Practical 7: Carbon dioxide and heat from respiring seeds

Aim

To show that germinating seeds release heat and carbon dioxide, using boiled (dead) seeds as a control.

Background

All living cells respire. Aerobic respiration releases energy from glucose using oxygen, and produces carbon dioxide and water. Some of the energy released is transferred as heat, so a respiring organism warms up if the heat cannot escape.

Germinating seeds respire quickly because they are growing, so they make a good choice for detecting heat and carbon dioxide.

A control of boiled seeds is needed. Boiling kills the seeds, so they cannot respire. Any change that happens only with the living seeds must be caused by respiration. Both sets of seeds are rinsed in disinfectant so that microorganisms on the seeds do not respire and affect the results.

Hydrogen-carbonate indicator is red/orange in air with the normal amount of carbon dioxide. It turns yellow when carbon dioxide rises. Limewater turns cloudy (milky) when carbon dioxide is present.

Hypothesis

Germinating seeds will release heat and carbon dioxide because they respire, but boiled seeds will not because they are dead.

Variables

Equipment

• Pea seeds soaked in water for 24 hours (about 100 g), so they have started to germinate
• Pea seeds boiled for 5 minutes and cooled (about 100 g)
• 2 vacuum flasks
• 2 thermometers, −10 to 110 °C
• Cotton wool
• Weak disinfectant solution: dilute sodium hypochlorite (about 1%), such as diluted Milton sterilising fluid
• 2 beakers and a sieve or tea strainer
• Top-pan balance
• Syringe or small pump, tubing and clips
• 4 boiling tubes with bungs and delivery tubes: two to hold 10 g of seeds each on gauze, and two to hold 10 cm³ of hydrogen-carbonate indicator (or limewater) each
• Stopclock or timer
• Eye protection and gloves

Risk assessment

Method

1. Soak the peas in water for 24 hours so that they begin to germinate. Boil a second batch of peas for 5 minutes to kill them, then cool to room temperature.
2. Put on eye protection. Rinse both batches of peas in the weak disinfectant (dilute sodium hypochlorite) for a few minutes, then rinse with clean water and drain them.
3. Weigh the same mass of each batch (for example 50 g) on the balance.
4. Put the germinating peas into the first vacuum flask and the boiled peas into the second. Label the flasks.
5. Put a thermometer in each flask, with the bulb in the middle of the peas, and plug the neck of each flask with cotton wool.
6. Record the starting temperature of each flask. Both should start at the same temperature.
7. Leave the flasks in the same place at room temperature, out of direct sunlight.
8. Record the temperature of each flask at the same time each day for four days.
9. For the carbon dioxide test, put 10 g of germinating peas on gauze in one boiling tube and 10 g of boiled peas on gauze in another. Add 10 cm³ of hydrogen-carbonate indicator to each of two further tubes, and join each seed tube to an indicator tube with tubing so that air drawn through the seeds then bubbles through the indicator.
10. Check the joints are airtight, then use a syringe or pump to draw air slowly through each set of apparatus for the same time (for example 5 minutes) and at the same speed.
11. Record the colour of the indicator in each tube at the start and after the 5 minutes. Repeat the air flow if there is no change, and record the colour again.
12. Wash your hands, then clear away, disposing of the seeds as directed by your teacher.
13. Plot a graph of temperature against time for both flasks.

Results

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

Drawing the graph

Line graph with two lines on the same axes: time (days) on the x-axis (0 to 4) and temperature (°C) on the y-axis. Plot the germinating peas and the boiled peas with different symbols or colours, joined with straight lines or smooth curves through the points (no line of best fit is needed because the line shows a trend over time). Label each line.