Aerobic respiration

1. Overview

Aerobic respiration is a fundamental metabolic process that occurs in the cells of almost all living organisms. It is the chemical process by which cells break down food molecules (primarily glucose) in the presence of oxygen to provide the energy required for all life processes, such as muscle contraction, cell division, and protein synthesis.

Key Definitions

• Aerobic Respiration: The chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
• Glucose: A simple sugar (carbohydrate) that acts as the primary fuel source for respiration.
• Mitochondria: The specialized organelles within a cell where the most significant stages of aerobic respiration occur.
• Metabolism: The sum of all chemical reactions that happen within a cell or organism.

Core Content

Aerobic respiration is a multi-step chemical reaction that happens continuously in almost every living cell. If respiration stops, the cell dies because it no longer has energy for maintenance.

The Purpose of Aerobic Respiration:
The main goal is to release energy. This energy is used for:

• Muscle contraction (movement).
• Maintaining a constant body temperature (in mammals and birds).
• Active transport (moving molecules against concentration gradients).
• Building large molecules (like proteins from amino acids).

The Process:

1. Glucose enters the cell from the bloodstream (after digestion).
2. Oxygen enters the cell from the bloodstream (after gas exchange in the lungs).
3. These molecules react inside the cytoplasm and the mitochondria.
4. The chemical bonds in glucose are broken, releasing energy.
5. Carbon dioxide and water are produced as waste products and leave the cell.

Word Equation:

glucose + oxygen → carbon dioxide + water

Structure and Function:

• Mitochondria: These are often called the "powerhouses" of the cell. Cells that require a lot of energy (like muscle cells or sperm cells) contain many more mitochondria than less active cells (like skin cells).
• Internal Membrane: The inner membrane of the mitochondrion is highly folded to increase surface area, allowing more respiration reactions to happen simultaneously.

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Extended Content (Extended curriculum only)

In the extended curriculum, you must be able to represent the chemical reaction using the correct molecular formulas and ensure the equation is balanced.

Balanced Chemical Equation:
The breakdown of one molecule of glucose requires six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Step-by-Step Balancing:

• Reactants: 6 Carbons, 12 Hydrogens, 18 Oxygens (6 from glucose + 12 from oxygen gas).
• Products: 6 Carbons, 12 Hydrogens (6x2), 18 Oxygens (12 from CO₂ + 6 from H₂O).

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Key Equations

Type	Equation
Word Equation	glucose + oxygen → carbon dioxide + water
Chemical Equation	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Symbols used:

• C₆H₁₂O₆: Glucose
• O₂: Oxygen gas
• CO₂: Carbon dioxide
• H₂O: Water

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Common Mistakes to Avoid

• ❌ Wrong: "Respiration is the same as breathing."
  • ✓ Right: Breathing (ventilation) is the physical movement of air; Respiration is the chemical reaction inside cells.
• ❌ Wrong: "Respiration produces energy."
  • ✓ Right: Respiration releases energy (energy cannot be created or destroyed, only transferred from chemical stores).
• ❌ Wrong: "Plants only photosynthesize, they don't respire."
  • ✓ Right: Plants respire 24 hours a day to stay alive; they only photosynthesize when light is available.
• ❌ Wrong: Writing the chemical equation without balancing it (e.g., C₆H₁₂O₆ + O₂ → CO₂ + H₂O).
  • ✓ Right: Always include the "6" before O₂, CO₂, and H₂O.

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Exam Tips

• Command Words: If the question asks you to "State" the equation, you can provide either the word or chemical equation unless the question specifies "balanced chemical equation."
• Question Types: Expect to see "fill in the blanks" for the equation or questions asking you to identify where in the cell respiration occurs.
• Real-world Context: Questions often link respiration to exercise. Remember that during exercise, the rate of aerobic respiration increases to provide more energy for muscle contraction, which is why your heart rate and breathing rate increase (to supply more O₂ and glucose).
• Typical Values: In paper 6 (Alternative to Practical), you might see data showing the uptake of oxygen by organisms. Remember that the more oxygen consumed, the higher the rate of aerobic respiration.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define aerobic respiration. [2]

(b) State the balanced chemical equation for aerobic respiration. [3]

Worked Solution:

(a)

1. Aerobic respiration is the breakdown of nutrient molecules...

2. ...using oxygen...

3. ...to release energy.

How to earn full marks:

• Must mention the breakdown of nutrient molecules.
• Must mention the use of oxygen.
• Must mention the release of energy.

(b)

1. Correct formula for glucose.
   $C_6H_{12}O_6$

2. Correct formula for oxygen, carbon dioxide, and water.
   $6O_2 \rightarrow 6CO_2 + 6H_2O$

3. Balanced equation.
   $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

How to earn full marks:

• Must have the correct formula for glucose.
• Must have the correct formulas for oxygen, carbon dioxide, and water and the correct stoichiometric coefficients.
• Must have the whole equation balanced. $\boxed{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$

Common Pitfall: Make sure you know the word equation and the balanced chemical equation for aerobic respiration. Don't forget to include the "energy" part in your definition, as that's the whole point of the process!

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A student is investigating aerobic respiration in yeast. They place a suspension of yeast and glucose solution in a sealed flask connected to a manometer, as shown in the diagram. A small tube of potassium hydroxide solution inside the flask absorbs any carbon dioxide that is produced. The apparatus is kept at a constant temperature.

(a) State two observations of the coloured liquid in the manometer that would indicate that aerobic respiration is taking place in the flask. [2]

(b) Explain why the level of the coloured liquid in the manometer changes during aerobic respiration. [4]

Worked Solution:

(a)

1. The coloured liquid rises in the arm of the manometer connected to the flask.
   The liquid moves towards the flask.

2. The coloured liquid falls in the open arm of the manometer.
   The difference between the two levels increases over time.

How to earn full marks:

• 1 mark for the liquid rising in the arm connected to the flask (or moving towards the flask).
• 1 mark for the liquid falling in the open arm (or the difference between the levels increasing over time).
• "Oxygen is used up" and "carbon dioxide is produced" are not observations, so they do not score in (a).

(b)

1. The yeast uses up oxygen in aerobic respiration.
   Oxygen is taken from the air inside the flask.

2. The carbon dioxide produced is absorbed by the potassium hydroxide.
   So no gas replaces the oxygen that has been used.

3. The volume of gas in the flask decreases, so the pressure inside the flask falls.
   There are fewer gas molecules in the flask.

4. The coloured liquid is pushed towards the flask.
   The air pressure on the open arm is now greater than the pressure in the flask, so it pushes the liquid towards the flask.

How to earn full marks:

• Must state that oxygen is used up by the yeast.
• Must state that the carbon dioxide is absorbed by the potassium hydroxide.
• Must link this to a decrease in gas volume or pressure in the flask.
• Must explain that the higher air pressure on the open arm pushes the liquid towards the flask.

Common Pitfall: Aerobic respiration releases as much carbon dioxide as the oxygen it uses (6 molecules of each for every glucose molecule). Without the potassium hydroxide, the volume of gas would hardly change and the liquid would barely move. Many students say the liquid moves away from the flask because carbon dioxide is produced, but here the carbon dioxide is absorbed, so the movement is caused only by the oxygen being used up.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

An athlete is running a marathon. During the race, their muscle cells respire both aerobically and anaerobically.

(a) Describe the role of aerobic respiration in providing energy for the athlete's muscles. [4]

(b) Explain how the rate of aerobic respiration changes in the athlete's muscles during the marathon, from the start to the end of the race. [4]

Worked Solution:

(a)

1. Glucose is the substrate for respiration.
   Glucose is broken down.

2. Oxygen is used.

3. Energy is released.

4. This energy is used to produce ATP (adenosine triphosphate).

5. ATP is then used to provide energy for muscle contraction.

How to earn full marks:

• Must mention glucose as the substrate and oxygen being used.
• Must state that energy is released and ATP is produced.
• Must link ATP to muscle contraction.

(b)

1. At the start of the race, the rate of aerobic respiration increases.

2. This is because the muscles need more energy for increased activity.

3. The body increases oxygen supply to the muscles.

4. Towards the end of the race, the rate may plateau or even decrease if the athlete is fatigued.

5. Because oxygen demand exceeds supply.

How to earn full marks:

• Must mention the initial increase in respiration rate.
• Must link this increase to increased energy demand and oxygen supply.
• Must state that the rate may plateau or decrease towards the end due to fatigue.

Common Pitfall: Don't confuse aerobic and anaerobic respiration. Aerobic respiration is the primary source of energy during sustained activity, but anaerobic respiration kicks in when oxygen supply is limited. Also, be sure to link the rate of respiration to the energy demands of the muscles.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

An experiment is set up to investigate the effect of temperature on the rate of aerobic respiration in germinating pea seeds. The volume of oxygen consumed by the seeds is measured at different temperatures over a period of one hour. The results are shown in the table below:

Temperature (°C)	Volume of Oxygen Consumed (cm³)
10	2.5
20	7.0
30	14.0
40	10.5
50	3.0

(a) Describe the trend shown by the data in the table. [2]

(b) Calculate the rate of oxygen consumption at 20°C in $cm^3/minute$. [2]

(c) Explain the effect of temperature on the rate of aerobic respiration in the germinating pea seeds. [5]

Worked Solution:

(a)

1. As the temperature increases from 10°C to 30°C, the volume of oxygen consumed increases.

2. Above 30°C, the volume of oxygen consumed decreases.

How to earn full marks:

• Must mention that the oxygen consumption increases with temperature initially.
• Must mention that the oxygen consumption decreases at higher temperatures.

(b)

1. Rate = Volume / Time

2. Convert time to minutes.
   Time = 1 hour = 60 minutes

3. Rate calculation.
   Rate = 7.0 cm³ / 60 minutes = 0.11666666 cm³/minute

4. Correct answer.
   Rate = $\boxed{0.12 \text{ } cm^3/minute}$ (rounded to 2 s.f.)

How to earn full marks:

• Must show the correct formula (volume/time).
• Must convert the time to minutes.
• Must calculate the rate correctly and include the units.

(c)

1. Aerobic respiration is controlled by enzymes.

2. Rate increases with temperature (up to optimum).
   As temperature increases, the rate of enzyme activity increases, leading to a higher rate of respiration.

3. There is an optimum temperature for enzyme activity.

4. Above the optimum temperature, the enzymes start to denature (change shape).

5. This reduces the rate of reaction as the substrate no longer fits the active site.

How to earn full marks:

• Must mention that enzymes control respiration.
• Must explain that the rate increases with temperature due to increased enzyme activity.
• Must explain that the rate decreases above the optimum temperature due to enzyme denaturation.

Common Pitfall: Remember that enzymes are involved in respiration, and their activity is affected by temperature. Don't just say "temperature increases respiration"; explain why using enzyme concepts like optimum temperature and denaturation. Also, pay close attention to units when calculating rates.