Cambridge O Level5090

Respiration

Biology 5090 Chapter Notes

What this chapter covers

RespirationAerobic respirationAnaerobic respiration
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In this chapter

  1. 1What is Respiration?
  2. 2Aerobic Respiration
  3. 3Anaerobic Respiration in Muscles
  4. 4Anaerobic Respiration in Yeast
  5. 5Comparing Respiration Types
  6. 6Investigating Respiration

1. What is Respiration?

Respiration is not the same as breathing. Breathing (or ventilation) is the physical process of moving air in and out of the lungs. Respiration is a chemical process that happens inside every living cell, including plant and animal cells, 24 hours a day. Its purpose is to break down nutrient molecules, like glucose, to release energy. This energy is vital for all life processes, such as muscle contraction, growth, cell division, and maintaining body temperature. In short, respiration is the chemical reaction that powers life.

Key term

Respiration: The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Examiner insight

Examiners look for the understanding that respiration happens in all living cells, including plants, and is a continuous process needed to release energy.

Common pitfall

Confusing respiration with breathing. Breathing is the mechanical process of getting gases to and from the gas exchange surface; respiration is the chemical reaction inside cells to release energy.

Worked example 12 marks

A student wrote, 'Plants photosynthesise in the light and respire in the dark.' Explain why this statement is only partially correct.

  1. 1

    Step 1: Identify the correct part of the statement. Plants do photosynthesise in the light.

  2. 2

    Step 2: Identify the incorrect part. Respiration is a process that releases energy for the plant to stay alive. This is needed all the time, not just in the dark.

  3. 3

    Step 3: Correct the statement. Plants respire continuously, both in the light and in the dark. Photosynthesis only occurs when there is light.

Recap

  • Respiration is a chemical reaction that releases energy from food.
  • It occurs in the mitochondria of all living cells.
  • Respiration is essential for all life processes, such as movement and growth.
  • Do not confuse respiration with breathing (gas exchange).

Quick check

  1. In which part of a cell does most aerobic respiration occur?1 mark
  2. State one life process that requires energy from respiration.1 mark

2. Aerobic Respiration

Aerobic respiration is the most efficient way for cells to release energy from glucose. It is called 'aerobic' because it requires oxygen. During this process, glucose is completely broken down into carbon dioxide and water, releasing a large amount of energy. The oxygen required is breathed in and transported to the cells by the blood. The carbon dioxide produced is a waste product that is removed from the body.

Word Equation: glucose + oxygen → carbon dioxide + water (+ energy released)

Balanced Chemical Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Key term

Aerobic Respiration: The chemical process of releasing energy from glucose using oxygen.

Examiner insight

Marks are often awarded for correctly stating both the word and balanced chemical equations. Ensure you know the difference between reactants and products.

Fun fact

Your body produces its own weight in ATP (the energy currency molecule made during respiration) every single day!

Worked example 13 marks

State the word equation for aerobic respiration and name the two reactants required.

  1. 1

    Step 1: Write the full word equation. The equation is glucose + oxygen → carbon dioxide + water.

  2. 2

    Step 2: Identify the reactants. The reactants are the substances on the left side of the equation.

  3. 3

    Step 3: State the reactants. The reactants are glucose and oxygen.

Worked example 22 marks

A person has a mass of 65 kg. Their body uses approximately 130 kJ of energy per kg of body mass per day. Calculate the total energy this person uses in one day.

  1. 1

    Step 1: Identify the given values. Mass = 65 kg. Energy use = 130 kJ per kg.

  2. 2

    Step 2: Set up the calculation. Total energy = Energy per kg × mass.

  3. 3

    Step 3: Calculate the result. Total energy = 130 kJ/kg × 65 kg = 8450 kJ.

Recap

  • Aerobic respiration requires oxygen.
  • It fully breaks down glucose into carbon dioxide and water.
  • A large amount of energy is released per molecule of glucose.
  • The balanced chemical equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Quick check

  1. What are the two waste products of aerobic respiration?2 marks

3. Anaerobic Respiration in Muscles

During intense exercise, such as sprinting, your body may not be able to supply oxygen to your muscle cells fast enough for aerobic respiration to meet the energy demand. In this situation, your muscles switch to anaerobic respiration. This is the breakdown of glucose without oxygen. It produces lactic acid and releases a small amount of energy. This is much less efficient than aerobic respiration. The build-up of lactic acid in muscles causes fatigue and pain ('stitch'). After the exercise, you continue to breathe deeply to take in extra oxygen. This extra oxygen is used to break down the lactic acid that has built up. This is known as repaying the 'oxygen debt'.

Word Equation: glucose → lactic acid (+ some energy released)

Key term

Oxygen Debt: The extra oxygen needed after strenuous exercise to break down the lactic acid that has accumulated in the muscles.

Examiner insight

Examiners reward clear explanations of oxygen debt, linking the build-up of lactic acid during exercise to the need for extra oxygen intake during recovery.

Common pitfall

Stating that anaerobic respiration produces 'lactic acid and energy'. While true, it's crucial to specify it's only a *small* amount of energy, which is a key comparison point with aerobic respiration.

Worked example 16 marks

An athlete's breathing rate is 12 breaths/min at rest and their average volume per breath is 0.5 dm³. After running, their rate is 20 breaths/min and volume is 3.5 dm³. Calculate the ventilation rate after running and explain why it is so high.

  1. 1

    Step 1: Calculate the ventilation rate after running. Ventilation rate = breathing rate × volume per breath. So, 20 breaths/min × 3.5 dm³/breath = 70 dm³/min.

  2. 2

    Step 2: Explain the need for a high rate. During running, muscles need more energy, so they respire more.

  3. 3

    Step 3: Link to anaerobic respiration. The energy demand is so high that anaerobic respiration occurs, producing lactic acid.

  4. 4

    Step 4: Explain oxygen debt. The high ventilation rate after exercise provides the extra oxygen needed to break down the toxic lactic acid.

  5. 5

    Step 5: Link to removing CO₂. The high rate also helps to remove the large amount of carbon dioxide produced by increased aerobic respiration.

Recap

  • Anaerobic respiration in muscles occurs when oxygen is limited.
  • It breaks down glucose into lactic acid.
  • It releases only a small amount of energy per glucose molecule.
  • Lactic acid build-up causes muscle fatigue and creates an oxygen debt.
  • Heavy breathing after exercise repays the oxygen debt.

Quick check

  1. What is the product of anaerobic respiration in human muscles?1 mark
  2. Why does anaerobic respiration release less energy than aerobic respiration?1 mark

4. Anaerobic Respiration in Yeast

Yeast is a single-celled fungus that can respire both aerobically and anaerobically. Anaerobic respiration in yeast is also known as fermentation. When yeast is in an environment without oxygen but with a food source like sugar (glucose), it breaks the glucose down into ethanol (alcohol) and carbon dioxide, releasing a small amount of energy. This process is commercially very important. In baking, the carbon dioxide produced makes bread dough rise. In brewing, the ethanol produced is the alcohol in drinks like beer and wine.

Word Equation: glucose → ethanol + carbon dioxide (+ some energy released)

Balanced Chemical Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Key term

Fermentation: The process of anaerobic respiration in microorganisms like yeast, which produces ethanol and carbon dioxide from glucose.

Fun fact

Some species of goldfish can survive in oxygen-free, frozen-over ponds for months by switching to anaerobic respiration, producing ethanol which they release through their gills!

Worked example 13 marks

Explain the role of yeast in bread-making.

  1. 1

    Step 1: State the ingredients. Yeast is mixed into the dough, which contains flour (starch) and sugar (glucose).

  2. 2

    Step 2: Describe the respiration process. The yeast respires anaerobically, breaking down the glucose.

  3. 3

    Step 3: Identify the key product. This produces carbon dioxide gas.

  4. 4

    Step 4: Explain the effect. The bubbles of carbon dioxide gas get trapped in the dough, causing it to rise and giving the bread a light, airy texture.

Worked example 23 marks

In the brewing industry, why is the fermenter vessel sealed from the air after the yeast is added?

  1. 1

    Step 1: Identify the desired process. The aim is to produce ethanol via anaerobic respiration (fermentation).

  2. 2

    Step 2: Explain the effect of oxygen. If oxygen were present, the yeast would respire aerobically.

  3. 3

    Step 3: State the outcome of aerobic respiration. Aerobic respiration produces carbon dioxide and water, not ethanol.

  4. 4

    Step 4: Conclude the reason for sealing. The vessel is sealed to create anaerobic conditions, forcing the yeast to produce ethanol.

Recap

  • Anaerobic respiration in yeast is called fermentation.
  • It breaks down glucose into ethanol and carbon dioxide.
  • This process is used in baking to make bread rise (due to CO₂).
  • It is also used in brewing to produce alcoholic drinks (due to ethanol).
  • Like all anaerobic respiration, it yields a small amount of energy.

Quick check

  1. Which product of yeast fermentation is essential for making beer?1 mark
  2. Which product of yeast fermentation is essential for making bread rise?1 mark

5. Comparing Respiration Types

It is essential to be able to compare and contrast aerobic and anaerobic respiration. The main differences relate to the requirement for oxygen, the products formed, the amount of energy released, and the extent to which glucose is broken down. Aerobic respiration is the default, most efficient process, while anaerobic respiration is a short-term alternative when oxygen is scarce.

Examiner insight

When asked to compare, using a table is often a clear and effective way to present your answer and ensure you cover multiple distinct points.

Worked example 13 marks

Complete the table to show three differences between aerobic respiration and anaerobic respiration in humans.

  1. 1

    Step 1: Consider the oxygen requirement. Aerobic needs oxygen; anaerobic does not.

  2. 2

    Step 2: Consider the products. Aerobic produces carbon dioxide and water; anaerobic in humans produces lactic acid.

  3. 3

    Step 3: Consider the energy yield. Aerobic releases a large amount of energy; anaerobic releases a small amount of energy.

  4. 4

    Step 4: Populate the table:

  5. 5

    Row 1: Aerobic - Oxygen required | Anaerobic - No oxygen required

  6. 6

    Row 2: Aerobic - Products are CO₂ and water | Anaerobic - Product is lactic acid

  7. 7

    Row 3: Aerobic - Large amount of energy released | Anaerobic - Small amount of energy released

Recap

  • Aerobic respiration requires oxygen, while anaerobic does not.
  • Aerobic respiration produces CO₂ and water; anaerobic produces lactic acid (in humans) or ethanol and CO₂ (in yeast).
  • Aerobic respiration releases a large amount of energy per glucose molecule.
  • Anaerobic respiration releases a much smaller amount of energy.
  • In aerobic respiration, glucose is completely broken down; in anaerobic, it is only partially broken down.

Quick check

  1. Which type of respiration releases more energy per molecule of glucose?1 mark

6. Investigating Respiration

We can investigate respiration in living organisms by measuring its inputs and outputs. Common experiments aim to show oxygen uptake, carbon dioxide production, or heat release. A respirometer is a piece of apparatus used to measure the rate of oxygen consumption. The organism (e.g., germinating seeds or small invertebrates) is placed in a sealed tube with a chemical like soda-lime, which absorbs any CO₂ produced. As the organism respires, it uses up oxygen, causing a pressure drop inside the tube. This pulls a coloured liquid along a narrow capillary tube, and the distance it moves over time gives a measure of the rate of respiration. To test for CO₂ production, gas from the respiring organisms can be bubbled through limewater (calcium hydroxide solution), which will turn from clear to cloudy/milky if CO₂ is present.

Key term

Respirometer: An apparatus used for measuring the rate of respiration of a living organism by measuring its rate of exchange of oxygen and/or carbon dioxide.

Common pitfall

Forgetting the role of soda-lime. Without it, the production of CO₂ would counteract the consumption of O₂, and little or no volume change would be observed in the respirometer.

Worked example 14 marks

In a respirometer experiment using germinating seeds and soda-lime, a coloured oil droplet moved 24 mm in 6 minutes.(i) In which direction did the droplet move and why?(ii) What was the purpose of the soda-lime?

  1. 1

    Step 1 (i): State the direction. The droplet moved towards the test tube containing the seeds.

  2. 2

    Step 2 (i): Explain the reason. The germinating seeds are respiring aerobically, so they are consuming oxygen from the air inside the tube. This reduces the volume of gas and therefore the pressure inside the tube, pulling the droplet inwards.

  3. 3

    Step 3 (ii): State the purpose of soda-lime. The soda-lime absorbs the carbon dioxide that is produced by the respiring seeds.

  4. 4

    Step 4 (ii): Explain why this is necessary. By removing the CO₂ produced, any change in gas volume is due only to the consumption of oxygen, allowing for an accurate measurement of the oxygen uptake rate.

Worked example 23 marks

When setting up an experiment to investigate respiration, why is it important to include a 'control'?

  1. 1

    Step 1: Define a control. A control is a setup that is identical to the main experiment in every way except for the one variable being investigated.

  2. 2

    Step 2: Give an example for a respiration experiment. For an experiment with germinating seeds, the control would use the same mass of dead (e.g., boiled) seeds.

  3. 3

    Step 3: Explain the purpose. The control is used for comparison. It shows that any observed result (e.g., oxygen uptake) is due to the biological process of respiration in the living organism, and not due to other factors like temperature changes or microbial activity.

Recap

  • Respiration can be investigated by measuring oxygen uptake, carbon dioxide production, or heat release.
  • A respirometer measures the rate of oxygen consumption.
  • Soda-lime is used in respirometers to absorb the carbon dioxide produced.
  • Limewater (calcium hydroxide solution) is used to test for the presence of carbon dioxide; it turns cloudy.
  • A control experiment, using dead organisms, is essential to prove the results are due to respiration.

Quick check

  1. What chemical is used to absorb carbon dioxide in a respirometer?1 mark
  2. What is the positive test for carbon dioxide using limewater?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the balanced chemical equation for aerobic respiration.3 marks
  2. Explain why a person continues to breathe heavily for several minutes after finishing a 100-metre sprint.4 marks
  3. Compare aerobic respiration with anaerobic respiration in yeast. You should include three distinct points of comparison in your answer.3 marks
  4. Describe an experiment you could carry out to show that germinating seeds produce carbon dioxide.4 marks
  5. A student sets up a respirometer with woodlice. They find the bubble in the capillary tube does not move. Suggest two possible reasons for this result.2 marks
  6. Explain the commercial importance of anaerobic respiration in yeast in both the baking and brewing industries.4 marks
  7. Distinguish between the terms 'respiration' and 'gas exchange'.2 marks
  8. During an investigation into respiration, a student measures an athlete's breathing rate as 15 breaths per minute and their average breath volume as 0.6 dm³. Calculate the athlete's ventilation rate in dm³ per minute.2 marks
  9. A respirometer containing maggots and soda-lime is placed in a water bath at 20°C. The volume of oxygen consumed in 5 minutes is 0.8 cm³. The experiment is repeated at 30°C and the volume of oxygen consumed is 1.6 cm³. Explain these results.4 marks
  10. Why does anaerobic respiration in muscles provide enough energy for a short sprint but not for a long-distance marathon?3 marks

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