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CBSE Class 7 Science Chapter 6 Respiration in Organisms — 20 MCQs with Answers

NCERT Class 7 Science Chapter 6 Respiration in Organisms explains how every cell in a living body obtains energy from food through respiration. Students often confuse breathing with cellular respiration or mix up aerobic and anaerobic pathways. Multiple-choice questions are a staple of CBSE periodic tests and final exams, testing both factual recall and application. This page presents 20 carefully designed MCQs — from straightforward definition-based items to higher-order thinking and assertion-reason formats — all grounded in NCERT textbook terminology and real exam patterns.

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Key takeaways

  • ✓Cellular respiration is the biochemical process that releases energy from glucose inside every living cell, not just breathing movements.
  • ✓Aerobic respiration requires oxygen and yields carbon dioxide, water and much more energy than anaerobic respiration.
  • ✓Anaerobic respiration in muscles produces lactic acid, while in yeast it yields ethanol and carbon dioxide, used in fermentation.
  • ✓Breathing (inhalation and exhalation) is the physical mechanism that brings oxygen in and expels carbon dioxide; it differs from cellular respiration.
  • ✓Fish use gills to extract dissolved oxygen from water, insects breathe through spiracles and tracheae, and earthworms respire through moist skin.
  • ✓Plants respire through stomata in leaves and lenticels in stems, taking in oxygen and releasing carbon dioxide all day and night.
  • ✓Regular MCQ practice with NCERT-aligned questions sharpens recall and teaches students to eliminate distractors efficiently in CBSE exams.

Understanding Cellular Respiration and Energy Release

Cellular respiration is the process by which living cells break down glucose in the presence (aerobic) or absence (anaerobic) of oxygen to release energy in the form of ATP. The NCERT textbook emphasises that respiration occurs continuously in every cell, whether the organism is at rest or active. Students must distinguish cellular respiration from breathing: breathing is the mechanical act of taking in air and expelling it, whereas respiration is the chemical breakdown of food inside cells. The chapter introduces the word equation for aerobic respiration: Glucose + Oxygen → Carbon dioxide + Water + Energy. Many exam questions test this equation, the role of mitochondria, and why energy release is essential for growth, movement and repair.
  • MCQ 1: Cellular respiration takes place in which part of the cell? (a) Nucleus (b) Cytoplasm and mitochondria (c) Cell wall (d) Vacuole | Answer: (b) Cytoplasm and mitochondria — Glycolysis occurs in cytoplasm, Krebs cycle in mitochondria.
  • MCQ 2: The main purpose of respiration in organisms is to (a) produce carbon dioxide (b) release energy from food (c) inhale oxygen only (d) store glucose. | Answer: (b) release energy from food — Energy in ATP drives all cellular activities.
  • MCQ 3: Which of the following is the correct word equation for aerobic respiration? (a) Glucose → Lactic acid + Energy (b) Glucose + Oxygen → Carbon dioxide + Water + Energy (c) Glucose → Ethanol + Carbon dioxide (d) Oxygen → Carbon dioxide + Water. | Answer: (b) Glucose + Oxygen → Carbon dioxide + Water + Energy — Standard NCERT equation for aerobic respiration.
  • MCQ 4: (Assertion) Respiration is different from breathing. (Reason) Breathing is a physical process; respiration is a biochemical process. (a) Both A and R true, R correct explanation (b) Both true, R not correct explanation (c) A true, R false (d) A false, R true. | Answer: (a) Both A and R true, R correct explanation — NCERT clearly differentiates the two terms.

Aerobic Respiration: Oxygen-Dependent Energy Production

Aerobic respiration occurs in the presence of oxygen and is the most efficient pathway, releasing about 38 ATP molecules per glucose molecule. The NCERT chapter explains that most organisms, including humans, plants and animals, rely on aerobic respiration for sustained energy. The process involves glycolysis (breakdown of glucose in cytoplasm) followed by the Krebs cycle and electron transport chain in mitochondria. Carbon dioxide and water are the by-products, expelled via exhalation in animals and through stomata or lenticels in plants. Exam questions often test the products, requirement of oxygen, and comparison with anaerobic respiration. Students should remember that aerobic respiration is continuous day and night in all living cells.
  • MCQ 5: In aerobic respiration, the end products are (a) Lactic acid and energy (b) Ethanol, carbon dioxide and energy (c) Carbon dioxide, water and energy (d) Only oxygen. | Answer: (c) Carbon dioxide, water and energy — These are expelled as waste after glucose oxidation.
  • MCQ 6: Aerobic respiration is more efficient than anaerobic because it (a) does not require glucose (b) produces much more ATP per glucose molecule (c) occurs only in plants (d) does not produce any waste. | Answer: (b) produces much more ATP per glucose molecule — Aerobic yields ~38 ATP, anaerobic only 2 ATP.
  • MCQ 7: During aerobic respiration in humans, oxygen is transported to cells by (a) Plasma (b) White blood cells (c) Haemoglobin in red blood cells (d) Platelets. | Answer: (c) Haemoglobin in red blood cells — Haemoglobin binds oxygen in lungs, releases it in tissues.
  • MCQ 8: Which organelle is called the powerhouse of the cell because aerobic respiration occurs there? (a) Ribosome (b) Chloroplast (c) Mitochondrion (d) Golgi body. | Answer: (c) Mitochondrion — The Krebs cycle and electron transport chain occur in mitochondrial membranes.

Anaerobic Respiration: Energy Without Oxygen

Anaerobic respiration occurs when oxygen is absent or insufficient. In human muscle cells during vigorous exercise, glucose is incompletely broken down into lactic acid, releasing only 2 ATP per glucose and causing muscle cramps if lactic acid accumulates. Yeast and some bacteria perform alcoholic fermentation, converting glucose into ethanol and carbon dioxide — a process exploited in baking and brewing. The NCERT chapter stresses that anaerobic respiration is less efficient and short-term. Students often encounter MCQs comparing aerobic and anaerobic pathways, identifying products, or explaining why athletes feel fatigued after intense sprints. Remember, anaerobic respiration is a survival adaptation, not the preferred energy route.
  • MCQ 9: In human muscles during heavy exercise, anaerobic respiration produces (a) Ethanol (b) Carbon dioxide only (c) Lactic acid (d) Oxygen. | Answer: (c) Lactic acid — Accumulation of lactic acid causes cramps and soreness.
  • MCQ 10: Yeast cells undergo anaerobic respiration and produce (a) Lactic acid and water (b) Ethanol and carbon dioxide (c) Only oxygen (d) Carbon dioxide and water. | Answer: (b) Ethanol and carbon dioxide — This is alcoholic fermentation, used in bread-making and brewing.
  • MCQ 11: Compared to aerobic respiration, anaerobic respiration (a) releases more energy (b) releases less energy (c) requires more oxygen (d) produces no waste. | Answer: (b) releases less energy — Only 2 ATP per glucose versus ~38 in aerobic.
  • MCQ 12: (HOTS) A student runs a 100 m sprint. Immediately after, his muscles ache. The reason is (a) Excess oxygen in muscles (b) Lactic acid buildup from anaerobic respiration (c) Too much glucose (d) Lack of haemoglobin. | Answer: (b) Lactic acid buildup from anaerobic respiration — Oxygen supply cannot meet demand, so muscles switch to anaerobic mode.

Breathing Mechanism in Humans

Breathing, or ventilation, is the physical process of inhaling oxygen-rich air into the lungs and exhaling carbon dioxide-rich air. The NCERT chapter describes the role of the diaphragm and rib muscles: when the diaphragm contracts and moves down, the chest cavity expands, air pressure drops, and air rushes in (inhalation). When the diaphragm relaxes and moves up, the chest cavity shrinks, pressure rises, and air is pushed out (exhalation). Oxygen diffuses from alveoli into blood capillaries, and carbon dioxide diffuses the opposite way. Understanding this mechanism helps students answer MCQs on the sequence of events, role of diaphragm, and gas exchange. Breathing rate increases during exercise to supply more oxygen for aerobic respiration in muscles.
  • MCQ 13: During inhalation, the diaphragm (a) Moves upward (b) Moves downward and contracts (c) Remains stationary (d) Expands sideways only. | Answer: (b) Moves downward and contracts — This increases chest cavity volume, reducing air pressure so air flows in.
  • MCQ 14: Exchange of gases (oxygen and carbon dioxide) in lungs occurs in tiny air sacs called (a) Bronchi (b) Trachea (c) Alveoli (d) Larynx. | Answer: (c) Alveoli — Thin-walled alveoli have rich blood supply for efficient diffusion.
  • MCQ 15: When we exhale, the air we breathe out is rich in (a) Oxygen (b) Carbon dioxide (c) Nitrogen only (d) Hydrogen. | Answer: (b) Carbon dioxide — Cells produce CO₂ during respiration; blood carries it to lungs for removal.
  • MCQ 16: (Application) During vigorous exercise, our breathing rate increases because (a) Lungs produce more oxygen (b) Muscles need more oxygen for aerobic respiration (c) Diaphragm stops working (d) We inhale carbon dioxide. | Answer: (b) Muscles need more oxygen for aerobic respiration — Increased demand triggers faster, deeper breaths.

Respiration in Plants: Stomata, Lenticels and Gas Exchange

Plants respire continuously, taking in oxygen and releasing carbon dioxide through stomata in leaves and lenticels in stems. Unlike photosynthesis, which occurs only during the day, respiration happens 24 hours a day in all living plant cells. The NCERT chapter clarifies that during the day, the rate of photosynthesis exceeds respiration, so net gas exchange appears to be oxygen release. At night, without photosynthesis, plants only respire, releasing carbon dioxide. Roots respire using oxygen present in air spaces between soil particles. If soil is waterlogged, roots may suffocate due to lack of oxygen. MCQs often test the difference between photosynthesis and respiration, the role of stomata and lenticels, and why it is incorrect to say plants do not respire.
  • MCQ 17: In plants, exchange of gases for respiration occurs through (a) Roots only (b) Stomata and lenticels (c) Flowers (d) Xylem vessels. | Answer: (b) Stomata and lenticels — Stomata are on leaves; lenticels are on woody stems.
  • MCQ 18: Respiration in plants takes place (a) Only at night (b) Only during the day (c) Throughout day and night (d) Only in flowers. | Answer: (c) Throughout day and night — All living cells respire continuously to release energy.
  • MCQ 19: Why do roots need air spaces in soil? (a) For photosynthesis (b) To absorb water (c) To get oxygen for respiration (d) To store food. | Answer: (c) To get oxygen for respiration — Roots are non-green and depend on oxygen in soil air.
  • MCQ 20: (Assertion) Plants produce oxygen during the day. (Reason) Photosynthesis occurs during the day, masking the oxygen consumption by respiration. (a) Both A and R true, R correct explanation (b) Both true, R not explanation (c) A true, R false (d) Both false. | Answer: (a) Both A and R true, R correct explanation — Net oxygen release during day is because photosynthesis rate > respiration rate.

Respiration in Aquatic Animals: Gills and Dissolved Oxygen

Fish and many aquatic animals extract dissolved oxygen from water using gills. The NCERT chapter describes gills as feathery, blood-rich structures that provide a large surface area for gas exchange. Water enters through the mouth, flows over the gill filaments where oxygen diffuses into blood and carbon dioxide diffuses out, then exits via gill slits. Unlike lungs that take oxygen from air, gills are adapted for extracting the small amount of oxygen dissolved in water. If water is polluted or warm (warm water holds less dissolved oxygen), fish may suffocate. MCQs test the structure and function of gills, the concept of dissolved oxygen, and why fish die when taken out of water (gills collapse in air and cannot extract oxygen efficiently).

Respiration in Insects: Spiracles and Tracheal System

Insects like grasshoppers, cockroaches and bees breathe through a network of tiny tubes called tracheae, which open to the outside via small pores called spiracles on the body surface. The NCERT chapter explains that air enters spiracles, travels through branching tracheae directly to cells, and oxygen diffuses into tissues without relying on blood for transport. Carbon dioxide exits the same way. This system is efficient for small organisms but limits insect size because diffusion alone cannot supply oxygen over very long distances. Exam questions often ask students to identify spiracles, compare insect respiration with human or fish respiration, and explain why insects have no lungs or gills.

Respiration in Earthworms and Amphibians: Skin Breathing

Earthworms respire through their moist skin. Oxygen dissolved in the moisture layer on the skin diffuses into blood vessels just below the surface, and carbon dioxide diffuses out. If the skin dries, the earthworm suffocates, which is why earthworms emerge during rain when soil is waterlogged and they risk drowning. Frogs, as amphibians, use lungs on land and also respire through moist skin underwater or when hibernating. The NCERT chapter highlights that moist skin is essential for gas exchange — dry skin blocks diffusion. MCQs may ask why earthworms die in sunlight (skin dries) or how frogs can stay underwater for extended periods (cutaneous respiration).

Common Misconceptions and Exam Traps in Respiration MCQs

Students frequently confuse breathing with respiration, assume plants do not respire, or think aerobic and anaerobic respiration occur in different organisms rather than under different conditions. Another trap is believing that photosynthesis and respiration are opposite processes — they are complementary but not opposites; respiration occurs in all living cells, photosynthesis only in green cells with chlorophyll. Assertion-reason MCQs exploit these gaps. A common error is choosing 'Both true but reason not the correct explanation' when the reason actually does explain the assertion. Practising a variety of MCQ formats — definition-based, application, HOTS, assertion-reason — trains students to read questions carefully, eliminate obviously wrong options first, and check if the NCERT textbook uses the exact term in the option.

Smart Strategies for Attempting MCQs in CBSE Science Papers

First, read the question stem completely before looking at options; underline keywords like 'not', 'except', 'most efficient'. Eliminate clearly incorrect options to improve odds. If two options seem correct, re-read the question for specific qualifiers such as 'in humans' or 'during anaerobic respiration'. In assertion-reason items, evaluate the assertion independently, then the reason, then their logical connection — many students lose marks by not checking if the reason truly explains the assertion even when both statements are factually correct. Manage time: CBSE papers allocate roughly one minute per MCQ, so do not spend three minutes on a single tough question. Mark it, move on, return later. For NCERT-based MCQs, exact textbook language often appears in the correct option — trust the book. Finally, avoid second-guessing: change an answer only if you spot a clear mistake.
  • Read every word in the question; note keywords like 'not', 'all', 'only', 'except' that reverse meaning.
  • Eliminate obviously wrong options first — even guessing between two choices gives 50 per cent odds.
  • In assertion-reason MCQs, check: (i) Is assertion true? (ii) Is reason true? (iii) Does reason explain assertion?
  • Do not spend more than 60-90 seconds per MCQ; mark difficult ones and revisit after finishing easier questions.
  • Revise NCERT definitions and labelled diagrams — exact terms and labels often become correct options in board exams.

How CBSETUTOR.ai Sharpens MCQ Skills with Instant Photo-Solve

Many Class 7 students lose marks not because they lack knowledge but because MCQ patterns — especially assertion-reason, application-based and HOTS — demand rapid analysis and elimination skills that traditional coaching rarely practises in depth. CBSETUTOR.ai offers every CBSE student from Class 6 to Class 12 a 24×7 AI tutor at a flat ₹999 per month, covering all subjects. Simply photograph any MCQ from your worksheet or textbook, upload it, and receive a step-by-step explanation: why the correct option is right, why each distractor is wrong, and which NCERT concept underpins the question. The AI highlights common traps and suggests memory hooks. For Chapter 6 Respiration in Organisms, you can upload sets of practice MCQs and get instant feedback, building speed and accuracy before periodic tests. A three-day free trial lets you explore the photo-solve feature, browse video solutions, and experience doubt-clearing without waiting for the next tuition class. Consistent MCQ practice with immediate AI feedback transforms guesswork into confident, reasoned elimination — exactly what CBSE board exams reward.

Frequently asked questions

What is the difference between breathing and respiration in Class 7 Science?+
Breathing is the physical act of inhaling oxygen and exhaling carbon dioxide using lungs and diaphragm. Respiration is the biochemical process inside cells that breaks down glucose to release energy, occurring continuously in every living cell. Breathing supplies the oxygen needed for cellular respiration.
Why is aerobic respiration more efficient than anaerobic respiration?+
Aerobic respiration uses oxygen to completely oxidise glucose, yielding about 38 ATP molecules per glucose. Anaerobic respiration occurs without oxygen, incompletely breaks down glucose into lactic acid or ethanol, and produces only 2 ATP per glucose — far less energy.
Do plants respire at night or only during the day?+
Plants respire 24 hours a day, taking in oxygen and releasing carbon dioxide. During the day, photosynthesis masks respiration because the plant produces more oxygen than it consumes. At night, without photosynthesis, only respiration occurs, so the plant releases carbon dioxide.
What causes muscle cramps during intense exercise?+
During vigorous activity, muscles may not receive enough oxygen for aerobic respiration. They switch to anaerobic respiration, which produces lactic acid. Accumulation of lactic acid in muscle tissue causes cramps, soreness and fatigue until the body clears it after exercise.
How do fish breathe underwater?+
Fish use gills to extract dissolved oxygen from water. Water flows over gill filaments, oxygen diffuses into blood vessels in the gills, and carbon dioxide diffuses out. Gills provide a large surface area for efficient gas exchange in an aquatic environment.
Why do earthworms come out of the soil when it rains heavily?+
Earthworms breathe through their moist skin, requiring oxygen from air spaces in soil. Heavy rain fills these spaces with water, reducing available oxygen and risking drowning. Earthworms surface to access atmospheric oxygen until soil drainage improves.
What are spiracles and which organisms have them?+
Spiracles are tiny pores on the body surface of insects like grasshoppers and cockroaches. They open into a network of tubes called tracheae that deliver oxygen directly to cells, bypassing the need for lungs or blood-based oxygen transport.
Is photosynthesis the opposite of respiration?+
No, photosynthesis and respiration are complementary, not opposites. Photosynthesis captures light energy to build glucose from carbon dioxide and water, releasing oxygen. Respiration breaks down glucose to release stored energy, consuming oxygen and producing carbon dioxide. Both occur in plants, in different organelles, serving different purposes.
How many MCQs typically appear from Chapter 6 in CBSE Class 7 Science exams?+
CBSE periodic tests and final exams for Class 7 Science usually include 3 to 5 MCQs from each chapter. Chapter 6 Respiration in Organisms often features questions on aerobic versus anaerobic respiration, breathing mechanism, and respiration in different organisms.
How can CBSETUTOR.ai help me score better in MCQs?+
CBSETUTOR.ai lets you photograph any MCQ and instantly receive a step-by-step explanation: why the correct answer is right, why distractors are wrong, and the NCERT concept behind the question. Regular practice with instant feedback sharpens elimination skills and builds exam confidence. A flat ₹999/month covers all subjects for Classes 6-12, with a three-day free trial.

Related resources

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