Gas Exchange and Respiration: Lungs, Breathing, Aerobic Respiration and Anaerobic Respiration

1. Which row correctly compares inspired air and expired air?

A inspired air has more carbon dioxide and less oxygen
B inspired air has less oxygen and more water vapour
C expired air has less oxygen, more carbon dioxide and more water vapour
D expired air has more oxygen, less carbon dioxide and less water vapour

2. Which feature of alveoli most directly reduces diffusion distance?

A large surface area
B one-cell-thick walls
C rich blood supply
D moist lining

3. Which statement correctly describes gas exchange at the alveoli?

A oxygen diffuses from blood into alveoli because blood has more oxygen
B carbon dioxide diffuses from alveoli into blood because alveoli have more carbon dioxide
C oxygen diffuses from alveoli into blood because alveolar air has a higher oxygen concentration
D carbon dioxide diffuses from blood into alveoli by active transport

4. Which structure prevents food from entering the trachea during swallowing?

A diaphragm
B epiglottis
C bronchus
D intercostal muscle

5. During inhalation, which row is correct?

A external intercostal muscles contract; diaphragm contracts; thorax volume increases
B external intercostal muscles relax; diaphragm relaxes; thorax volume increases
C internal intercostal muscles contract; diaphragm relaxes; thorax volume decreases
D external intercostal muscles contract; diaphragm relaxes; thorax volume decreases

6. What happens to air pressure inside the lungs during inhalation?

A it becomes higher than atmospheric pressure
B it becomes lower than atmospheric pressure
C it stays exactly equal to atmospheric pressure throughout
D it becomes zero because the lungs are empty

7. Which statement about exhalation at rest is correct?

A diaphragm contracts and flattens
B external intercostal muscles contract strongly
C diaphragm relaxes and becomes dome-shaped
D ribs move upwards and outwards

8. Which equation represents aerobic respiration?

A glucose → lactic acid
B glucose + oxygen → carbon dioxide + water
C carbon dioxide + water → glucose + oxygen
D glucose → ethanol + carbon dioxide

9. Which equation represents anaerobic respiration in yeast?

A glucose → lactic acid
B glucose + oxygen → carbon dioxide + water
C glucose → ethanol + carbon dioxide
D ethanol + oxygen → glucose + carbon dioxide

10. Which statement about anaerobic respiration in human muscle is correct?

A it releases more energy per glucose molecule than aerobic respiration
B it produces ethanol and carbon dioxide
C it produces lactic acid and releases less energy than aerobic respiration
D it requires oxygen but produces no carbon dioxide

11. A student runs hard for 2 minutes. After stopping, breathing rate remains high for several minutes.
    What is the best explanation?

A oxygen is needed to remove lactic acid and repay oxygen debt
B oxygen is needed to make more lactic acid
C carbon dioxide is needed to break down glucose aerobically
D breathing remains high because the lungs have permanently enlarged

12. Which process occurs in mitochondria?

A photosynthesis
B aerobic respiration
C diffusion of oxygen through stomata
D emulsification of fats

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Which structure is correctly matched with its function?

A trachea: site of most gas exchange
B bronchioles: tubes leading air towards alveoli
C alveoli: produce mucus to trap dust only
D ribs: contract to change thorax volume

14. A person has damaged cilia in the air passages.
    Which effect is most likely?

A mucus is moved out of the airways less effectively
B oxygen can no longer diffuse across alveoli
C haemoglobin can no longer bind oxygen
D diaphragm contraction permanently stops

15. Which substance in cigarette smoke reduces the oxygen-carrying capacity of blood?

A nicotine
B tar
C carbon monoxide
D water vapour

16. Which substance in cigarette smoke is most directly linked with addiction?

A nicotine
B tar
C carbon monoxide
D carbon dioxide

17. Which substance in cigarette smoke is most directly linked with lung cancer?

A tar
B oxygen
C nitrogen
D water vapour

18. Which condition involves breakdown of alveolar walls, reducing surface area for gas exchange?

A bronchitis
B emphysema
C anaemia
D scurvy

19. Which symptom is most likely caused by mucus and inflamed bronchi in chronic bronchitis?

A persistent coughing
B increased oxygen diffusion
C stronger alveolar walls
D faster removal of carbon monoxide

20. Which row correctly matches disease and main effect?

A emphysema: reduced alveolar surface area
B lung cancer: increased number of healthy alveoli
C bronchitis: stronger cilia and less mucus
D asthma: permanent digestion of diaphragm

21. A person breathes into limewater. It turns milky faster than when air is bubbled through it.
    What does this show?

A expired air contains more carbon dioxide than atmospheric air
B expired air contains no oxygen
C inspired air contains more carbon dioxide than expired air
D limewater tests for oxygen

22. Hydrogencarbonate indicator turns yellow when carbon dioxide concentration is high. A respiring insect is placed in a sealed tube with the indicator.
    What result is expected?

A indicator turns purple because oxygen increases
B indicator turns yellow because carbon dioxide increases
C indicator remains red because insects do not respire
D indicator turns blue-black because starch is produced

23. A respirometer contains germinating seeds and soda lime.
    What is the purpose of soda lime?

A absorb oxygen produced by seeds
B absorb carbon dioxide produced by seeds
C provide glucose for anaerobic respiration
D absorb water from the seeds only

24. In a respirometer with soda lime, the air bubble moves towards the seeds.
    What is the best explanation?

A oxygen is used and carbon dioxide is absorbed, reducing gas volume
B carbon dioxide is produced and accumulates, increasing gas volume
C oxygen is produced by photosynthesis, increasing gas volume
D nitrogen is used up by aerobic respiration

25. Which control is most suitable for a respirometer using germinating seeds?

A boiled seeds of the same mass
B more germinating seeds
C green leaves in bright light
D yeast with sugar solution

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. A respirometer bubble moves 36 mm in 9 minutes.
    What is the rate of bubble movement?

A 0.25 mm/min
B 4 mm/min
C 27 mm/min
D 324 mm/min

27. A respirometer capillary tube has a cross-sectional area of 0.2 mm². The bubble moves 50 mm in 5 minutes.
    What is the rate of oxygen uptake?

A 2 mm³/min
B 10 mm³/min
C 20 mm³/min
D 50 mm³/min

28. A person’s breathing rate increases from 12 breaths per minute to 30 breaths per minute during exercise. Tidal volume increases from 0.5 dm³ to 2.0 dm³.
    What happens to ventilation rate?

A increases from 6 dm³/min to 60 dm³/min
B increases from 12.5 dm³/min to 32 dm³/min
C decreases from 60 dm³/min to 6 dm³/min
D remains constant at 30 dm³/min

29. Which statement about ventilation rate is correct?

A ventilation rate = breathing rate × tidal volume
B ventilation rate = heart rate × stroke volume
C ventilation rate = oxygen concentration ÷ carbon dioxide concentration
D ventilation rate = lung volume ÷ rib length

30. During vigorous exercise, muscle cells may respire anaerobically because

A oxygen supply cannot meet demand quickly enough
B glucose is absent from muscle cells
C mitochondria stop existing
D carbon dioxide becomes the only fuel

31. Which statement about lactic acid is correct?

A it is produced during aerobic respiration in yeast
B it can build up in muscles during anaerobic respiration
C it is produced in alveoli during gas exchange
D it is the main product of photosynthesis

32. Which statement about oxygen debt is correct?

A it is the extra oxygen needed after exercise to break down lactic acid
B it is the oxygen stored permanently inside the stomach
C it is the oxygen used to make ethanol in muscle cells
D it is the oxygen removed from blood by smoking tar

33. Which adaptation of alveoli helps maintain a steep oxygen concentration gradient?

A thick walls
B poor blood supply
C continuous blood flow through capillaries
D dry internal surface

34. Why are alveoli moist?

A to dissolve gases before diffusion
B to prevent any water loss from the lungs
C to trap glucose molecules only
D to make oxygen move by osmosis

35. Which statement correctly compares gas exchange and breathing?

A gas exchange is movement of air into lungs; breathing is diffusion across surfaces
B gas exchange is diffusion of gases; breathing is ventilation of lungs
C both mean the same as aerobic respiration
D both occur only inside mitochondria

36. Which row correctly identifies gas movements in body tissues?

A oxygen diffuses from cells to blood; carbon dioxide diffuses from blood to cells
B oxygen diffuses from blood to cells; carbon dioxide diffuses from cells to blood
C both oxygen and carbon dioxide diffuse from blood to cells
D both oxygen and carbon dioxide diffuse from cells to blood

37. Which vessel carries blood with the highest carbon dioxide concentration to the lungs?

A pulmonary vein
B pulmonary artery
C aorta
D coronary artery

38. Which statement about respiration is correct?

A respiration is the same as breathing
B respiration releases energy from food molecules in cells
C respiration only occurs in lungs
D respiration only happens during exercise

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. A student says, “Anaerobic respiration is useless because it releases less energy than aerobic respiration.”
    Which response is best?

A correct, because anaerobic respiration never helps organisms survive
B incorrect, because it can release energy without oxygen for a short time
C correct, because anaerobic respiration produces no ATP
D incorrect, because anaerobic respiration releases more energy than aerobic respiration

40. Which process removes carbon dioxide from the body?

A diffusion from blood into alveoli followed by exhalation
B active transport from alveoli into blood followed by inhalation
C osmosis through the diaphragm
D emulsification by bile in the lungs

41. Which part of the brain helps control breathing rate by responding to carbon dioxide levels?

A cerebrum only
B medulla
C retina
D pancreas

42. An increase in carbon dioxide concentration in the blood usually causes

A a decrease in breathing rate
B an increase in breathing rate and depth
C permanent closure of alveoli
D inhibition of all muscle contraction

43. Why does carbon dioxide concentration strongly affect breathing?

A carbon dioxide dissolves to form acidic conditions, affecting blood pH
B carbon dioxide carries oxygen in red blood cells
C carbon dioxide is converted directly into haemoglobin
D carbon dioxide blocks all diffusion through the ribs

44. Which statement about inhaled and exhaled nitrogen is correct?

A nitrogen is completely used in respiration
B nitrogen percentage changes very little between inhaled and exhaled air
C exhaled air contains no nitrogen
D nitrogen is converted into lactic acid in alveoli

45. Which change would decrease the rate of diffusion across an alveolar surface?

A thinner alveolar wall
B larger surface area
C smaller concentration gradient
D faster blood flow

46. Which statement about cilia and mucus in the trachea is correct?

A mucus traps particles and cilia move mucus towards the throat
B cilia trap particles and mucus pumps oxygen into blood
C mucus digests oxygen and cilia absorb glucose
D both are found only in alveolar capillaries

47. A person has fewer functioning alveoli after severe lung damage.
    Which effect is most likely?

A reduced surface area for gas exchange
B increased oxygen diffusion into blood
C faster removal of carbon dioxide at rest and during exercise
D no effect because gas exchange occurs in the trachea

48. Which row correctly compares aerobic and anaerobic respiration in humans?

A aerobic: no oxygen needed; anaerobic: oxygen needed
B aerobic: carbon dioxide and water produced; anaerobic: lactic acid produced
C aerobic: less energy released; anaerobic: more energy released
D aerobic: occurs only in lungs; anaerobic: occurs only in blood plasma

49. Which statement about yeast respiration is correct?

A yeast produces lactic acid during anaerobic respiration
B yeast produces ethanol and carbon dioxide during anaerobic respiration
C yeast cannot respire without oxygen
D yeast uses carbon dioxide as its only fuel

50. A yeast and glucose mixture is kept warm in a sealed tube. Gas bubbles are produced and limewater turns milky.
    Which conclusion is most valid?

A yeast is producing carbon dioxide during anaerobic respiration
B yeast is producing oxygen during photosynthesis
C glucose is being converted into lactic acid only
D limewater is detecting ethanol

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. Answer: C
   A is wrong because inspired air has less carbon dioxide and more oxygen.
   B is wrong because inspired air has more oxygen and less water vapour than expired air.
   C is correct because expired air has less oxygen, more carbon dioxide and more water vapour.
   D is wrong because it reverses the normal comparison.

2. Answer: B
   A is wrong because large surface area increases total diffusion, but does not specifically reduce diffusion distance.
   B is correct because one-cell-thick alveolar walls give a very short diffusion distance.
   C is wrong because rich blood supply maintains the concentration gradient.
   D is wrong because moist lining helps gases dissolve before diffusion.

3. Answer: C
   A is wrong because oxygen diffuses from alveoli into blood, not from blood into alveoli.
   B is wrong because carbon dioxide diffuses from blood into alveoli.
   C is correct because alveolar air has a higher oxygen concentration than deoxygenated blood.
   D is wrong because carbon dioxide moves by diffusion, not active transport.

4. Answer: B
   A is wrong because the diaphragm helps breathing.
   B is correct because the epiglottis covers the trachea during swallowing.
   C is wrong because the bronchus carries air to the lungs.
   D is wrong because intercostal muscles move the ribs.

5. Answer: A
   A is correct because during inhalation, external intercostal muscles contract, the diaphragm contracts and flattens, and thorax volume increases.
   B is wrong because relaxation of both would reduce thorax volume.
   C is wrong because internal intercostal contraction is linked with forced exhalation.
   D is wrong because the diaphragm contracts during inhalation, not relaxes.

6. Answer: B
   A is wrong because higher lung pressure would push air out.
   B is correct because during inhalation, lung pressure falls below atmospheric pressure, so air enters.
   C is wrong because pressure must become lower for air to move in.
   D is wrong because lung pressure does not become zero.

7. Answer: C
   A is wrong because diaphragm contraction and flattening happen during inhalation.
   B is wrong because external intercostal muscles contract during inhalation.
   C is correct because during resting exhalation, the diaphragm relaxes and becomes dome-shaped.
   D is wrong because ribs move downwards and inwards during exhalation.

8. Answer: B
   A is wrong because glucose → lactic acid is anaerobic respiration in muscles.
   B is correct because aerobic respiration is glucose + oxygen → carbon dioxide + water.
   C is wrong because that is photosynthesis.
   D is wrong because glucose → ethanol + carbon dioxide is anaerobic respiration in yeast.

9. Answer: C
   A is wrong because glucose → lactic acid happens in human muscle cells.
   B is wrong because this is aerobic respiration.
   C is correct because yeast anaerobically respires glucose into ethanol and carbon dioxide.
   D is wrong because ethanol is not converted into glucose in yeast respiration.

10. Answer: C
    A is wrong because anaerobic respiration releases less energy per glucose molecule.
    B is wrong because ethanol and carbon dioxide are produced by yeast, not human muscle.
    C is correct because anaerobic respiration in human muscle produces lactic acid and releases less energy than aerobic respiration.
    D is wrong because anaerobic respiration does not require oxygen.

11. Answer: A
    A is correct because extra oxygen is needed after exercise to break down/remove lactic acid and repay oxygen debt.
    B is wrong because oxygen is used to remove lactic acid, not make more of it.
    C is wrong because oxygen, not carbon dioxide, is needed for aerobic respiration.
    D is wrong because the lungs do not permanently enlarge after running.

12. Answer: B
    A is wrong because photosynthesis occurs in chloroplasts.
    B is correct because mitochondria are the site of aerobic respiration.
    C is wrong because stomata are in leaves, not mitochondria.
    D is wrong because emulsification occurs in digestion, not mitochondria.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

13. Answer: B
    A is wrong because most gas exchange occurs in alveoli, not the trachea.
    B is correct because bronchioles carry air towards alveoli.
    C is wrong because alveoli are mainly for gas exchange, not mucus production only.
    D is wrong because ribs do not contract; intercostal muscles contract.

14. Answer: A
    A is correct because cilia normally move mucus and trapped particles towards the throat; damaged cilia reduce this movement.
    B is wrong because oxygen can still diffuse across alveoli if alveoli are functional.
    C is wrong because haemoglobin binding oxygen is not directly controlled by airway cilia.
    D is wrong because diaphragm contraction is controlled by muscles/nerves, not cilia.

15. Answer: C
    A is wrong because nicotine is addictive and affects heart rate/blood pressure.
    B is wrong because tar damages cilia and is linked with cancer.
    C is correct because carbon monoxide binds strongly with haemoglobin, reducing oxygen transport.
    D is wrong because water vapour does not reduce oxygen-carrying capacity.

16. Answer: A
    A is correct because nicotine is the addictive substance in cigarette smoke.
    B is wrong because tar is mainly linked with cancer and cilia damage.
    C is wrong because carbon monoxide reduces oxygen transport.
    D is wrong because carbon dioxide is not the main addictive substance.

17. Answer: A
    A is correct because tar contains carcinogens linked with lung cancer.
    B is wrong because oxygen is not the cancer-causing component of smoke.
    C is wrong because nitrogen is not the key harmful carcinogen in cigarette smoke.
    D is wrong because water vapour is not the main cancer risk.

18. Answer: B
    A is wrong because bronchitis involves inflammation and mucus in bronchi.
    B is correct because emphysema breaks down alveolar walls, reducing surface area for gas exchange.
    C is wrong because anaemia is reduced oxygen transport by blood.
    D is wrong because scurvy is vitamin C deficiency.

19. Answer: A
    A is correct because inflamed bronchi and excess mucus cause persistent coughing.
    B is wrong because mucus and inflammation reduce ventilation, not increase oxygen diffusion.
    C is wrong because bronchitis does not strengthen alveolar walls.
    D is wrong because carbon monoxide removal is not improved.

20. Answer: A
    A is correct because emphysema reduces alveolar surface area.
    B is wrong because lung cancer does not increase healthy alveoli.
    C is wrong because bronchitis damages cilia and increases mucus.
    D is wrong because asthma does not digest the diaphragm.

21. Answer: A
    A is correct because limewater turns milky faster with expired air, showing expired air has more carbon dioxide.
    B is wrong because expired air still contains oxygen.
    C is wrong because expired air has more carbon dioxide than inspired air.
    D is wrong because limewater tests for carbon dioxide, not oxygen.

22. Answer: B
    A is wrong because oxygen decreases during respiration; hydrogencarbonate indicator is used for carbon dioxide changes.
    B is correct because respiring insects release carbon dioxide, making the indicator yellow.
    C is wrong because insects respire.
    D is wrong because blue-black is the iodine result for starch.

23. Answer: B
    A is wrong because seeds use oxygen; they do not produce oxygen in this set-up.
    B is correct because soda lime absorbs carbon dioxide produced by respiration.
    C is wrong because soda lime does not provide glucose.
    D is wrong because its key role here is carbon dioxide absorption.

24. Answer: A
    A is correct because seeds use oxygen and soda lime absorbs carbon dioxide, reducing total gas volume and pulling the bubble towards the seeds.
    B is wrong because carbon dioxide does not accumulate when soda lime is present.
    C is wrong because germinating seeds are not photosynthesising significantly in this set-up.
    D is wrong because nitrogen is not used in aerobic respiration.

25. Answer: A
    A is correct because boiled seeds of the same mass control for non-respiration factors while not respiring.
    B is wrong because more germinating seeds would increase respiration instead of acting as a control.
    C is wrong because green leaves in light may photosynthesise.
    D is wrong because yeast with sugar respires and is not a suitable non-respiring control.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

26. Answer: B
    Rate = distance ÷ time = 36 mm ÷ 9 min = 4 mm/min.
    A is too small.
    B is correct.
    C is wrong because it subtracts instead of dividing.
    D is wrong because it multiplies instead of dividing.

27. Answer: A
    Volume moved = distance × cross-sectional area = 50 mm × 0.2 mm² = 10 mm³.
    Rate = 10 mm³ ÷ 5 min = 2 mm³/min.
    A is correct.
    B, C and D are too large.

28. Answer: A
    Initial ventilation rate = 12 × 0.5 = 6 dm³/min.
    Final ventilation rate = 30 × 2.0 = 60 dm³/min.
    A is correct.
    B, C and D use the wrong calculation.

29. Answer: A
    A is correct because ventilation rate = breathing rate × tidal volume.
    B is wrong because heart rate × stroke volume gives cardiac output.
    C is wrong because gas concentration ratio is not ventilation rate.
    D is wrong because rib length is irrelevant. Tiny maths trap, big exam damage.

30. Answer: A
    A is correct because during vigorous exercise, oxygen supply may not meet muscle demand quickly enough.
    B is wrong because glucose is usually still available.
    C is wrong because mitochondria do not stop existing.
    D is wrong because carbon dioxide is not a fuel.

31. Answer: B
    A is wrong because yeast anaerobic respiration produces ethanol and carbon dioxide.
    B is correct because lactic acid can build up in muscles during anaerobic respiration.
    C is wrong because lactic acid is not produced in alveoli during gas exchange.
    D is wrong because glucose is the main product of photosynthesis.

32. Answer: A
    A is correct because oxygen debt is the extra oxygen needed after exercise to break down/remove lactic acid.
    B is wrong because oxygen is not stored permanently in the stomach.
    C is wrong because human muscles do not make ethanol.
    D is wrong because tar does not remove oxygen from blood; carbon monoxide reduces oxygen carrying.

33. Answer: C
    A is wrong because thick walls increase diffusion distance.
    B is wrong because poor blood supply would reduce the concentration gradient.
    C is correct because continuous blood flow removes oxygenated blood and maintains a steep gradient.
    D is wrong because gases must dissolve in moisture before diffusion.

34. Answer: A
    A is correct because the moist alveolar surface allows gases to dissolve before diffusing.
    B is wrong because lungs still lose water vapour during breathing.
    C is wrong because alveoli are not for trapping glucose.
    D is wrong because oxygen moves by diffusion, not osmosis.

35. Answer: B
    A is wrong because it reverses the meanings.
    B is correct because gas exchange is diffusion of gases, while breathing is ventilation of the lungs.
    C is wrong because respiration is energy release in cells.
    D is wrong because breathing and gas exchange are not inside mitochondria.

36. Answer: B
    A is wrong because oxygen moves from blood to respiring cells.
    B is correct because oxygen diffuses from blood to cells and carbon dioxide diffuses from cells to blood.
    C is wrong because carbon dioxide usually moves from cells to blood.
    D is wrong because oxygen usually moves from blood to cells.

37. Answer: B
    A is wrong because pulmonary vein carries oxygenated blood from lungs to heart.
    B is correct because pulmonary artery carries deoxygenated blood rich in carbon dioxide from the heart to lungs.
    C is wrong because the aorta carries oxygenated blood to the body.
    D is wrong because the coronary artery carries oxygenated blood to heart muscle.

38. Answer: B
    A is wrong because respiration is not the same as breathing.
    B is correct because respiration releases energy from food molecules in cells.
    C is wrong because respiration occurs in cells throughout the body, not only lungs.
    D is wrong because respiration occurs all the time.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

39. Answer: B
    A is wrong because anaerobic respiration can be useful when oxygen is limited.
    B is correct because anaerobic respiration releases some energy without oxygen for a short time.
    C is wrong because anaerobic respiration still produces ATP, just less than aerobic respiration.
    D is wrong because anaerobic respiration releases less energy than aerobic respiration.

40. Answer: A
    A is correct because carbon dioxide diffuses from blood into alveoli and is removed during exhalation.
    B is wrong because carbon dioxide is not actively transported from alveoli into blood for removal.
    C is wrong because osmosis involves water, not carbon dioxide.
    D is wrong because bile acts in the digestive system, not lungs.

41. Answer: B
    A is wrong because the cerebrum is not the main breathing control centre.
    B is correct because the medulla helps control breathing rate by responding to carbon dioxide levels/pH changes.
    C is wrong because the retina detects light.
    D is wrong because the pancreas controls blood glucose using hormones.

42. Answer: B
    A is wrong because increased carbon dioxide usually stimulates breathing.
    B is correct because higher carbon dioxide causes breathing rate and depth to increase.
    C is wrong because alveoli do not permanently close because carbon dioxide rises.
    D is wrong because muscle contraction is not completely inhibited.

43. Answer: A
    A is correct because carbon dioxide dissolves to form acidic conditions, lowering blood pH and stimulating breathing control centres.
    B is wrong because haemoglobin mainly carries oxygen; carbon dioxide is mostly transported in plasma as hydrogencarbonate ions.
    C is wrong because carbon dioxide does not become haemoglobin.
    D is wrong because ribs are not diffusion surfaces.

44. Answer: B
    A is wrong because nitrogen is not used in respiration.
    B is correct because nitrogen percentage changes very little between inhaled and exhaled air.
    C is wrong because exhaled air still contains nitrogen.
    D is wrong because nitrogen is not converted into lactic acid.

45. Answer: C
    A is wrong because thinner walls increase diffusion rate.
    B is wrong because larger surface area increases diffusion rate.
    C is correct because a smaller concentration gradient decreases diffusion rate.
    D is wrong because faster blood flow helps maintain the gradient.

46. Answer: A
    A is correct because mucus traps particles and cilia move mucus towards the throat.
    B is wrong because cilia do not trap particles mainly, and mucus does not pump oxygen.
    C is wrong because mucus does not digest oxygen or absorb glucose.
    D is wrong because cilia and mucus are in airways such as the trachea, not only alveolar capillaries.

47. Answer: A
    A is correct because fewer functioning alveoli reduce surface area for gas exchange.
    B is wrong because oxygen diffusion into blood would decrease.
    C is wrong because carbon dioxide removal would become less efficient.
    D is wrong because gas exchange mainly occurs in alveoli, not trachea.

48. Answer: B
    A is wrong because aerobic respiration needs oxygen; anaerobic does not.
    B is correct because aerobic respiration produces carbon dioxide and water, while anaerobic respiration in humans produces lactic acid.
    C is wrong because aerobic respiration releases more energy.
    D is wrong because aerobic respiration occurs in cells, especially mitochondria, not only lungs.

49. Answer: B
    A is wrong because lactic acid is produced in human muscle anaerobic respiration, not yeast.
    B is correct because yeast anaerobic respiration produces ethanol and carbon dioxide.
    C is wrong because yeast can respire anaerobically without oxygen.
    D is wrong because glucose, not carbon dioxide, is the fuel.

50. Answer: A
    A is correct because yeast in a sealed tube with glucose produces carbon dioxide during anaerobic respiration, turning limewater milky.
    B is wrong because yeast does not photosynthesise.
    C is wrong because yeast produces ethanol and carbon dioxide, not lactic acid only.
    D is wrong because limewater detects carbon dioxide, not ethanol.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.