K325 / 2027

Respiration in humans

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Which route correctly takes inhaled air towards its main gas-exchange surface?

  1. Alveoli -> larynx -> bronchioles -> trachea
  2. Larynx -> trachea -> bronchi -> bronchioles -> alveoli
  3. Trachea -> bronchioles -> capillaries -> bronchi -> alveoli
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Larynx -> trachea -> bronchi -> bronchioles -> alveoli

Gas exchange occurs across the thin alveolar and capillary walls at the end of this route.

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The diaphragm flattens and the ribs move up and out. Which sequence follows?

  1. Chest volume decreases -> lung pressure rises -> air is pushed in
  2. Lung pressure rises -> oxygen is actively transported in
  3. Chest volume increases -> lung pressure falls -> air enters
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Chest volume increases -> lung pressure falls -> air enters

Air enters because pressure in the expanding lungs falls below atmospheric pressure.

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Which set of changes happens when you breathe out at rest?

  1. Diaphragm relaxes and domes up; chest volume falls; pressure rises
  2. Ribs move up and out; chest volume falls; pressure rises
  3. Diaphragm relaxes and domes up; chest volume rises; pressure falls
  4. Diaphragm contracts and flattens; chest volume rises; pressure falls
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Diaphragm relaxes and domes up; chest volume falls; pressure rises

A smaller chest volume raises the pressure in the lungs above air pressure, so air moves out.

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Why does a continuous capillary blood supply help alveolar gas exchange?

  1. It makes the alveolar wall thicker.
  2. It carries inhaled air directly to the heart, which pumps the oxygen round the body.
  3. It brings oxygen-poor blood and removes oxygen-rich blood, maintaining a gradient.
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It brings oxygen-poor blood and removes oxygen-rich blood, maintaining a gradient.

Maintaining the difference between alveolar air and incoming blood supports continued net oxygen diffusion.

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Cilia are damaged and mucus accumulates in the airways. Which explanation best links cause and effect?

  1. Carbon monoxide makes cilia carry more oxygen.
  2. Nicotine thickens the mucus into fibrin, which sticks the cilia together.
  3. Tar damages cilia, reducing movement of mucus towards the throat.
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Tar damages cilia, reducing movement of mucus towards the throat.

Poorer clearance leaves mucus and trapped material in the airways.

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Which correctly describes aerobic respiration in a human cell?

  1. Oxygen is converted directly into glucose by breathing.
  2. Glucose is broken down to lactic acid, and oxygen and energy are released.
  3. Glucose and oxygen are used; carbon dioxide and water form; energy is released.
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Glucose and oxygen are used; carbon dioxide and water form; energy is released.

The balanced equation is C6H12O6 + 6O2 -> 6CO2 + 6H2O.

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Which is a correct word equation for anaerobic respiration in human muscle?

  1. glucose + oxygen -> lactic acid + energy released
  2. glucose -> lactic acid + energy released
  3. glucose -> ethanol + carbon dioxide + energy released
  4. lactic acid + oxygen -> glucose + energy released
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glucose -> lactic acid + energy released

Human muscle without enough oxygen breaks glucose down to lactic acid, releasing a little energy.

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Why does a runner keep breathing rapidly and deeply after vigorous exercise?

  1. To supply extra oxygen needed for recovery and removal of the oxygen debt
  2. Because the cells no longer need any oxygen at rest
  3. To breathe the lactic acid made in the muscles directly out of the lungs
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To supply extra oxygen needed for recovery and removal of the oxygen debt

The additional oxygen supports recovery after anaerobic activity, so ventilation remains elevated for a time.

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Invented data: a runner's breathing rate is 14 breaths/min at rest, 40 at the end of a sprint and 20 after three minutes. Why is it still above 14 after three minutes?

  1. The heart has stopped pumping, so breathing must make up for it
  2. The muscles are still respiring anaerobically to make carbon dioxide
  3. The lungs are still breathing out the lactic acid made in the sprint
  4. Extra oxygen is still needed to break down lactic acid
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Extra oxygen is still needed to break down lactic acid

The oxygen debt is repaid after exercise by oxidising lactic acid, so breathing stays fast.

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Question 1 of 8

Measure oxygen uptake with a respirometer

5 marks

In an invented respirometer experiment, the coloured drop moves 30 mm towards germinating seeds in 10 min. In a second tube with glass beads, it does not move. (a) Explain why the drop moves, including the role of sodium hydroxide. (b) Calculate the rate of movement with its unit. (c) Explain what the glass-bead tube shows. (d) State one safety precaution. [5]

Oxygen in, carbon dioxide absorbed, so gas volume falls. Rate = distance / time.

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