Q80. Which of the following properties of alveoli is/are helpful in gaseous exchange during respiration in human beings?
I. Very thin epithelial lining.
II. Elastic fibres in the alveolar wall.
III. Many surrounding blood capillaries.
Select the correct answer.
[UPSC NDA-II (13 September 2026)]
(a) I and II only
(b) I and III only
(c) III only
(d) I, II and III
Answer: (D) I, II and III
Explanation: The alveolar epithelial layer is extremely thin, reducing diffusion distance. Numerous surrounding capillaries maintain concentration gradients for oxygen and carbon dioxide. Elastic fibres permit stretching and recoil during ventilation. Together these three characteristics support efficient lung gas exchange.
Q79. In which of the following parts of organisms does gaseous exchange during respiration NOT take place?
[UPSC NDA-II (13 September 2026)]
(a) Skin of earthworms
(b) Alveoli of humans
(c) Gills of Pila
(d) Bronchi of humans
Answer: (D) Bronchi of humans
Explanation: Earthworms exchange gases across moist skin, aquatic snails such as Pila use gills, and human alveoli exchange oxygen and carbon dioxide with blood. Bronchi mainly conduct air towards the respiratory region; their walls are not the principal gas-exchange surface.
Q78. Which one of the following respiratory volumes of the human lung is the lowest?
[UPSC NDA (2026, GAT)]
(a) Tidal volume
(b) Residual volume
(c) Inspiratory reserve volume
(d) Expiratory reserve volume
Answer: (A) Tidal volume
Explanation: Tidal volume is the air moved during quiet breathing, generally about 500 mL in an average adult. Residual volume remains after forced expiration and is typically over a litre. Inspiratory and expiratory reserve volumes also generally exceed resting tidal volume.
Q77. Which valve guards the opening between the right ventricle and the pulmonary artery?
[UPSC NDA (2026, GAT)]
(a) Semilunar valve
(b) Bicuspid valve
(c) Mitral valve
(d) Tricuspid valve
Answer: (A) Semilunar valve
Explanation: The pulmonary semilunar valve stands at the exit of the right ventricle into the pulmonary trunk. It prevents blood flowing backward during ventricular relaxation. The tricuspid valve lies between the right atrium and ventricle; the mitral valve is on the left.
Q76. What is the process called in which organisms take in oxygen and use it to break down food to release energy for cellular activities?
[RRB Group D (6 August 2026, Shift 1)]
(a) Photosynthesis
(b) Transpiration
(c) Respiration
(d) Fermentation
Answer: (C) Respiration
Explanation: Cellular respiration releases usable energy from organic nutrients such as glucose and conserves part of it as ATP. In aerobic respiration, oxygen participates as the final electron acceptor. Photosynthesis makes carbohydrates, while transpiration is water loss from plant surfaces.
Q75. Which process releases energy in plants through the breakdown of glucose?
[RRB Group D (3 August 2026, Shift 1)]
(a) Excretion
(b) Respiration
(c) Transpiration
(d) Photosynthesis
Answer: (B) Respiration
Explanation: Plants perform cellular respiration continuously in living cells, using carbohydrate molecules to generate ATP. Photosynthesis stores light energy in glucose and normally requires light, whereas respiration can occur day and night. The two processes have distinct purposes and reactions.
Q74. Which respiratory process occurs without oxygen and releases relatively little energy?
[RRB Group D (10 August 2026, Shift 1)]
(a) Transpiration
(b) Aerobic respiration
(c) Anaerobic respiration
(d) Photosynthesis
Answer: (C) Anaerobic respiration
Explanation: Anaerobic energy pathways do not require molecular oxygen. Fermentation follows glycolysis and regenerates NAD+, allowing limited ATP formation: typically two ATP per glucose from glycolysis. Aerobic respiration supports much greater ATP production through electron transport and oxidative phosphorylation.
Q73. Compared with anaerobic respiration, the energy released during aerobic respiration is generally ______.
[RRB Group D (20 August 2026, Shift 3)]
(a) Slower
(b) Less
(c) Greater
(d) Equal
Answer: (C) Greater
Explanation: With oxygen available, cells can oxidise glucose more completely to carbon dioxide and water. The electron transport chain powers synthesis of considerably more ATP than glycolysis followed by fermentation. Actual ATP yield depends on the organism and cellular conditions.
Q72. Which cell organelle is the major site of aerobic respiration in eukaryotic cells?
[RRB Group D (9 August 2026, Shift 2)]
(a) Ribosome
(b) Mitochondria
(c) Nucleus
(d) Endoplasmic reticulum
Answer: (B) Mitochondria
Explanation: Mitochondria contain the enzymes of the citric acid cycle in their matrix and the electron transport complexes in their inner membrane. Glycolysis occurs separately in the cytosol. This division explains why mitochondria are described as the main ATP-producing organelles.
Q71. Which of the following processes does NOT consume atmospheric oxygen?
[RRB Group D (5 August 2026, Shift 2)]
(a) Photosynthesis by green plants
(b) Respiration by animals
(c) Combustion of fuels
(d) Respiration by plants
Answer: (A) Photosynthesis by green plants
Explanation: Oxygenic photosynthesis uses carbon dioxide and water, captures light energy, and produces oxygen rather than consuming it as a reactant. By contrast, aerobic respiration in plants and animals consumes oxygen. Combustion also ordinarily requires an oxidising agent such as atmospheric oxygen.
Q70. Which gas is essential for aerobic respiration in animals?
[RRB Group D (4 August 2026, Shift 3)]
(a) Hydrogen
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen
Answer: (B) Oxygen
Explanation: Oxygen acts as the terminal electron acceptor in mitochondrial electron transport, combining with electrons and hydrogen ions to form water. Without adequate oxygen, oxidative phosphorylation slows or stops. Cells may temporarily rely more heavily on glycolysis to obtain limited ATP.
Q69. If oxygen supply to muscles becomes inadequate during strenuous activity, what may happen to pyruvate?
[RRB Group D (10 August 2026, Shift 3)]
(a) It becomes ethanol and carbon dioxide
(b) It is fully oxidised aerobically to carbon dioxide and water
(c) It is converted into lactic acid by an anaerobic pathway
(d) It changes directly into ATP without by-products
Answer: (C) It is converted into lactic acid by an anaerobic pathway
Explanation: When oxygen delivery cannot meet muscle energy demand, glycolytic pyruvate can be reduced to lactate. This regenerates NAD+ needed to sustain glycolysis. Human muscle cells do not normally convert pyruvate into ethanol, a product associated with yeast fermentation.
Q68. Which human organ is most directly affected by inhaling tobacco smoke?
[RRB Group D (14 August 2026, Shift 1)]
(a) Pancreas
(b) Brain
(c) Intestine
(d) Lungs
Answer: (D) Lungs
Explanation: Inhaled smoke first enters the respiratory passages and lungs, exposing airway cells and alveoli to particulates and toxic chemicals. Persistent exposure increases the risk of chronic obstructive pulmonary disease, lung cancer and impaired gas exchange, although smoking damages other organs too.
Q67. Why does carbon monoxide from tobacco smoke impair the transport of oxygen in blood?
[RRB Group D (21 August 2026, Shift 1)]
(a) It destroys every red blood cell immediately
(b) It binds strongly to haemoglobin, reducing oxygen carriage
(c) It increases oxygen binding to platelets
(d) It converts oxygen into nitrogen
Answer: (B) It binds strongly to haemoglobin, reducing oxygen carriage
Explanation: Carbon monoxide binds haemoglobin with far greater affinity than oxygen, forming carboxyhaemoglobin. This occupies oxygen-binding sites and hinders oxygen delivery to tissues. Carbon monoxide can additionally make haemoglobin release its remaining oxygen less readily, worsening cellular oxygen deprivation.
Q66. Which tiny balloon-like structures are present at the ends of the smallest air passages in human lungs?
[RRB Group D (11 August 2026, Shift 2)]
(a) Bronchi
(b) Villi
(c) Bronchioles
(d) Alveoli
Answer: (D) Alveoli
Explanation: Alveoli are minute air sacs grouped around alveolar ducts. Their extensive surface area, very thin walls and adjacent capillaries allow rapid diffusion of oxygen into blood and carbon dioxide out of blood. Villi are absorptive projections of the small intestine.
Q65. Which three-carbon molecule is formed during the initial breakdown of glucose in respiration?
[RRB Group D (18 August 2026, Shift 3)]
(a) Fructose-6-phosphate
(b) Acetyl coenzyme A
(c) Pyruvate
(d) Glucose-6-phosphate
Answer: (C) Pyruvate
Explanation: Glycolysis converts one six-carbon glucose molecule into two three-carbon pyruvate molecules in the cytosol. The pathway produces a net gain of two ATP and reduces NAD+ to NADH. Pyruvate then enters aerobic oxidation or anaerobic fermentation pathways.
Q64. Which statement about respiration is INCORRECT?
[RRB Group D (6 August 2026, Shift 3)]
(a) Yeast can produce ethanol and carbon dioxide
(b) Yeast produces water as the principal end-product of alcoholic fermentation
(c) The first stage of glucose breakdown occurs in cytoplasm
(d) Anaerobic respiration can occur without oxygen
Answer: (B) Yeast produces water as the principal end-product of alcoholic fermentation
Explanation: Alcoholic fermentation in yeast chiefly yields ethanol and carbon dioxide from pyruvate, while regenerating NAD+ for glycolysis. Water is not the defining major product of this pathway. Glycolysis occurs in cytoplasm and does not itself require molecular oxygen.
Q63. What happens to gases as blood passes through capillaries around pulmonary alveoli?
[RRB Group D (9 August 2026, Shift 3)]
(a) Both oxygen and carbon dioxide enter blood from alveoli
(b) Carbon dioxide enters blood and oxygen enters alveoli
(c) No exchange occurs
(d) Oxygen enters blood while carbon dioxide enters alveoli
Answer: (D) Oxygen enters blood while carbon dioxide enters alveoli
Explanation: Oxygen diffuses from alveolar air across the respiratory membrane into capillary blood because of partial-pressure differences. Carbon dioxide diffuses in the opposite direction and is exhaled. Effective exchange also depends on adequate alveolar ventilation and pulmonary blood flow.
Q62. Which of the following does NOT normally occur during inhalation?
[RRB Group D (12 August 2026, Shift 2)]
(a) Blood releases oxygen to the alveolar air
(b) Air enters the lungs
(c) The thoracic cavity expands
(d) The diaphragm moves downward and becomes flatter
Answer: (A) Blood releases oxygen to the alveolar air
Explanation: During inspiration, diaphragm contraction enlarges the thoracic cavity and lowers intrapulmonary pressure, drawing air into lungs. At the alveoli, oxygen normally enters pulmonary blood from inhaled air rather than being released by blood into that air.
Q61. Which pigment of human blood carries oxygen?
[RRB Group D (4 August 2026, Shift 3)]
(a) Myosin
(b) Haemoglobin
(c) Insulin
(d) Chlorophyll
Answer: (B) Haemoglobin
Explanation: Haemoglobin is an iron-containing protein concentrated inside red blood cells. Its haem groups reversibly bind oxygen in the lungs and release it at tissues. Myosin is a muscle protein, insulin is a pancreatic hormone, and chlorophyll is a photosynthetic pigment.
Q60. Consider statements about aerobic respiration:
1. Glycolysis occurs in mitochondria.
2. Krebs cycle occurs in the mitochondrial matrix.
3. Oxygen is the final electron acceptor.
Which are correct?
[SSC CGL Tier I (17 September 2025, Shift 1)]
(a) Only 1 and 2
(b) Only 2 and 3
(c) Only 1 and 3
(d) All 1, 2 and 3
Answer: (B) Only 2 and 3
Explanation: Statement 1 is false because glycolysis takes place in the cytosol. The Krebs cycle operates in the mitochondrial matrix, and molecular oxygen receives electrons at the end of the respiratory chain, forming water. Therefore only statements 2 and 3 are correct.
Q59. Which structures in the upper respiratory tract help remove inhaled dust and other particles?
[SSC CHSL (29 November 2025, Shift 3)]
(a) Alveoli
(b) Bronchi
(c) Pleura
(d) Cilia
Answer: (D) Cilia
Explanation: Cilia are microscopic hair-like projections on respiratory epithelial cells. Their coordinated beating moves mucus, containing trapped dust and microorganisms, towards the throat for removal. This mucociliary clearance mechanism protects lower airways, although alveolar macrophages provide additional defence deeper in lungs.
Q58. What is the process called by which organisms take oxygen from outside and break down food to release energy?
[SSC CHSL (13 November 2025, Shift 2)]
(a) Respiration
(b) Digestion
(c) Photosynthesis
(d) Fermentation
Answer: (A) Respiration
Explanation: The question describes aerobic respiration, in which food molecules are oxidised to release chemical energy. Digestion breaks complex food into simpler substances but does not itself represent cellular energy production. Photosynthesis builds sugars, whereas fermentation proceeds without oxygen as a reactant.
Q57. In which blood cells is haemoglobin present?
[SSC CHSL (29 November 2025, Shift 1)]
(a) Red blood cells
(b) Platelets
(c) Monocytes
(d) Nerve cells
Answer: (A) Red blood cells
Explanation: Mature human red blood cells contain abundant haemoglobin for oxygen transport. Haemoglobin gives blood its characteristic red colour through iron-containing haem groups. Platelets assist clotting, monocytes are white blood cells involved in defence, and neurons transmit electrical signals.
Q56. Which medium carries carbon dioxide produced in body tissues towards the lungs?
[SSC CHSL (17 November 2025, Shift 2)]
(a) Sweat
(b) Mucus
(c) Blood
(d) Lymph only
Answer: (C) Blood
Explanation: Blood transports carbon dioxide from tissues to lungs, predominantly as bicarbonate ions in plasma and partly attached to haemoglobin or dissolved. Carbonic anhydrase in red cells rapidly assists bicarbonate formation. In lung capillaries these reactions reverse, permitting carbon dioxide exhalation.
Q55. Consider the statements:
I. Haemoglobin is the respiratory pigment of humans.
II. Haemoglobin has very low affinity for oxygen.
Which is correct?
[SSC CHSL (12 November 2025, Shift 3)]
(a) Only I
(b) Only II
(c) Both I and II
(d) Neither I nor II
Answer: (A) Only I
Explanation: Haemoglobin is the main oxygen-binding respiratory pigment in human red blood cells, so statement I is correct. It binds oxygen readily at the high oxygen partial pressure of lung capillaries and releases it more easily in oxygen-poor tissues; statement II is incorrect.
Q54. How many carbon atoms are present in a pyruvate molecule?
[UPSC CDS-I (13 April 2025)]
(a) Two
(b) Three
(c) Four
(d) Five
Answer: (B) Three
Explanation: Pyruvate is a three-carbon compound produced by glycolysis. One six-carbon glucose yields two pyruvate molecules, preserving all six original carbon atoms at this stage. Under aerobic conditions, pyruvate is further oxidised through acetyl-CoA formation and the citric acid cycle.
Q53. The amount of air breathed in and out during normal quiet respiration by human lungs is referred to as what?
[UPSC NDA-II (14 September 2025)]
(a) Vital capacity
(b) Tidal volume
(c) Inspiratory reserve volume
(d) Total lung capacity
Answer: (B) Tidal volume
Explanation: Tidal volume represents the amount of air moved with each ordinary resting breath, often estimated as roughly 500 mL in a typical adult. Vital capacity is the maximal exhalable volume after full inspiration, while total lung capacity includes residual air.
Q52. Which instrument is used to measure pulmonary volumes and capacities?
[RRB NTPC Undergraduate (20 August 2025, Shift 2)]
(a) Respirometer
(b) Sphygmomanometer
(c) Spirometer
(d) Barometer
Answer: (C) Spirometer
Explanation: A spirometer records the volume of air moving into and out of the lungs, allowing assessment of tidal volume and various capacities. A simple spirometer cannot directly measure residual volume. Sphygmomanometers measure blood pressure, while barometers measure atmospheric pressure.
Q51. Which statements about animal respiration are correct?
1. Dolphins and whales breathe through blowholes.
2. Earthworms breathe through moist skin.
[UPPSC PCS Prelims (2025)]
(a) Neither statement
(b) Only statement 1
(c) Only statement 2
(d) Both statements
Answer: (D) Both statements
Explanation: Whales and dolphins are mammals that take atmospheric air through blowholes on the head, not through gills. Earthworms rely on diffusion across a moist, vascularised skin surface. Oxygen dissolves in skin moisture before passing into underlying blood vessels.
Q50. In which cellular organelle does the Krebs cycle occur?
[SSC MTS (30 October 2024, Shift 3)]
(a) Lysosome
(b) Ribosome
(c) Nucleus
(d) Mitochondria
Answer: (D) Mitochondria
Explanation: In eukaryotic cells, the citric acid or Krebs cycle occurs within the mitochondrial matrix. Its reactions oxidise acetyl groups, release carbon dioxide, and generate NADH and FADH2. These reduced carriers subsequently deliver electrons to the mitochondrial respiratory chain.
Q49. Which virus is most commonly responsible for the common cold?
[SSC MTS (4 November 2024, Shift 2)]
(a) Coronavirus
(b) Hepatitis A virus
(c) Rhinovirus
(d) Influenza virus
Answer: (C) Rhinovirus
Explanation: Rhinoviruses are major causes of common cold, typically producing a runny nose, sneezing and sore throat. Some coronaviruses also cause colds, but rhinovirus is the classic commonest answer. Influenza causes a different respiratory illness, often with more prominent fever and body aches.
Q48. At which time of day is dissolved oxygen in a pond usually lowest?
[SSC MTS (15 October 2024, Shift 1)]
(a) At dawn
(b) During afternoon
(c) At midnight
(d) In the evening
Answer: (A) At dawn
Explanation: Aquatic plants and algae photosynthesise during daylight and add oxygen to water. At night, photosynthesis stops but respiration by plants, animals and microorganisms continues to consume oxygen. Consequently, dissolved oxygen often reaches its daily minimum shortly before sunrise.
Q47. What commonly happens to dissolved oxygen in a water body when thermal pollution raises its temperature?
[SSC MTS (28 October 2024, Shift 1)]
(a) Dissolved oxygen decreases
(b) Water turbidity must become zero
(c) pH always becomes exactly neutral
(d) Dissolved oxygen increases without limit
Answer: (A) Dissolved oxygen decreases
Explanation: Warm water can hold less dissolved oxygen than cool water at similar conditions. Higher temperatures can also accelerate metabolic rates and oxygen consumption by aquatic organisms. The combined effects may stress fish and other animals that depend on dissolved oxygen for respiration.
Q46. Consider the statements:
I. Pranayama may help improve breathing efficiency and lung function.
II. During inhalation, the ribs move upwards and the diaphragm moves downwards.
Which statement(s) are correct?
[SSC CHSL (16 March 2023, Shift 3)]
(a) Both I and II
(b) Only II
(c) Neither I nor II
(d) Only I
Answer: (A) Both I and II
Explanation: Breathing exercises such as pranayama can help improve respiratory control and some measures of pulmonary function. During normal inspiration, external intercostal muscles raise the ribs while contraction of the diaphragm moves it downward, increasing thoracic volume and drawing air into lungs.
Q45. Which of the following is NOT a three-carbon compound formed or involved in glycolysis?
[SSC MTS (11 September 2023, reported shift question)]
(a) Pyruvic acid
(b) Dihydroxyacetone phosphate
(c) 2-Phosphoglycerate
(d) Fructose-6-phosphate
Answer: (D) Fructose-6-phosphate
Explanation: Fructose-6-phosphate retains six carbon atoms and appears in the early phase of glycolysis. Dihydroxyacetone phosphate and 2-phosphoglycerate are three-carbon intermediates, while pyruvate is a three-carbon final product. Glycolysis splits six-carbon sugar intermediates into two three-carbon streams.
Q44. In which blood component is the respiratory pigment haemoglobin found?
[RRB Group D (11 October 2022, Shift 2)]
(a) White blood cells
(b) Red blood cells
(c) Platelets
(d) Plasma
Answer: (B) Red blood cells
Explanation: Red blood cells are specialised for oxygen transport because their cytoplasm contains large amounts of haemoglobin. Each haemoglobin molecule has four haem groups capable of reversible oxygen binding. Blood plasma transports most carbon dioxide as bicarbonate but contains little normal free haemoglobin.
Q43. What is the air remaining in the lungs even after a forceful expiration called?
[RRB Group D (5 September 2022, Shift 2)]
(a) Residual volume
(b) Respiration volume
(c) Pulmonary volume
(d) Alveolar volume
Answer: (A) Residual volume
Explanation: Residual volume is the gas that remains in the lungs after maximum forced exhalation. It prevents complete lung collapse and allows ongoing gas exchange between breaths. It cannot be directly measured by simple spirometry because no airflow exits during its measurement.
Q42. The respiration reaction in humans is an example of which type of chemical reaction?
[RRB Group D (28 September 2022, Shift 1)]
(a) Exothermic reaction
(b) Displacement reaction
(c) Endothermic reaction
(d) Precipitation reaction
Answer: (A) Exothermic reaction
Explanation: Overall oxidation of glucose during cellular respiration releases energy, making respiration an exothermic process. Cells capture some released energy as ATP rather than releasing it all at once as heat. Displacement and precipitation describe different chemical reaction categories.
Q41. Which of the following animals generally has the fastest breathing rate?
[RRB Group D (8 September 2022, Shift 1)]
(a) Fish
(b) Human
(c) Elephant
(d) Pigeon
Answer: (A) Fish
Explanation: Fish often move water over their gills rapidly because water holds far less available oxygen per unit volume than air. Their ventilatory movements can be frequent, though rates vary greatly with species, size, temperature and activity. The examination compares typical textbook examples.
Q40. Which of the following is the approximate resting respiratory rate of an adult human?
[RRB Group D (12 September 2022, Shift 1)]
(a) 72–75 breaths/minute
(b) 70–72 breaths/minute
(c) 12–15 breaths/minute
(d) 15–18 breaths/minute
Answer: (D) 15–18 breaths/minute
Explanation: The paper keys about 15–18 breaths per minute as the typical resting rate. Clinical references commonly give a broader adult range of about 12–20 per minute, varying with age and circumstances. A resting respiratory rate must not be confused with pulse rate.
Q39. Which of the following statements is INCORRECT?
[RRB Group D (14 September 2022, Shift 2)]
(a) Conversion of pyruvate into lactate may occur in muscle cells
(b) Carbon dioxide is more soluble in water than oxygen
(c) Alveoli are surrounded by blood vessels
(d) Respiratory pigment is present in human white blood cells
Answer: (D) Respiratory pigment is present in human white blood cells
Explanation: Haemoglobin, the principal respiratory pigment of humans, occurs inside red blood cells rather than white blood cells. The other statements describe familiar respiratory facts: capillary networks surround alveoli, carbon dioxide dissolves relatively readily, and muscles can produce lactate from pyruvate.
Q38. Which structure helps prevent the collapse of the human trachea?
[RRB Group D (6 September 2022, Shift 1)]
(a) Tendons
(b) Ligaments
(c) Cartilage rings
(d) Skeletal muscles
Answer: (C) Cartilage rings
Explanation: The tracheal wall is supported by incomplete C-shaped rings of hyaline cartilage that maintain an open airway during pressure changes. Their open posterior portion permits expansion of the neighbouring oesophagus during swallowing. Mucus and cilia protect the airway but do not provide rigid support.
Q37. Which statement correctly describes aerobic breakdown of one pyruvate molecule?
[RRB Group D (12 September 2022, Shift 1)]
(a) It proceeds without oxygen being required by the respiratory chain
(b) It produces less ATP than fermentation
(c) Its three carbon atoms can ultimately be released as three CO2 molecules
(d) It forms only carbon dioxide and no other products
Answer: (C) Its three carbon atoms can ultimately be released as three CO2 molecules
Explanation: Pyruvate contains three carbon atoms. Conversion to acetyl-CoA releases one carbon dioxide; oxidation of the resulting two-carbon acetyl group through the citric acid cycle releases two more. Aerobic oxidation also generates reduced coenzymes and ATP-related energy, not only carbon dioxide.
Q36. Which gas is required for aerobic respiration?
[RRB Group D (30 August 2022, Shift 3)]
(a) Carbon dioxide
(b) Hydrogen
(c) Oxygen
(d) Nitrogen
Answer: (C) Oxygen
Explanation: Oxygen receives electrons at the terminal stage of aerobic electron transport. Its reduction to water helps maintain the flow of electrons through the respiratory complexes, which drive ATP synthesis. Carbon dioxide is mainly a waste product of complete glucose oxidation.
Q35. Which organism is commonly used for alcoholic fermentation?
[RRB Group D (16 September 2022, Shift 3)]
(a) Paramecium
(b) Euglena
(c) Amoeba
(d) Yeast
Answer: (D) Yeast
Explanation: Yeasts such as Saccharomyces cerevisiae convert sugars into ethanol and carbon dioxide when fermentative conditions prevail. This process regenerates NAD+ for continued glycolysis and is exploited in brewing and bread making. Paramecium, Euglena and Amoeba are protists, not the usual industrial fermenters.
Q34. Why do the lungs contain numerous alveoli?
[RRB Group D (7 October 2022, Shift 3)]
(a) To increase the surface area for gas exchange
(b) To pump blood through the heart
(c) To increase the number of ribs
(d) To give rigid mechanical support to the body
Answer: (A) To increase the surface area for gas exchange
Explanation: The very large combined surface area of millions of alveoli allows efficient diffusion of oxygen and carbon dioxide. Their walls are exceptionally thin and closely associated with pulmonary capillaries. Alveoli facilitate gas exchange; the heart is responsible for pumping blood.
Q33. Which process is represented by glucose + oxygen → carbon dioxide + water + energy?
[RRB Group D (1 September 2022, Shift 3)]
(a) Metabolism of minerals
(b) Digestion alone
(c) Aerobic respiration
(d) Translocation
Answer: (C) Aerobic respiration
Explanation: This is the overall simplified equation for aerobic cellular respiration. Glucose is oxidised, oxygen is reduced, and released energy is partly captured in ATP. The pathway actually involves many enzyme-controlled steps, including glycolysis, the citric acid cycle and oxidative phosphorylation.
Q32. Where does the conversion of glucose into pyruvate take place?
[RRB Group D (24 August 2022, Shift 3)]
(a) Mitochondria
(b) Cytoplasm
(c) Nucleus
(d) Ribosome
Answer: (B) Cytoplasm
Explanation: Glycolysis occurs in the cytosol of both prokaryotic and eukaryotic cells. It transforms glucose into two pyruvate molecules, producing a net two ATP and two NADH per glucose. Later aerobic stages proceed in mitochondria in eukaryotic organisms.
Q31. Which substance is a common initial product in both aerobic and anaerobic breakdown of glucose?
[RRB Group D (23 August 2022, Shift 2)]
(a) Water
(b) Pyruvate
(c) Lactate
(d) Ethanol
Answer: (B) Pyruvate
Explanation: Both aerobic respiration and common fermentation pathways begin with glycolysis and therefore produce pyruvate from glucose. With oxygen available, pyruvate enters further oxidation. Under fermentative conditions, cells convert pyruvate or its derivatives to products such as lactate or ethanol.
Q30. Conversion of pyruvate into ethanol and carbon dioxide is associated with which process?
[RRB Group D (18 September 2022, Shift 3)]
(a) Diffusion
(b) Translocation
(c) Fermentation
(d) Transpiration
Answer: (C) Fermentation
Explanation: In alcoholic fermentation, pyruvate first loses carbon dioxide to form acetaldehyde, which is then reduced to ethanol. This restores NAD+ so glycolysis can continue under oxygen-limited conditions. Yeasts commonly perform the process, producing bubbles of carbon dioxide in bread dough.
Q29. Where does anaerobic glucose breakdown to lactate take place in human cells?
[RRB Group D (30 September 2022, Shift 2)]
(a) Only in striated muscle tissue
(b) In the cytoplasm
(c) Inside mitochondria
(d) Only inside veins
Answer: (B) In the cytoplasm
Explanation: Glycolysis and conversion of pyruvate into lactate occur in the cytosol, not in mitochondrial compartments. Lactate formation regenerates NAD+, which supports glycolytic ATP production. This pathway is used by red blood cells and may increase markedly in active muscles.
Q28. Which statement about alveoli is INCORRECT?
[RRB Group D (22 August 2022, Shift 3)]
(a) They are surrounded by extensive blood capillaries
(b) Their walls are very thin
(c) They collectively provide a large gas-exchange area
(d) They are finger-like projections of the small intestine
Answer: (D) They are finger-like projections of the small intestine
Explanation: Alveoli are microscopic lung air sacs, not the finger-like intestinal villi. Their thin epithelium, expansive collective surface and close contact with blood capillaries optimise respiratory gas diffusion. Villi instead increase the absorptive surface area of the small intestine.
Q27. During intense running, what happens when muscles receive inadequate oxygen relative to their demand?
[RRB Group D (29 August 2022, Shift 3)]
(a) Pyruvate is converted into ethanol in human muscle
(b) Pyruvate is converted into lactate only when oxygen is abundant
(c) Pyruvate is converted directly into glucose in each muscle cell
(d) Pyruvate can be converted into lactate
Answer: (D) Pyruvate can be converted into lactate
Explanation: Rapid glycolysis in strongly exercising muscle may produce lactate when energy demand exceeds the available oxidative capacity. Lactate formation permits NAD+ regeneration. Although older textbooks link lactate directly to muscle cramps, exercise-associated cramps have more complex causes and are not simply lactate accumulation.
Q26. Which substance is a principal product of alcoholic fermentation besides carbon dioxide and a small ATP yield?
[RRB Group D (11 October 2022, Shift 1)]
(a) Pyruvate
(b) Water
(c) Lactic acid
(d) Ethanol
Answer: (D) Ethanol
Explanation: Yeast fermentation converts glucose-derived pyruvate into ethanol and carbon dioxide after glycolysis. The pathway allows NAD+ recycling rather than adding further ATP beyond glycolysis. Ethanol formation distinguishes alcoholic fermentation from lactic acid fermentation, which predominantly generates lactate.
Q25. Why does keeping fruits in cold storage generally increase their shelf life?
[UPPSC PCS Prelims (2021)]
(a) It completely prevents sunlight from reaching them
(b) It necessarily increases carbon dioxide production
(c) It slows their rate of respiration
(d) It always increases moisture in fruits
Answer: (C) It slows their rate of respiration
Explanation: Low temperatures reduce the rates of enzyme-controlled metabolic reactions, including fruit respiration. Slower respiration consumes stored food reserves less quickly and delays ripening and senescence. Temperature must be controlled carefully, however, because some tropical fruits develop chilling injury.
Q24. Complete the analogy: Tongue : Taste :: Lungs : ______.
[RRB NTPC (3 February 2021)]
(a) Cognition
(b) Circulation
(c) Perspiration
(d) Respiration
Answer: (D) Respiration
Explanation: The tongue contains taste receptors involved in gustation, while lungs are organs of breathing and pulmonary gas exchange. They allow oxygen to enter the bloodstream and carbon dioxide to leave it. Cellular respiration subsequently uses oxygen within tissues to generate ATP.
Q23. Complete the analogy: Kidney : Urine :: Lungs : ______.
[RRB NTPC (13 March 2021)]
(a) Cough
(b) Carbon dioxide
(c) Breathe
(d) Oxygen
Answer: (B) Carbon dioxide
Explanation: Kidneys eliminate nitrogenous wastes in urine, whereas lungs remove carbon dioxide produced by cellular metabolism. Carbon dioxide travels from tissues to lungs mainly in the blood as bicarbonate. Exhalation also removes water vapour, an additional component of expired air.
Q22. The trachea belongs to which human body system?
[RRB NTPC (31 July 2021)]
(a) Excretory system
(b) Circulatory system
(c) Endocrine system
(d) Respiratory system
Answer: (D) Respiratory system
Explanation: The trachea, or windpipe, is a conducting airway connecting the larynx with the two main bronchi. Rings of cartilage keep it patent, while mucus and cilia help trap and remove inhaled particles. It is part of the respiratory system.
Q21. Insects generally take in air through which structures?
[RRB NTPC (4 March 2021)]
(a) Lungs
(b) Gills
(c) Spiracles
(d) Moist skin
Answer: (C) Spiracles
Explanation: Insects possess external respiratory openings called spiracles. These connect to branching air-filled tracheae and tracheoles, which deliver oxygen toward body tissues. The system reduces reliance on blood for oxygen carriage and is distinct from vertebrate lungs and aquatic gills.
Q20. In human blood, oxygen is chiefly bound to which component?
[WBCS Preliminary Examination (2020)]
(a) Red blood cells
(b) White blood cells
(c) Vitamin B12
(d) Vitamin E
Answer: (A) Red blood cells
Explanation: Most oxygen in arterial blood is reversibly attached to haemoglobin within red blood cells. A smaller amount travels physically dissolved in plasma. The haemoglobin-oxygen interaction allows a much higher oxygen-carrying capacity than dissolved oxygen alone could provide.
Q19. Which among the following fermentation processes is aerobic?
[RPSC Second Grade Teacher (18 February 2019)]
(a) Lactic acid fermentation
(b) Glycolysis
(c) Alcoholic fermentation
(d) Acetic acid fermentation
Answer: (D) Acetic acid fermentation
Explanation: Industrial acetic acid fermentation by acetic acid bacteria requires oxygen to oxidise ethanol into acetic acid. Despite the traditional word fermentation, the conversion is aerobic. Alcoholic and lactic fermentation regenerate NAD+ without using oxygen as the terminal electron acceptor.
Q18. ATP is produced in aerobic respiration through which phosphorylation mechanisms?
[RPSC Second Grade Teacher (18 February 2019)]
(a) Oxidative phosphorylation only
(b) Both oxidative and substrate-level phosphorylation
(c) Substrate-level phosphorylation only
(d) Oxidative and photophosphorylation
Answer: (B) Both oxidative and substrate-level phosphorylation
Explanation: Glycolysis and the citric acid cycle generate small amounts of ATP or GTP by substrate-level phosphorylation. The majority of ATP in aerobic eukaryotic respiration normally arises through oxidative phosphorylation, where the respiratory electron transport chain establishes a proton gradient.
Q17. Which set correctly describes major products of the tricarboxylic acid cycle?
[RPSC Second Grade Teacher (4 July 2019)]
(a) Carbon dioxide and water only
(b) Carbon dioxide and NADH only
(c) NADH and FADH2 only
(d) Carbon dioxide, NADH and FADH2
Answer: (D) Carbon dioxide, NADH and FADH2
Explanation: Each turn of the citric acid cycle releases two carbon dioxide molecules and generates reduced electron carriers, including NADH and FADH2. It also produces one GTP or ATP equivalent. These carriers supply electrons for the later oxidative phosphorylation stage.
Q16. Complete the analogy: Heart : Circulation :: Lungs : ______.
[SSC CHSL (8 July 2019)]
(a) Ribs
(b) Oxygen
(c) Excretion
(d) Respiration
Answer: (D) Respiration
Explanation: The heart pumps blood through pulmonary and systemic circuits, making it central to circulation. The lungs ventilate and exchange respiratory gases, replenishing blood oxygen and eliminating carbon dioxide. The analogy matches each organ to its primary associated physiological system or function.
Q15. Which substance has traditionally been associated with muscle fatigue after intense exercise?
[WBCS Preliminary Examination (2018)]
(a) Carbon dioxide
(b) Creatinine
(c) Lactic acid
(d) Ethyl alcohol
Answer: (C) Lactic acid
Explanation: This traditional exam answer is lactic acid, referring to elevated lactate during vigorous glycolysis. Current exercise science distinguishes lactate production from the complex causes of fatigue and muscle cramping. Lactate also serves as a useful fuel and metabolic intermediate.
Q14. About how much oxygen can 100 mL of normal human blood carry?
[WBCS Preliminary Examination (2018)]
(a) 40 mL
(b) 10 mL
(c) 20 mL
(d) 30 mL
Answer: (C) 20 mL
Explanation: Normal whole blood typically carries roughly 20 mL oxygen per 100 mL when haemoglobin is nearly fully saturated. This follows from around 15 grams of haemoglobin per decilitre and an oxygen capacity of about 1.34 mL per gram.
Q13. What is the main respiratory organ of a whale?
[WBCS Preliminary Examination (2018)]
(a) Integument
(b) Gills
(c) Lungs
(d) Trachea alone
Answer: (C) Lungs
Explanation: Whales are mammals, not fish, and breathe atmospheric air using lungs. They surface periodically to inhale and exhale through blowholes. Their specialised diving physiology permits prolonged breath-holding, but they cannot extract dissolved oxygen from seawater using gills.
Q12. Alveoli are tiny balloon-like structures found in which organ?
[SSC CHSL (10 March 2018)]
(a) Lungs
(b) Kidneys
(c) Liver
(d) Heart
Answer: (A) Lungs
Explanation: Alveoli are terminal pulmonary air sacs where oxygen crosses into blood and carbon dioxide leaves it. They are lined by thin epithelial cells and closely surrounded by capillaries. Surfactant helps prevent alveolar collapse by reducing surface tension within the sacs.
Q11. In human muscle cells, pyruvate may be converted into lactic acid when which substance is insufficient?
[SSC CHSL (6 March 2018)]
(a) Water
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen
Answer: (B) Oxygen
Explanation: When oxygen availability or oxidative capacity is inadequate relative to metabolic demand, cells may reduce pyruvate to lactate. This reaction regenerates NAD+ and allows glycolysis to continue producing a small amount of ATP without depending directly on oxygen.
Q10. Which of the following is NOT a part of the breathing mechanism?
[UPSC NDA-I (22 April 2018)]
(a) Bronchi
(b) Bowman’s capsule
(c) Diaphragm
(d) Trachea
Answer: (B) Bowman’s capsule
Explanation: Bowman’s capsule is part of a kidney nephron, surrounding the glomerulus where blood filtration begins. Bronchi and trachea conduct air, and the diaphragm is the primary inspiratory muscle. Thus only Bowman’s capsule belongs to the urinary rather than respiratory system.
Q9. What are the overall end-products of aerobic respiration?
[RPSC (1 July 2017)]
(a) Carbon dioxide, water and energy
(b) Carbon dioxide and energy only
(c) Sugar, energy and water
(d) Oxygen, carbon dioxide and water
Answer: (A) Carbon dioxide, water and energy
Explanation: Complete aerobic oxidation of glucose produces carbon dioxide and water and releases energy, much of which cells capture as ATP. Carbon dioxide arises primarily during pyruvate oxidation and the citric acid cycle; oxygen is consumed at the electron transport chain.
Q8. Which pathway is common to aerobic respiration and alcoholic fermentation?
[RPSC (1 July 2017)]
(a) Fermentation itself
(b) Oxidative phosphorylation
(c) Glycolysis
(d) Citric acid cycle
Answer: (C) Glycolysis
Explanation: Glycolysis is the shared cytosolic pathway that converts glucose into pyruvate. With oxygen, pyruvate undergoes further oxidation and the respiratory electron transport chain can produce additional ATP. Under alcoholic fermentation, pyruvate is converted into ethanol and carbon dioxide.
Q7. Which organelle is chiefly associated with aerobic cellular respiration?
[UPPSC Staff Nurse Examination (December 2017)]
(a) Glyoxysomes
(b) Ribosomes
(c) Lysosomes
(d) Mitochondria
Answer: (D) Mitochondria
Explanation: Mitochondria perform pyruvate oxidation, the Krebs cycle and oxidative phosphorylation in aerobic eukaryotic cells. Glycolysis nevertheless occurs in the cytoplasm, so not every respiratory reaction is mitochondrial. Ribosomes synthesise proteins, while lysosomes chiefly participate in intracellular digestion.
Q6. Mammals respire by which principal organs?
[MPPSC Preliminary Examination (2016)]
(a) Gills
(b) Trachea alone
(c) Skin
(d) Lungs
Answer: (D) Lungs
Explanation: Mammals exchange respiratory gases in their lungs, where extensive alveolar surfaces support diffusion between air and blood. They ventilate the lungs using breathing muscles, notably the diaphragm. Gills are typical of fish and some aquatic invertebrates, not mammals.
Q5. Bacteria that can survive and grow both in the presence and absence of free oxygen are known as what?
[RPSC Assistant Engineer Examination (2016)]
(a) Facultative anaerobes
(b) Obligate aerobes
(c) Obligate anaerobes
(d) Non-pathogenic bacteria
Answer: (A) Facultative anaerobes
Explanation: Facultative anaerobes can use oxygen-dependent respiration when oxygen is available but switch to anaerobic energy pathways when it is absent. Obligate aerobes require oxygen, while obligate anaerobes grow without it and may be harmed by exposure to oxygen.
Q4. Which processes add carbon dioxide to Earth’s carbon cycle?
1. Volcanic action
2. Respiration
3. Photosynthesis
4. Decay of organic matter.
[UPSC Civil Services Preliminary Examination (2014)]
(a) 1 and 3 only
(b) 2 only
(c) 1, 2 and 4 only
(d) 1, 2, 3 and 4
Answer: (C) 1, 2 and 4 only
Explanation: Respiration releases carbon dioxide as organic carbon is oxidised; decomposers also release it during breakdown of dead material, and volcanic activity emits geological carbon dioxide. Photosynthesis instead usually removes carbon dioxide from the atmosphere by fixing it into organic compounds.
Q3. Birds differ from bats in the absence of which structure?
[WBCS Preliminary Examination (2013)]
(a) Warm blood
(b) Four-chambered heart
(c) Trachea
(d) Diaphragm
Answer: (D) Diaphragm
Explanation: Birds lack a muscular diaphragm and ventilate relatively rigid lungs using movements of the rib cage and air sacs. Bats are mammals and use a diaphragm for breathing. Both groups are warm-blooded vertebrates with a trachea and four-chambered heart.
Q2. Consider: 1. Photosynthesis 2. Respiration 3. Decay of organic matter 4. Volcanic action. Which add carbon dioxide to Earth’s carbon cycle?
[UPSC Civil Services Preliminary Examination (2011)]
(a) 1 and 4 only
(b) 2 and 3 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4
Answer: (C) 2, 3 and 4 only
Explanation: Carbon dioxide enters the atmosphere through cellular respiration, biological decomposition and volcanic emissions. Green plant photosynthesis fixes atmospheric carbon dioxide into carbohydrates instead. The question tests the direction of carbon transfer, not whether a process involves carbon molecules.
Q1. The proportion of which air component remains almost unchanged between inhalation and exhalation during ordinary human respiration?
[RAS/RTS Preliminary Examination (2003)]
(a) Carbon dioxide
(b) Oxygen
(c) Water vapour
(d) Nitrogen
Answer: (D) Nitrogen
Explanation: Inhaled and exhaled air differ mainly in oxygen, carbon dioxide and moisture. Nitrogen is largely physiologically inert under normal breathing conditions and undergoes negligible net exchange, so its proportion stays nearly the same. Small percentage variations may reflect changes in other gases.