Cambridge IGCSE0610

Respiration

Biology 0610 Chapter Notes

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RespirationAerobic respirationAnaerobic respiration
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In this chapter

  1. 1Respiration: Releasing Energy for Life
  2. 2Aerobic Respiration: The Main Event
  3. 3Anaerobic Respiration: The Emergency Backup
  4. 4Gas Exchange: Fuelling Respiration
  5. 5Investigating Respiration

1. Respiration: Releasing Energy for Life

Respiration is the fundamental chemical process that occurs in every living cell of every organism, from the smallest bacterium to the largest whale. Its purpose is to break down nutrient molecules, most commonly glucose, to release the energy stored within their chemical bonds. This energy isn't released all at once like a fire; instead, it's captured in small, manageable packets in a molecule called Adenosine Triphosphate (ATP). Think of ATP as the cell's universal energy currency, used to power all other life processes. So, while we often associate respiration with breathing, breathing (gas exchange) is just the system for getting oxygen in and carbon dioxide out. The real work of respiration happens inside our cells.

Nutrient (e.g. Glucose) → Energy (ATP) + Waste Products

Key term

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

Examiner insight

Examiners reward students who clearly state that respiration occurs in *all* living cells, not just in animals or in the lungs. This demonstrates a precise understanding of the topic.

Common pitfall

Confusing cellular respiration with breathing (gas exchange). Breathing is the physical act of moving air into and out of the lungs, while respiration is the chemical reaction inside cells to release energy.

Worked example 12 marks

Define the term 'respiration' and state where it occurs. (2 marks)

  1. 1

    Step 1: Define respiration. Respiration is the chemical process of breaking down nutrient molecules in cells to release energy.

  2. 2

    Step 2: State the location. This process occurs in all living cells, specifically starting in the cytoplasm and, for the main energy-releasing stages, in the mitochondria.

Worked example 23 marks

Explain why respiration is essential for living organisms. (3 marks)

  1. 1

    Step 1: State the primary purpose. Respiration is essential because it releases energy from food.

  2. 2

    Step 2: Explain how the energy is used. This energy, in the form of ATP, is required for all life processes, also known as metabolic processes.

  3. 3

    Step 3: Give specific examples. Examples include muscle contraction for movement, active transport of substances across cell membranes, and the synthesis of large molecules for growth and repair.

Recap

  • Respiration is a chemical reaction that releases energy from food.
  • It happens in all living cells, all the time.
  • The energy released is transferred to a molecule called ATP.
  • ATP is the energy currency that powers all other life processes.
  • Respiration is not the same as breathing.

Quick check

  1. What is the name of the molecule that acts as the main energy currency in a cell?1 mark

2. Aerobic Respiration: The Main Event

Aerobic respiration is the most efficient way for cells to release energy from glucose. The 'aerobic' part means 'with oxygen'. In this process, glucose is completely broken down into carbon dioxide and water, releasing a large amount of energy (around 32-38 ATP molecules per glucose molecule). This multi-step chemical reaction takes place primarily inside tiny organelles called mitochondria, which are often called the 'powerhouses' of the cell. Because it's so efficient, aerobic respiration is the primary method of energy release for most multicellular organisms, including humans, during normal activity.

Word Equation: glucose + oxygen → carbon dioxide + water (+ large amount of energy)

Balanced Chemical Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP)

Key term

Mitochondria: The organelles within a cell where the majority of aerobic respiration takes place, releasing energy from food molecules.

Fun fact

Your brain, while only about 2% of your body weight, consumes about 20% of the oxygen and calories you take in, all to power the aerobic respiration needed for its constant electrical activity.

Worked example 13 marks

a) Write the balanced chemical equation for aerobic respiration. (2 marks)b) Name the organelle where most of these reactions occur. (1 mark)

  1. 1

    a) The balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. One mark for correct reactants and products, one mark for correct balancing (all the 6s).

  2. 2

    b) The organelle is the mitochondrion (or mitochondria).

Worked example 23 marks

Explain why a very active cell, such as a muscle cell or a sperm cell, contains a large number of mitochondria. (3 marks)

  1. 1

    Step 1: State the function of the cell. Active cells like muscle or sperm cells require a large amount of energy for their function (movement).

  2. 2

    Step 2: Link mitochondria to energy. This energy is released by aerobic respiration.

  3. 3

    Step 3: Link number of mitochondria to respiration rate. Most aerobic respiration occurs in mitochondria. Therefore, having a large number of mitochondria allows for a high rate of respiration to provide sufficient ATP to meet the cell's high energy demand.

Recap

  • Aerobic respiration requires oxygen.
  • It involves the complete breakdown of glucose.
  • The products are carbon dioxide and water.
  • A large amount of energy (ATP) is released per glucose molecule.
  • It mainly occurs in the mitochondria.

Quick check

  1. Name the two reactants of aerobic respiration.2 marks
  2. Name the two waste products of aerobic respiration.2 marks

3. Anaerobic Respiration: The Emergency Backup

Anaerobic respiration is a way of releasing energy from glucose *without* using oxygen. This process is used when oxygen supply is insufficient to meet the energy demand, such as during intense exercise in humans. It involves the incomplete breakdown of glucose and releases only a small fraction of the energy (just 2 ATP molecules) compared to aerobic respiration. There are two main types to know:

  1. In Animals (e.g., human muscles): During a sprint, your muscles can't get oxygen fast enough. They switch to anaerobic respiration, converting glucose into lactic acid. This allows for a quick burst of energy but the build-up of lactic acid causes muscle fatigue and pain. The lactic acid must later be broken down using oxygen, creating an 'oxygen debt'.
  1. In Yeast and some Plants: This process is called fermentation. Yeast breaks down glucose into ethanol (an alcohol) and carbon dioxide. This is a vital process in baking (CO₂ makes bread rise) and brewing (ethanol makes alcoholic drinks).

In Animals/Muscles: glucose → lactic acid (+ small amount of energy)

In Yeast/Plants (Fermentation): glucose → ethanol + carbon dioxide (+ small amount of energy)

Key term

Oxygen Debt: The extra oxygen required after strenuous exercise to be taken into the body to break down the accumulated lactic acid.

Examiner insight

Examiners often test the comparison between aerobic and anaerobic respiration. Be prepared to compare reactants, products, location, and the amount of energy released for both processes.

Common pitfall

Stating that lactic acid produces CO₂. Anaerobic respiration in animal muscles produces only lactic acid and energy. Only fermentation in yeast produces CO₂.

Worked example 13 marks

A sprinter's muscles produce lactic acid during a 100m race. Explain why. (3 marks)

  1. 1

    Step 1: Relate exercise to energy demand. The 100m race is a short, intense exercise, requiring a lot of energy very quickly.

  2. 2

    Step 2: Relate demand to oxygen supply. The heart and lungs cannot supply oxygen to the muscles fast enough to meet this high energy demand through aerobic respiration alone.

  3. 3

    Step 3: Explain the switch to anaerobic respiration. To supplement the energy supply, muscle cells respire anaerobically, breaking down glucose into lactic acid to produce the extra ATP needed.

Worked example 22 marks

Yeast is used in bread-making. Explain the role of anaerobic respiration in this process. (2 marks)

  1. 1

    Step 1: Identify the process. The yeast respires anaerobically in the dough.

  2. 2

    Step 2: Identify the key product and its effect. This fermentation produces carbon dioxide gas. The bubbles of carbon dioxide get trapped in the dough, causing it to rise and giving the bread a light, airy texture.

Recap

  • Anaerobic respiration does not use oxygen.
  • It involves the incomplete breakdown of glucose.
  • It releases a small amount of energy.
  • In animals, it produces lactic acid, leading to an oxygen debt.
  • In yeast, it produces ethanol and carbon dioxide (fermentation).

Quick check

  1. Name the toxic substance produced during anaerobic respiration in muscles.1 mark
  2. Name the two products of anaerobic respiration in yeast.2 marks

4. Gas Exchange: Fuelling Respiration

For aerobic respiration to occur, cells need a constant supply of oxygen and a way to remove the waste product, carbon dioxide. This swapping of gases is called gas exchange. To be effective, any gas exchange surface (like the lungs in humans or gills in fish) must be adapted to maximise the rate of diffusion. The key features are:

  1. Large surface area: To allow enough gas to diffuse at once. The human lungs have a surface area of a tennis court!
  2. Thin walls: Usually just one cell thick, providing a very short distance for gases to diffuse across.
  3. Moist surface: Oxygen and carbon dioxide must dissolve in a fluid before they can diffuse across the membrane.
  4. Rich blood supply: To maintain a steep concentration gradient by rapidly transporting oxygen away from the surface and bringing carbon dioxide to it.
  5. Good ventilation: (In larger animals) A mechanism like breathing moves air, maintaining a steep concentration gradient for oxygen and carbon dioxide between the air and the blood.

Key term

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

Common pitfall

Simply listing the features of the alveoli without explaining *why* each feature is an adaptation. For example, writing 'It has thin walls' earns fewer marks than 'It has thin walls, which provides a short diffusion distance for gases'.

Worked example 14 marks

The alveoli are the gas exchange surfaces in human lungs. Explain how they are adapted for this function. (4 marks)

  1. 1

    Step 1: Large surface area. There are millions of alveoli, which collectively provide a huge surface area for diffusion.

  2. 2

    Step 2: Thin walls. The walls of the alveoli (and the surrounding capillaries) are only one cell thick, creating a very short diffusion pathway for gases.

  3. 3

    Step 3: Rich blood supply. Each alveolus is covered in a dense network of capillaries, which quickly carries oxygen away and brings CO₂ to be excreted, maintaining a steep concentration gradient.

  4. 4

    Step 4: Moist surface. The inner surface is covered in a thin layer of moisture, allowing oxygen to dissolve before it diffuses into the blood.

Recap

  • Gas exchange supplies the oxygen needed for aerobic respiration.
  • It also removes the waste product, carbon dioxide.
  • Efficient gas exchange surfaces have a large surface area and are thin and moist.
  • A good blood supply and ventilation are needed to maintain concentration gradients.
  • In humans, gas exchange occurs in the alveoli of the lungs.

Quick check

  1. State two features of an efficient gas exchange surface.2 marks
  2. Why is a short diffusion distance important for gas exchange?1 mark

5. Investigating Respiration

Scientists can measure the rate of respiration by detecting the consumption of a reactant (oxygen) or the production of a waste product (carbon dioxide).

1. Demonstrating CO₂ Production: We can use hydrogencarbonate indicator. This indicator is normally red, but turns yellow in the presence of high CO₂ concentrations and purple in low CO₂ concentrations. If you place germinating seeds or small insects in a sealed tube with hydrogencarbonate indicator, it will slowly turn from red to yellow as they respire and release CO₂.

2. Measuring Oxygen Consumption: This is done using a respirometer. The apparatus consists of a sealed container holding the living organism (e.g., woodlice or germinating seeds) and a chemical called soda lime or potassium hydroxide. The soda lime absorbs all the CO₂ produced by the organism. A thin, graduated capillary tube containing a drop of coloured liquid is attached. As the organism uses up oxygen for aerobic respiration, the pressure inside the sealed container drops, causing the coloured liquid to move along the tube. The distance the liquid moves over a set time is a measure of the rate of oxygen consumption, and therefore the rate of respiration.

Rate of respiration (in a respirometer) = Distance moved by liquid / Time taken

Key term

Respirometer: An apparatus used to measure the rate of respiration of a living organism by measuring its rate of oxygen consumption.

Examiner insight

For respirometer questions, examiners frequently ask about control variables. Key controls include setting up an identical respirometer with dead organisms or glass beads to account for any pressure changes due to temperature fluctuations, and keeping the temperature constant using a water bath.

Worked example 11 mark

In an experiment using a respirometer with maggots, what is the purpose of the soda lime? (1 mark)

  1. 1

    The soda lime is there to absorb the carbon dioxide produced by the maggots during respiration.

Worked example 25 marks

A student used a respirometer to measure the respiration rate of germinating peas at 20°C. The drop of liquid moved 24 mm in 10 minutes.a) Calculate the rate of oxygen consumption in mm/min. (2 marks)b) The student repeated the experiment at 30°C. Predict and explain the result. (3 marks)

  1. 1

    a) Rate = Distance / Time. Rate = 24 mm / 10 min = 2.4 mm/min. (1 mark for calculation, 1 mark for correct units).

  2. 2

    b) Prediction: The rate of oxygen consumption will be faster at 30°C. Explanation: Respiration is controlled by enzymes. At 30°C, the enzymes and substrate molecules have more kinetic energy, leading to more frequent collisions and a higher rate of reaction, thus a faster rate of respiration.

Recap

  • Respiration can be investigated by measuring oxygen intake or carbon dioxide output.
  • Hydrogencarbonate indicator turns from red to yellow in the presence of CO₂.
  • A respirometer is used to measure the rate of oxygen consumption.
  • Soda lime is used in a respirometer to absorb the CO₂ produced.
  • The rate of respiration is affected by factors like temperature and the type of organism.

Quick check

  1. What colour change would you expect in hydrogencarbonate indicator if a snail is left in a sealed tube with it?1 mark
  2. Why is it important to use a water bath when investigating the effect of temperature on respiration?1 mark

End-of-chapter exercise

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

  1. Write the balanced chemical equation for aerobic respiration and state the name of the organelle where most of these reactions occur.3 marks
  2. Compare aerobic and anaerobic respiration in humans in terms of: a) oxygen requirement, b) products, and c) amount of energy released.5 marks
  3. Explain why the build-up of lactic acid in muscles during exercise can lead to an 'oxygen debt'.4 marks
  4. Describe how the structure of an alveolus is adapted for efficient gas exchange.4 marks
  5. The production of beer involves the use of yeast in large, sealed containers. Explain why the conditions are kept anaerobic and name the two key products of this process.3 marks
  6. A student designed an experiment to investigate the rate of respiration in woodlice. They placed five woodlice in a sealed container with a substance to absorb carbon dioxide. A sensor measured the oxygen concentration. State two variables the student should have kept constant to ensure a fair test.2 marks
  7. Explain why a person's breathing rate and heart rate remain high for a short period after finishing a session of strenuous exercise.4 marks
  8. Describe an experiment you could carry out to show that germinating seeds produce carbon dioxide.4 marks
  9. State three different processes in a plant that require energy released from respiration.3 marks
  10. An athlete is preparing for two races: a 100m sprint and a 10,000m marathon. Explain how the primary method of respiration used to supply energy will differ between the two races.6 marks

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