloured liquid (for example food-colouring solution) to make a drop in the capillary tube
• Glass beads (for the control tube), the same volume as the seeds
• Water baths (or large beakers of water with a thermometer) at 10, 20, 30, 40 and 50 °C; ice to make the 10 °C bath
• Thermometer
• Stopwatch
• Syringe, to reset the position of the liquid
• Petroleum jelly, to seal the bung
• Eye protection and gloves

Risk assessment

Method

1. Put on eye protection and gloves. Place a layer of soda lime about 1 cm deep in the bottom of a boiling tube.
2. Place a plug of cotton wool or gauze on top of the soda lime so that the seeds cannot touch it.
3. Add 10 g of germinating seeds on top of the cotton wool.
4. Put the bung with the capillary tube in the boiling tube. Seal the join with petroleum jelly so that no air can get in or out.
5. Set up a control tube in the same way, but use glass beads of the same volume instead of seeds.
6. Place both tubes in a water bath at 10 °C, with the capillary tubes pointing out of the water. Leave them for 5 minutes to reach the temperature of the water.
7. Use a syringe to put a drop of coloured liquid into the open end of each capillary tube so that it sits against the scale. Record the starting position of the liquid.
8. Start the stopwatch. After 5 minutes, read the new position of the liquid and calculate the distance it moved in mm.
9. Repeat steps 7 and 8 twice more at 10 °C, resetting the liquid each time, and record all three distances.
10. Repeat steps 6 to 9 with water baths at 20 °C, 30 °C, 40 °C and 50 °C, keeping the temperature of the water bath constant.
11. Calculate the mean distance moved at each temperature. Divide by 5 to find the rate in mm/min.
12. Plot a graph of rate of respiration (mm/min) against temperature (°C).

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Draw a scatter graph with temperature (°C) on the x-axis and rate of respiration (mm/min) on the y-axis. Plot the mean rate at each temperature and draw a smooth curve of best fit (not dot-to-dot). The curve should rise to a maximum and then fall.