Key Takeaways for Respiration

1. Aerobic Respiration

  • Definition: Chemical reaction in mitochondria that releases energy in the presence of oxygen.
  • Equation:
    • Word: glucose + oxygen → carbon dioxide + water (+ energy)
    • Symbol:
      C6H12O6+6O2→6CO2+6H2OC6​H12​O6​+6O2​→6CO2​+6H2​O
  • Key Features:
    • Exothermic (releases energy).
    • Produces 38 ATP molecules (high energy yield).
    • Occurs continuously in plants and animals.
  • Example: Muscle cells during moderate exercise.

Tip:

  • Remember: Aerobic = “with air” (oxygen).
  • Mitochondria are the “powerhouses” of the cell. Cells with high energy demands (e.g., sperm, muscle) have more mitochondria.

2. Anaerobic Respiration

  • Definition: Energy release without oxygen, producing less ATP.
  • In Humans:
    • Word: glucose → lactic acid (+ 5% energy).
    • Causes oxygen debt: Excess post-exercise oxygen consumption to break down lactic acid in the liver.
  • In Yeast (Fermentation):
    • Word: glucose → ethanol + carbon dioxide.
    • Used in brewing (alcohol) and baking (CO₂ makes bread rise).
  • Example: Sprinters rely on anaerobic respiration during a race.

Key Differences:

FeatureAerobicAnaerobic (Humans)
Oxygen required?YesNo
Energy per glucose38 ATP2 ATP (5% of aerobic)
ProductsCO₂ + H₂OLactic acid

Tip:

  • Oxygen debt is repaid by heavy breathing after exercise.
  • Lactic acid → CO₂ + H₂O in the liver via oxidation:
    Lactic acid+O2→CO2+H2OLactic acid+O2​→CO2​+H2​O

3. Metabolism

  • Definition: Sum of all chemical reactions in the body (e.g., respiration, protein synthesis).
  • Breakdown Reactions:
    • Example: Proteins → amino acids → urea (excreted by kidneys).
  • Synthesis Reactions:
    • Example: Glucose → starch (plants) or glycogen (animals).
  • Role of Enzymes: Catalyse metabolic reactions.

Tip:

  • Urea is produced in the liver and excreted in urine.
  • Plants respire and photosynthesise!

4. Experiments & Data Analysis

  • Respirometers: Measure oxygen consumption (e.g., invertebrates in soda lime tubes).
    • Key Control: Use dead organisms to account for atmospheric changes.
  • Yeast & Temperature:
    • Optimal temperature for fermentation: ~35°C (enzymes denature above 45°C).
    • Ethanol production decreases at high temps (yeast dies).
  • Data Skills:
    • Mean: Average of repeats.
    • Range: Highest – lowest value.
    • Anomalies: Outliers (e.g., Afreen’s 4 cm³ at 50°C).

Example Calculation:

  • Terry’s mean at 35°C:
    623+645+682+654+6525=651.25623+645+682+654+652​=651.2

Tip:

  • Liquid paraffin in experiments prevents oxygen entering (ensures anaerobic conditions).

5. Common Exam Questions

  1. Compare Aerobic & Anaerobic Respiration (6 marks):
    • Include oxygen requirement, energy yield, products, and examples.
  2. Explain Oxygen Debt (3 marks):
    • Link to lactic acid breakdown in the liver post-exercise.
  3. Balanced Symbol Equation for Aerobic Respiration (2 marks):
    • Ensure coefficients are correct:
      C6H12O6+6O2→6CO2+6H2OC6​H12​O6​+6O2​→6CO2​+6H2​O

Trick: Use OIL RIG to remember redox reactions:

  • Oxidation Is Loss (of electrons).
  • Reduction Is Gain.

Need to Memorise?

  • Aerobic equation: C6H12O6 + 6O2 → 6CO2 + 6H2O.
  • Anaerobic in yeast: glucose → ethanol + CO₂.
  • Metabolism = breakdown + synthesis.

Common Mistake:

  • Respiration ≠ Breathing: Respiration is chemical; breathing (ventilation) is gas exchange.

50 GCSE Biology Questions on Respiration

Section A: Aerobic Respiration

  1. What is the primary purpose of respiration in living organisms?
  2. Write the word equation for aerobic respiration.
  3. Give the balanced symbol equation for aerobic respiration.
  4. Why is respiration described as an exothermic reaction?
  5. Name the cellular organelle where aerobic respiration occurs.
  6. Define the term ‘aerobic’.
  7. Explain why muscle and sperm cells contain more mitochondria.
  8. What are the reactants in aerobic respiration?
  9. What are the products of aerobic respiration?
  10. Why do plants need to respire if they photosynthesise?

Section B: Anaerobic Respiration

  1. What is anaerobic respiration?
  2. Write the word equation for anaerobic respiration in humans.
  3. Write the word equation for anaerobic respiration in yeast.
  4. Why is less energy released in anaerobic respiration?
  5. Define ‘oxygen debt’ and explain how it is repaid.
  6. What causes muscle fatigue during intense exercise?
  7. Where is lactic acid broken down in the body?
  8. Compare the energy yield of aerobic and anaerobic respiration.
  9. Why does bread not taste alcoholic despite yeast fermentation?
  10. What is the chemical formula for ethanol?

Section C: Metabolism

  1. Define ‘metabolism’.
  2. Give an example of a synthesis reaction.
  3. Give an example of a breakdown reaction.
  4. How are excess amino acids processed in the liver?
  5. What is urea, and how is it excreted?
  6. Explain the role of enzymes in metabolism.
  7. How is glucose stored in animals and plants?
  8. What is the role of the thyroid gland in metabolism?

Section D: Experiments & Data Analysis

  1. What is the purpose of soda lime in a respirometer?
  2. Why does the water drop move in a respirometer during an experiment?
  3. Describe a control for the yeast fermentation experiment.
  4. Why was liquid paraffin used in the yeast experiment?
  5. Calculate the cardiac output if heart rate = 125 bpm and stroke volume = 145 cm³.
  6. Identify an anomaly in Afreen’s data at 50°C (Table 10.3).
  7. Calculate the mean volume of gas produced by Terry at 35°C (Table 10.3).
  8. Explain why yeast dies at high temperatures (>45°C).
  9. What gas is produced in fermentation, and how can it be tested?
  10. Why was a thermos flask used in the yeast experiment?

Section E: Comparison & Application

  1. Compare aerobic and anaerobic respiration in humans (6 marks).
  2. Explain why photosynthesis and respiration are not exact opposites.
  3. Describe the energy flow in photosynthesis and respiration.
  4. Why do heart and breathing rates increase during exercise?
  5. Explain the economic importance of yeast fermentation.
  6. How does oxygen move into cells for respiration?
  7. What happens to lactic acid after exercise?
  8. Why do roots die in waterlogged soil?
  9. Explain the term ‘oxidation’ using the acronym OIL RIG.
  10. Why is respiration vital for all living organisms?
  11. Describe how glycogen levels change during exercise.
  12. What is the main energy source for cellular reactions?

Detailed Answers

  1. To release energy from glucose for cellular processes (e.g., movement, growth).
  2. Glucose + oxygen → carbon dioxide + water (+ energy).
  3. C6H12O6+6O2→6CO2+6H2OC6​H12​O6​+6O2​→6CO2​+6H2​O
  4. It releases energy to the surroundings (exothermic).
  5. Mitochondria.
  6. Occurring in the presence of oxygen.
  7. High energy demands require more ATP production.
  8. Glucose and oxygen.
  9. Carbon dioxide and water.
  10. Respiration releases energy for life processes (occurs day and night).
  11. Energy release without oxygen.
  12. Glucose → lactic acid (+ 5% energy).
  13. Glucose → ethanol + carbon dioxide.
  14. Glucose is not fully broken down (no oxygen for complete oxidation).
  15. Oxygen debt = oxygen needed to oxidise lactic acid. Repaid via heavy breathing post-exercise.
  16. Lactic acid buildup inhibits muscle contraction.
  17. In the liver, converted to CO₂ and water.
  18. Aerobic: 38 ATP/glucose. Anaerobic: 2 ATP/glucose (5%).
  19. Ethanol evaporates during baking; yeast is killed by heat.
  20. C2H5OHC2​H5​OH.
  21. Sum of all chemical reactions in a cell/organism.
  22. Glucose → starch (plants) or glycogen (animals).
  23. Proteins → amino acids → urea.
  24. Deamination removes amine groups, forming urea.
  25. Urea: waste from protein breakdown. Excreted in urine.
  26. Enzymes catalyse reactions (lower activation energy).
  27. Animals: glycogen. Plants: starch/cellulose.
  28. Regulates metabolic rate via hormone secretion.
  29. Absorbs CO₂ to measure oxygen consumption accurately.
  30. Oxygen is consumed, reducing pressure, drawing water inward.
  31. Use dead organisms to account for non-respiratory gas changes.
  32. Prevent oxygen entry (ensure anaerobic conditions).
  33. Cardiac output = 125 × 145 = 18,125 cm³/min.
  34. 4 cm³ at 50°C (Repeat 3) is anomalously low.
  35. Mean = (623 + 645 + 682 + 654 + 652)/5 = 651.2 bubbles.
  36. Enzymes denature, disrupting metabolic reactions.
  37. CO₂; test with limewater (turns cloudy).
  38. Insulate to minimise heat loss, ensuring accurate temperature readings.
  39. Include oxygen requirement, energy yield, products, and examples (see Key Takeaways).
  40. Photosynthesis stores energy; respiration releases it. Equations are reversed but energy flow differs.
  41. Light → chemical (photosynthesis) → ATP (respiration).
  42. To deliver more oxygen/glucose to muscles and remove CO₂/lactic acid.
  43. Used in brewing (alcohol) and baking (CO₂ rises bread).
  44. Diffusion from capillaries into cells.
  45. Oxidised to CO₂ and water in the liver.
  46. Roots respire anaerobically, producing toxic ethanol.
  47. Oxidation Is Loss (of electrons); Reduction Is Gain.
  48. Provides ATP for all life processes (MRS GREN).
  49. Glycogen → glucose to fuel respiration.
  50. Glucose (from food or stored glycogen).

Exam Tip: Memorise equations using flashcards and practice data analysis (means, ranges, anomalies).