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Skip to 0 minutes and 6 seconds BEVERLEY GOODGER: In this practical, the uptake of oxygen by living organisms will be investigated using a respirometer to measure the rate of respiration. Respirometers range from relatively simple pieces of equipment used in school science labs with small organisms to elaborate devices the size of a room used to measure respiration rates in humans living near normal lives over a period of several days. In this practical, a very simple respirometer will be used to investigate and compare the rates of respiration in germinating seeds and non-germinating seeds. And glass beads will be used as a control. 1. You first need to find out the volume of the germinating seeds [see technique in other video].

Skip to 0 minutes and 47 seconds Then, use glass beads to make the volume of the non-germinating seeds and the glass beads the same. 2. To assemble a respirometer, place an absorbent cotton ball in the bottom of each respirometer vial.

Skip to 1 minute and 2 seconds Use a dropping pipette to saturate the cotton wool with two cubic centimetres of 15% potassium hydroxide. Caution– avoid skin contact with potassium hydroxide. Be certain that the respirometer vials are dry on the inside. Do not get potassium hydroxide on the sides of the respirometer. 4. Place a small wad of dry non-absorbent cotton wool or a disc of foam like this one, on the top of the potassium hydroxide soaked absorbent cotton wool. This will prevent the potassium hydroxide solution from contacting the peas. It is important that the amount of cotton wool or foam and the potassium hydroxide solution is the same for each respirometer. 5. Place the 10 germinating peas in the respirometer vial one.

Skip to 1 minute and 51 seconds Place the 10 dry peas and glass beads in respirometer vial two. Place glass beads only in the respirometer vial three.

Skip to 2 minutes and 5 seconds Insert a stopper fitted with a calibrated pipette into each respirometer vial. The stopper must fit tightly. If the respirometers leak during the experiment, you will have to start again.

Skip to 2 minutes and 20 seconds Place a set of three respirometers in a water bath with their pipette tips resting on the lip of the tray or on a piece of tape across the water bath. The end of the tube, containing the peas or beads, may need to be weighted down. 8.Wait five minutes before proceeding. This is to allow time for the respirometers to reach thermal equilibrium with the water. If any of the respirometers begin to fill with water, the experiment will have to restart. 9. After the equilibrium period, immerse all the respirometers, including the pipette tips, in the water bath. Position the respirometers so that it’s easy to read the scales on the pipettes.

Skip to 3 minutes and 0 seconds The laminated paper can be placed into the water bath under the pipettes to make reading them easier. Do not put anything else into the water bath or take anything out until the readings have been completed. 10. Allow the respirometers to equilibrate for another five minutes. Then, observe the initial volume reading on the scale to the nearest 0.01 cubic centimetres. Record the data in a table for time zero. 11. Repeat your observations, and record them every five minutes for 20 minutes. Rates of oxygen consumption in the three respirometers can now be compared and explained. You should see water move up the pipette of the tube containing the germinating peas, showing oxygen consumption as the seeds respire.

Skip to 3 minutes and 47 seconds Don’t forget to use appropriate precautions for clearing away the tubes and the potassium hydroxide.

Using a respirometer

In this activity we will look at three practicals which can support students understanding of respiration and challenge misconceptions.

As you watch the videos, we’d like you to make a note of any misconceptions which could be addressed by each of the three practical investigations. You’ll choose one of these practicals, and the misconceptions it addresses, to devise your own diagnostic question in step 2.11

Using a respirometer

This video demonstrates how the rate of aerobic respiration in germinating pea seeds can be compared to the rate of aerobic respiration in dormant pea seeds. The rate of oxygen consumption will be used to determine the rate of aerobic respiration and will be measured using a very simple respirometer.

It will also allow some common misconceptions to be tested: for example, that seeds are dead and therefore won’t respire, or if they are not dead, they are from plants and therefore will only respire at night.

This simple respirometer can be used to investigate cellular respiration in living organisms. The rate of respiration of germinating and non-germinating peas in this experiment is determined by the consumption of oxygen. Potassium Hydroxide (KOH) is used to alter the equilibrium. Potassium hydroxide removes the carbon dioxide and oxygen is used by cellular respiration thus decreasing the volume of gas in the respirometer.

The recorded observations show that after 20 minutes, a large volume of water has moved into the pipette in respirometer A, a small volume of water has moved into the pipette in respirometer B and no water has moved into the pipette in respirometer C.

The germinating peas in tube A have used up the most oxygen and have the highest rate of respiration. The dry (dormant) seeds in tube B have used up a smaller volume of oxygen, demonstrating that they are carrying out respiration at a slow rate, The tube of glass beads was set up as a control, to investigate whether something other than the respiring peas was using up the oxygen in the tube. No water entered the pipette in the control experiment, so the glass beads were an effective control.


Note in the comments below the misconceptions that this practical could help to address.

‘Like’ the misconceptions identified by other learners if you agree.

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Teaching Biology: Inspiring Students with Plant Science

National STEM Learning Centre

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