3.2.3Inquiry Process
Cell Respiration- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 109 of 160
Curriculum Equation Key معادلة التنفس الخلوي الهوائي الموزونة - Balanced Cellular Respiration EquationFast Exam Solution
C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP
Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP) | جلوكوز + أكسجين ← ثاني أكسيد كربون + ماء + طاقة
Location & Specifications: الميتوكوندريا (Mitochondria) والسيتوسول.
Section 1 of 5Factors Affecting the Rate
Respiration rate depends on how much energy cells need and on the conditions for their enzymes.
Metabolic activity
- Metabolic activity Cells that are very active, such as muscle cells, respire faster than less active cells such as fat storage cells.
Size of organism
- Size of organism Small organisms have a high surface area to volume ratio and lose heat quickly, so their rate of respiration per gram is higher.
Oxygen supply
- Oxygen supply If oxygen is scarce, cells shift towards anaerobic respiration and the aerobic rate falls.
Substrate supply
- Substrate supply With less glucose available, respiration is slower, and a lack of substrate limits the rate.
Temperature and pH
- Temperature and pH The rate rises up to the optimum for the respiratory enzymes and then falls as they denature;
- carbon dioxide can lower pH.
Section 2 of 5The Respirometer
1
Living organisms such as germinating seeds or small invertebrates are sealed in a chamber with air and a capillary tube attached.
2
Potassium hydroxide solution absorbs carbon dioxide made by the organisms, so only oxygen uptake changes the gas pressure.
3
As oxygen is used, the pressure in the chamber falls and coloured liquid in the capillary moves towards the organisms.
4
The distance moved by the liquid in a fixed time is measured against a scale, and repeated readings give a mean.
5
The apparatus is kept in a water bath, because a small change in temperature alters the gas pressure and would give false readings.
Section 3 of 5Calculating and Evaluating
Calculating and Evaluating
Volume formula
- Volume formula Volume of oxygen used = π r² h, where r is the radius of the capillary bore and h is the distance moved by the liquid.
Rate
- Rate Divide the volume by the time to give a rate, for example in mm³ min⁻¹.
Control tube
- Control tube A second tube with glass beads in place of organisms corrects for changes in pressure that are not due to respiration.
Technology
- Technology Oxygen sensors and carbon dioxide monitors with dataloggers record results continuously without exposing organisms to strong alkali.
Ethics
- Ethics Seeds give good data, so animals should only be used if necessary and should be handled humanely.
| Variable | How it is handled |
|---|---|
| Temperature | Controlled with a thermostatic water bath |
| Mass of organisms | Kept equal in every tube |
| Time | Same period for each reading |
| Repeats | At least three readings per condition |
Section 4 of 5Worked Example
Question
- A respirometer contained germinating seeds.
- The capillary tube had an internal diameter of 0.50 mm.
- The liquid in the capillary moved 18 mm in 6.0 minutes.
- Calculate the rate of oxygen uptake in mm³ min⁻¹.
- Radius = 0.50 / 2 = 0.25 mm.
- Cross-sectional area = π r² = 3.14 x 0.25² = 0.196 mm².
- Volume of oxygen used = 0.196 x 18 = 3.53 mm³.
- Rate = 3.53 / 6.0 = 0.59 mm³ min⁻¹.
Answer The seeds took up oxygen at about 0.59 mm³ per minute.
Section 5 of 5Exam Checklist & Tips
3.2.3 – Key Points for the Exam
- Respirometer Apparatus that measures the rate of oxygen uptake of living organisms.
- Substrate The molecule, such as glucose, that is used in respiration.
- Datalogger A device that records sensor readings over time.
- Surface area to volume ratio The ratio that affects how fast an organism loses heat and exchanges gases.
Common Mistakes
- Forgetting the potassium hydroxide, so carbon dioxide production hides the oxygen uptake.
- Using the diameter in the formula instead of the radius.
- Leaving out a temperature-controlled water bath and so recording changes caused by pressure.
- Confusing the rate of cell respiration with the breathing rate of the organism.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.