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5.4Chapter 5: Respiration and the Respiratory System

Investigating respiration — Alhussan Biology - Stage 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 12 1 quiz

📚 Alhussan Biology - Stage 8 Hub / Topical Revision
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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

Part 1 of 7Limewater and the test for carbon dioxide

Limewater
  • Limewater a dilute solution of calcium hydroxide, a clear, colourless liquid.
Test
  • Test bubbling carbon dioxide through it turns it milky white, and the more carbon dioxide there is, the faster it goes cloudy.
Hazard
  • Hazard limewater carries a hazard symbol (irritant), so eye protection is worn.
Prediction
  • Prediction predictions are based on scientific knowledge, here aerobic respiration producing waste carbon dioxide.

Part 2 of 7Investigation 1: do people produce carbon dioxide?

Method
  • Method each student breathes out gently and steadily through a straw into a test tube of limewater.
Strength
  • Strength it is simple and quick, and shows that exhaled air has enough carbon dioxide to turn limewater milky.
Weakness
  • 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.

Part 3 of 7Investigation 2: adding a control

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
  • Strength comparing two conditions shows where the extra carbon dioxide comes from.

Part 4 of 7Investigation 3: do plants respire?

Aim
  • 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
  • Conditions the three tubes are left in the same place for a week.
Weakness
  • Weakness microorganisms on the seeds could also respire, so the seeds should be disinfected.
Evaluation
  • Evaluation the control experiments in Investigations 2 and 3 are the strongest, because they compare two conditions.

Part 5 of 7A simple respirometer

What can be measured
  • What can be measured oxygen used per minute, carbon dioxide made or temperature rise, since respiration cannot be seen directly.
Respirometer
  • Respirometer apparatus that measures the oxygen used by small living organisms such as germinating seeds, woodlice or maggots.
A simple respirometer
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
  • 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
  • Rate distance divided by time, in mm per minute.

Part 6 of 7Variables and controls

TypeExample
IndependentTemperature of the water bath, or type of organism
DependentDistance moved by the drop per minute
ControlMass of organisms, volume of apparatus, time, absorbent
1Control tube
  • 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.
2Hydrogencarbonate indicator
  • 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.

Part 7 of 7Evaluating and improving

Pattern
  • Pattern the rate rises with temperature up to about 40 °C because enzymes work faster, and germinating seeds respire faster than dormant seeds.
Reliability
  • Reliability repeat, take a mean and ignore anomalies only after checking them.
Apparatus
  • Apparatus make sure there are no leaks and allow time for the apparatus to reach the water bath temperature.
Safety
  • Safety wear eye protection for corrosive chemicals such as potassium hydroxide, handle live animals gently and return them to their habitat.

Key terms

TermMeaning
RespirometerApparatus for measuring the oxygen used by living organisms.
LimewaterA dilute solution of calcium hydroxide that turns milky with carbon dioxide.
Soda limeA chemical that absorbs carbon dioxide.
ControlA set-up used to check that results are caused by the variable tested.
AnomalyA result that does not fit the pattern.
Capillary tubeA very narrow tube.

Avoid these mistakes

1
Putting organisms in direct contact with the absorbent chemical — use a gauze platform.
2
Not allowing the apparatus to reach a steady temperature — wait before starting.
3
Forgetting units for the rate — give mm per minute.
4
Using different masses of organism in different tubes — control the mass.

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