Cambridge IGCSE0970

Respiration

Biology 0970 Chapter Notes

What this chapter covers

RespirationAerobic respirationAnaerobic respiration
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In this chapter

  1. 1Respiration: Releasing Energy for Life
  2. 2Aerobic Respiration: Using Oxygen
  3. 3Anaerobic Respiration: Without Oxygen
  4. 4Gas Exchange: Supplying Oxygen
  5. 5Investigating Respiration: The Respirometer

1. Respiration: Releasing Energy for Life

Respiration is not breathing! It's a fundamental chemical process that happens inside the cells of all living organisms. Its purpose is to break down nutrient molecules, like glucose, to release the energy stored within them. This energy isn't released all at once like a fire; instead, it's captured in small, manageable packets in a molecule called ATP (adenosine triphosphate). ATP acts like a rechargeable battery for the cell, providing the power for everything from muscle contraction and nerve impulses to building new molecules and maintaining body temperature.

Glucose + Oxygen → Carbon Dioxide + Water + Energy (released as ATP)

Key term

ATP (Adenosine Triphosphate): The molecule that acts as the universal energy currency, storing and transferring chemical energy for metabolic processes within cells.

Examiner insight

Examiners expect you to be precise. Stating that respiration 'creates' or 'makes' energy is incorrect. Always state that energy is 'released' from glucose and 'transferred' by ATP.

Common pitfall

Confusing respiration with breathing. Breathing (or ventilation) is the mechanical process of moving air into and out of the lungs, whereas respiration is the chemical reaction that releases energy in cells.

Worked example 12 marks

The overall process of aerobic respiration can be summarised by an equation.(a) Name the two reactants in this equation.(b) Name the two waste products.

  1. 1

    Step 1: Recall the word equation for aerobic respiration: Glucose + Oxygen → Carbon Dioxide + Water + Energy.

  2. 2

    Step 2: Identify the substances on the left side of the arrow. These are the reactants.

  3. 3

    Answer (a): The reactants are glucose and oxygen.

  4. 4

    Step 3: Identify the substances produced on the right side of the arrow, excluding the main purpose (energy). These are the waste products.

  5. 5

    Answer (b): The waste products are carbon dioxide and water.

Recap

  • Respiration is a chemical reaction that releases energy from food.
  • It occurs in every living cell, not just in the lungs.
  • The energy released is transferred to a molecule called ATP.
  • ATP powers all life processes, such as movement, growth, and keeping warm.
  • Respiration is essential for all living organisms.

Quick check

  1. What is the primary purpose of cellular respiration?1 mark
  2. In which part of an animal cell does most respiration take place?1 mark

2. Aerobic Respiration: Using Oxygen

Aerobic means 'with air', and this is the most efficient type of respiration. It occurs when plenty of oxygen is available. In this process, glucose is completely broken down into carbon dioxide and water, releasing a large amount of energy—around 38 ATP molecules per molecule of glucose. This process takes place in tiny organelles inside the cell called mitochondria. Because it is so efficient, aerobic respiration is the primary way that complex organisms, including humans, get their energy.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + (large amount of ATP)

Key term

Mitochondria: The organelles within a cell, often called the 'powerhouses', where the majority of ATP is produced during aerobic respiration.

Worked example 13 marks

A cell aerobically respires 2 molecules of glucose. Calculate how many molecules of carbon dioxide and oxygen will be involved in this reaction.

  1. 1

    Step 1: Write down the balanced chemical equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

  2. 2

    Step 2: Interpret the ratio from the equation. For every 1 molecule of glucose (C₆H₁₂O₆), 6 molecules of oxygen (O₂) are used and 6 molecules of carbon dioxide (CO₂) are produced.

  3. 3

    Step 3: Apply this ratio to the 2 molecules of glucose in the question.

  4. 4

    Step 4: Calculate the oxygen used: 2 molecules of glucose × 6 = 12 molecules of O₂.

  5. 5

    Step 5: Calculate the carbon dioxide produced: 2 molecules of glucose × 6 = 12 molecules of CO₂.

  6. 6

    Answer: 12 molecules of oxygen will be used, and 12 molecules of carbon dioxide will be produced.

Recap

  • Aerobic respiration requires oxygen.
  • It takes place in the mitochondria of cells.
  • Glucose is completely broken down into carbon dioxide and water.
  • It releases a large amount of energy (many ATP molecules).
  • The balanced chemical equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Quick check

  1. State the two products of aerobic respiration, other than energy.2 marks

3. Anaerobic Respiration: Without Oxygen

What happens when oxygen is in short supply? Cells can switch to anaerobic respiration. This is the incomplete breakdown of glucose, which releases much less energy than aerobic respiration (only 2 ATP molecules per glucose). The products also differ. In animal muscle cells during strenuous exercise, glucose is converted to lactic acid. This build-up of lactic acid causes muscle fatigue and creates an 'oxygen debt'. In microorganisms like yeast, the process is called fermentation, and glucose is converted into ethanol and carbon dioxide. This is how bread is made to rise and how alcoholic beverages are produced.

In muscles: Glucose → Lactic Acid + (small amount of ATP)

In yeast: Glucose → Ethanol + Carbon dioxide + (small amount of ATP)

Key term

Oxygen Debt: The extra oxygen required by the body after strenuous exercise to break down the accumulated lactic acid and restore normal oxygen levels.

Common pitfall

Mixing up the products of anaerobic respiration. Remember: Animals produce Lactic Acid. Yeast produces Ethanol and CO₂.

Fun fact

The soreness you feel in your muscles a day or two after intense exercise isn't caused by lactic acid itself (it's cleared within hours), but by the microscopic muscle damage and inflammation that the exercise caused.

Worked example 14 marks

A sprinter finishes a 100m race and continues to breathe heavily for several minutes. Explain why, in terms of respiration.

  1. 1

    Step 1: During the sprint, the body's demand for energy is extremely high and cannot be met by aerobic respiration alone, as oxygen supply is limited.

  2. 2

    Step 2: The muscle cells switch to anaerobic respiration to provide additional ATP quickly.

  3. 3

    Step 3: Anaerobic respiration in muscles produces lactic acid as a waste product.

  4. 4

    Step 4: After the race, the sprinter has an 'oxygen debt'. They must continue to breathe heavily to take in extra oxygen.

  5. 5

    Step 5: This extra oxygen is used to break down the accumulated lactic acid (converting it back to glucose in the liver) and repay the debt.

  6. 6

    Answer: The heavy breathing repays the oxygen debt created by anaerobic respiration during the sprint, which is needed to break down the lactic acid that has built up in the muscles.

Recap

  • Anaerobic respiration does not require oxygen.
  • It releases a small amount of energy compared to aerobic respiration.
  • In animals, it produces lactic acid and leads to oxygen debt.
  • In yeast, it produces ethanol and carbon dioxide (fermentation).
  • It is an inefficient but fast way to produce ATP.

Quick check

  1. Name the product of anaerobic respiration in human muscle.1 mark
  2. Name the two products of anaerobic respiration in yeast.2 marks

4. Gas Exchange: Supplying Oxygen

For aerobic respiration to occur, cells need a constant supply of oxygen and a way to remove the waste carbon dioxide. This process of swapping gases is called gas exchange. It happens across a specialised respiratory surface. In humans, this surface is the millions of tiny air sacs in the lungs called alveoli. An efficient gas exchange surface has five key features: it is thin (often one cell thick) for a short diffusion path; it has a large surface area to maximise diffusion; it's moist so gases can dissolve; it has a rich blood supply to transport gases away quickly and maintain a steep concentration gradient; and it is well ventilated to bring fresh oxygen in and remove carbon dioxide.

Key term

Alveoli: Tiny, balloon-like air sacs in the lungs where the exchange of oxygen and carbon dioxide between the air and the blood takes place.

Examiner insight

To get full marks, don't just list the features. You must explain *how* each feature helps to speed up the rate of diffusion. For example, 'large surface area' gets one mark, but 'large surface area provides more space for diffusion to happen at the same time' gets the explanation mark.

Worked example 15 marks

Explain how the structure of the alveoli in the human lungs is adapted for efficient gas exchange.

  1. 1

    Step 1: Large surface area. There are millions of alveoli, which collectively provide a huge surface area (around 70m², the size of a tennis court) for gases to diffuse across simultaneously.

  2. 2

    Step 2: Thin walls. The walls of the alveoli and the surrounding capillaries are each only one cell thick. This creates a very short diffusion distance for oxygen and carbon dioxide, allowing for rapid exchange.

  3. 3

    Step 3: Rich blood supply. Each alveolus is covered in a dense network of capillaries. Blood constantly flows, carrying oxygen away and bringing carbon dioxide, which maintains a steep concentration gradient for both gases, driving diffusion.

  4. 4

    Step 4: Moist surface. The inner surface of the alveoli is covered in a thin layer of fluid. This allows oxygen to dissolve before it diffuses across the membrane into the blood.

  5. 5

    Step 5: Ventilation. The process of breathing constantly replaces the air in the alveoli, bringing in air with a high oxygen concentration and removing air with a high carbon dioxide concentration, which also helps maintain the concentration gradient.

Recap

  • Gas exchange supplies oxygen for respiration and removes carbon dioxide.
  • Efficient exchange surfaces are thin, moist, have a large surface area, and a good blood supply.
  • In humans, gas exchange occurs in the alveoli of the lungs.
  • Breathing ventilates the lungs, maintaining a steep concentration gradient.
  • Gases move by diffusion from an area of high concentration to an area of low concentration.

Quick check

  1. List three features of an efficient gas exchange surface.3 marks

5. Investigating Respiration: The Respirometer

Scientists measure the rate of respiration using an apparatus called a respirometer (or respirometer). It works by measuring the change in gas volume around a living organism in a sealed container. To measure the rate of aerobic respiration, we specifically measure oxygen consumption. The respirometer contains the living organisms (e.g., germinating seeds, maggots) and a chemical like potassium hydroxide solution or soda lime. This chemical absorbs all the carbon dioxide produced by the organism. Therefore, any decrease in the volume of gas inside the sealed chamber is due to the oxygen being consumed. This volume change is measured by observing the movement of a coloured drop of liquid in a narrow, calibrated tube (capillary tube) over a set period.

Rate of oxygen uptake = Volume of oxygen consumed / Time taken

Volume of oxygen consumed (mm³) = π × (radius of capillary tube in mm)² × distance moved by liquid (mm)

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 the potassium hydroxide/soda lime. Without it, the CO₂ produced would cancel out the volume change from the O₂ consumed, and the liquid would not move significantly.

Worked example 14 marks

A student used a respirometer to measure the rate of respiration in woodlice. The coloured liquid in the capillary tube moved 20 mm in 5 minutes. The radius of the capillary tube was 0.5 mm. Calculate the rate of oxygen consumption in mm³/min.

  1. 1

    Step 1: Calculate the volume of oxygen consumed. The capillary tube is a cylinder, so the volume is the cross-sectional area (πr²) multiplied by the distance moved (l).

  2. 2

    Step 2: Substitute the values into the volume formula: Volume = π × (0.5 mm)² × 20 mm.

  3. 3

    Step 3: Calculate the cross-sectional area: π × (0.5)² = π × 0.25 ≈ 0.785 mm².

  4. 4

    Step 4: Calculate the volume: 0.785 mm² × 20 mm = 15.7 mm³.

  5. 5

    Step 5: Calculate the rate of oxygen consumption. Rate = Volume / Time.

  6. 6

    Step 6: Substitute the values: Rate = 15.7 mm³ / 5 min.

  7. 7

    Step 7: Calculate the final answer: Rate ≈ 3.14 mm³/min.

  8. 8

    Answer: The rate of oxygen consumption is 3.14 mm³/min.

Recap

  • A respirometer measures the rate of respiration.
  • It typically measures the rate of oxygen consumption.
  • Potassium hydroxide or soda lime is used to absorb the CO₂ produced.
  • The decrease in gas volume causes a coloured liquid to move along a capillary tube.
  • The rate can be calculated by measuring the distance the liquid moves over time.

Quick check

  1. What is the function of soda lime in a respirometer experiment?1 mark
  2. Why is it important to use a control, such as glass beads of the same mass as the organism, in a respirometer experiment?2 marks

End-of-chapter exercise

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

  1. State the word equation for aerobic respiration and the word equation for anaerobic respiration in yeast.3 marks
  2. Compare aerobic and anaerobic respiration in terms of oxygen requirement, energy yield, and products in humans.4 marks
  3. Explain why a person's breathing rate and heart rate remain high for a period after they have finished a session of strenuous exercise.4 marks
  4. Describe an experiment you could carry out to investigate the effect of temperature on the rate of respiration in germinating peas, using a respirometer.6 marks
  5. Explain why anaerobic respiration in yeast is important to both the baking and brewing industries.4 marks
  6. The walls of the alveoli are one-cell thick. Explain how this feature aids the function of the lungs.2 marks
  7. A respirometer was set up with 20 maggots. The apparatus was left for 10 minutes. The coloured liquid moved 30mm. The internal radius of the capillary tube was 1mm. Calculate the rate of oxygen consumption by the maggots in mm³ per minute.4 marks
  8. Some organisms, such as parasitic worms that live in the intestines, respire anaerobically. Suggest why they are adapted to respire in this way.3 marks
  9. During prolonged exercise, a person's blood glucose concentration may fall. Explain why this happens.3 marks
  10. A student argues, 'Plants photosynthesise, animals respire.' Explain the inaccuracies in this statement.4 marks

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