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Biology Unit 5
Topic 7: Run for your life
6. Describe how to investigate rate of respiration
You will need:
Respirometer. This is comprised of: two boiling tubes, one basket or cage (metal or plastic),
one 1 cm3syringe, two bungs, and one manometer (a capillary Utube fixed to a scale on a
board and filled with coloured fluid)
Potassium hydroxide solution, 15% (= 2.7 M), about 15 cm3
A small quantity (about 5 cm3) of a small organism such as woodlice or germinating seeds.
SAFETY: Take care with potassium hydroxide which is corrosive. Wear goggles and wash any splashes
off the skin immediately.
1. Use a funnel to pour 5 cm3 of potassium hydroxide solution (corrosive) into each boiling tube
2. Fill the basket or cage with small organism and put it into boiling tube B. Make sure that they are not
touching the potassium hydroxide. Add water to tube A to match the volume of the small organisms in
tube B see diagram.
3. Fit tube A with a bung with a 3way tap connected to the same items.
4. Fit tube B with a bung holding a 1 cm3 syringe and a connecting tube as shown in the diagram.
Alternatively fit a bung with a 3way tap connected to the syringe and tube.
5. Draw some coloured fluid into the manometer Utube. The fluid must be free of bubbles and come to
about the middle of the scale on each side.
6. Connect the apparatus together.
7. Use the syringe to adjust the manometer so that the fluid levels are the same on both sides.
8. Record the exact position of the syringe piston, the position of the menisci on both sides of the
manometer, and the time.
1. Record new positions of the manometer fluid at regular intervals for 30 minutes. When it nears the end
of the scale on one side, restore it to its original position and note the new position of the syringe piston.
2. Find the amount of oxygen absorbed by small organisms in a period of 30 minutes at 20 °C.
Text Book: p. 131
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Biology Unit 5
3. Remove the potassium hydroxide solution from both tubes and wash them out with water.
4. Replace the basket containing seeds or invertebrates in tube B, an equivalent volume of water in the
other tube and the bungs in both. Set up the respirometer at 20 °C again and record any increase or
decrease in gas volume over the next 30 minutes.
5. Calculate the volume of carbon dioxide produced.
6. Calculate the respiratory quotient.…read more