BIOLOGY · Stage 8 · Chapter 5: Respiration and the Respiratory System
Biology 5.4 Investigating respiration — Experimental Respirometry
5 min read 7 parts Lesson 4 of 12
Lesson summary
Biology 5.4 Investigating respiration — Experimental Respirometry — Alhussan International Academy — Biology (Stages 7–9) notes: Plan and evaluate experiments that show or measure respiration, including controls, variables, limewater tests and respirometers.Limewater — a dilute s…
- Prediction
- Conditions
- Evaluation
- What can be measured
- Control tube
- Hydrogencarbonate indicator
Section 1 of 7Limewater and the test for carbon dioxide
- Limewater
- a dilute solution of calcium hydroxide, a clear, colourless liquid.
- Test
- bubbling carbon dioxide through it turns it milky white, and the more carbon dioxide there is, the faster it goes cloudy.
- Hazard
- limewater carries a hazard symbol (irritant), so eye protection is worn.
- Prediction
- predictions are based on scientific knowledge, here aerobic respiration producing waste carbon dioxide.
Section 2 of 7Investigation 1: do people produce carbon dioxide?
- Method
- each student breathes out gently and steadily through a straw into a test tube of limewater.
- Strength
- it is simple and quick, and shows that exhaled air has enough carbon dioxide to turn limewater milky.
- Weakness
- it has no comparison with inhaled air, so it does not show that exhaled air has more carbon dioxide than the air breathed in.
Section 3 of 7Investigation 2: adding a control
- Control
- a big syringe or plastic bottle of the air everyone breathes in is bubbled through limewater in a test tube.
- If the limewater stays clear, the concentration of carbon dioxide in inhaled air is lower than in exhaled air.
- Strength
- comparing two conditions shows where the extra carbon dioxide comes from.
Section 4 of 7Investigation 3: do plants respire?
- Aim
- show that plants respire as well as animals by testing whether germinating seeds produce carbon dioxide.
- Tubes:
- Tube A has no seeds, as a control, so no milky limewater is expected.
- Tube B has dry, dormant seeds, as another control, which make so little carbon dioxide that limewater is unaffected.
- Tube C has germinating seeds, expected to make enough carbon dioxide to turn limewater milky.
- Conditions
- the three tubes are left in the same place for a week.
- Weakness
- microorganisms on the seeds could also respire, so the seeds should be disinfected.
- Evaluation
- the control experiments in Investigations 2 and 3 are the strongest, because they compare two conditions.
Section 5 of 7A simple respirometer
- What can be measured
- oxygen used per minute, carbon dioxide made or temperature rise, since respiration cannot be seen directly.
- Respirometer
- apparatus that measures the oxygen used by small living organisms such as germinating seeds, woodlice or maggots.
- Place the living organisms on a gauze platform above a carbon dioxide absorber (soda lime or potassium hydroxide solution) in a boiling tube.
- Seal the tube, attach a capillary tube with a drop of coloured liquid, and place it in a water bath at constant temperature.
- Mark the drop's position, wait a measured time such as 5 minutes and record the distance moved.
- Principle
- the organisms use oxygen, the carbon dioxide they release is absorbed, so the gas volume falls and the drop moves towards the tube.
- Rate
- distance divided by time, in mm per minute.
Section 6 of 7Variables and controls
| Type | Example |
|---|---|
| Independent | Temperature of the water bath, or type of organism |
| Dependent | Distance moved by the drop per minute |
| Control | Mass of organisms, volume of apparatus, time, absorbent |
- Control tube
- glass beads or dead organisms of the same mass show whether changes in temperature or air pressure move the drop, and the result is subtracted.
- Hydrogencarbonate indicator
- it is red at normal carbon dioxide levels, turns yellow as carbon dioxide increases and purple as it decreases.
- The time for the colour change at different temperatures measures respiration, but the colour judgement is subjective.
Section 7 of 7Evaluating and improving
- Pattern
- the rate rises with temperature up to about 40 °C because enzymes work faster, and germinating seeds respire faster than dormant seeds.
- Reliability
- repeat, take a mean and ignore anomalies only after checking them.
- Apparatus
- make sure there are no leaks and allow time for the apparatus to reach the water bath temperature.
- Safety
- wear eye protection for corrosive chemicals such as potassium hydroxide, handle live animals gently and return them to their habitat.
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