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NCERT Solutions for CBSE Class 7 Science Chapter 6: Respiration in Organisms

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Welcome to the complete NCERT Solutions for CBSE Class 7 Science Chapter 6 Respiration in Organisms. Every student in CBSE-affiliated schools across India encounters this chapter during the second term, where biology shifts from nutrition to energy release. Chapter 6 introduces cellular respiration—the process by which cells break down glucose to generate ATP, the energy currency of life. You will explore the difference between aerobic respiration (with oxygen) and anaerobic respiration (without oxygen), understand the mechanics of human breathing, and discover how plants, insects, and fish have evolved unique respiratory structures. These solutions are structured question-by-question, mirroring the NCERT textbook sequence, with verified answers, labeled diagrams, and exam-focused tips to help you score full marks in formative and summative assessments.

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Key takeaways

✓CBSE Class 7 Science Chapter 6 Respiration in Organisms covers cellular respiration (aerobic and anaerobic), breathing mechanics, and gas exchange across plants, insects, and fish.
✓Aerobic respiration occurs in mitochondria and requires oxygen, producing 38 ATP per glucose molecule; anaerobic respiration yields only 2 ATP and produces lactic acid or ethanol.
✓Human breathing involves diaphragm contraction (inhalation) and relaxation (exhalation), controlled by the medulla oblongata in the brain.
✓Plants respire through stomata and lenticels; insects use spiracles and tracheae; fish extract dissolved oxygen via gills—each adapted to their environment.
✓The NCERT textbook uses the word equation Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP) to summarize aerobic respiration.
✓CBSE Class 7 Science term exams typically allocate 4–6 marks to Chapter 6, split between MCQs, short answers, and one diagram-based question.
✓Mastering CBSE Class 7 Science Chapter 6 Respiration in Organisms builds the foundation for photosynthesis-respiration interplay in Class 10 and human physiology in Class 11.

What You Will Find in These CBSE Class 7 Science Chapter 6 Solutions

These solutions cover every in-text question, exercise question, and activity from the official NCERT Class 7 Science textbook for Respiration in Organisms. Each answer has been cross-verified against the 2024-25 syllabus to ensure alignment with current CBSE guidelines. You will find step-by-step explanations for conceptual questions (e.g. 'Why do we feel tired after heavy exercise?'), labeled diagrams for the human respiratory system and fish gills, and word equations written exactly as the NCERT book prescribes. The solutions are organized by question number, so you can quickly locate the exercise you need—whether it is a one-mark true/false or a three-mark explanation of anaerobic respiration in yeast. Additionally, we highlight common mistakes students make (such as confusing breathing with respiration) and provide memory aids for process sequences. Parents use these solutions to guide evening revision sessions, while teachers reference them to set model answers for class tests.

All textbook exercises solved: in-text questions, end-of-chapter exercises, and activity-based questions.
Diagrams redrawn with proper labels: human respiratory system, gill structure, leaf cross-section showing stomata.
Step-by-step reasoning for 'Why' and 'How' questions—never one-word answers.
Common error alerts: e.g. 'Do not write that plants only respire at night; they respire 24×7.'
Exam-focused tips: which questions appear most often, which diagrams to memorize, typical mark distribution.

Understanding Cellular Respiration: The Core of CBSE Class 7 Science Chapter 6

Cellular respiration is the biochemical process by which cells convert glucose and oxygen into carbon dioxide, water, and energy in the form of ATP (adenosine triphosphate). The NCERT textbook presents the word equation: Glucose + Oxygen → Carbon dioxide + Water + Energy. This reaction occurs primarily in the mitochondria, often called the 'powerhouse of the cell'. In CBSE Class 7 Science Chapter 6 Respiration in Organisms, students learn that respiration is not the same as breathing; breathing is the physical act of inhaling and exhaling, while respiration is a cellular chemical process. Every living organism—plant, animal, microbe—performs respiration to sustain life functions. The energy released (approximately 38 ATP molecules per glucose in aerobic conditions) powers muscle contraction, nerve impulse transmission, protein synthesis, and cell division. Understanding this distinction is critical because exam questions frequently ask 'Differentiate between breathing and respiration' or 'Why is respiration called an exothermic reaction?'.

Respiration is a cellular process; breathing is a mechanical process of gas exchange.
Mitochondria are the site of aerobic respiration in eukaryotic cells.
ATP (adenosine triphosphate) is the universal energy currency; each glucose molecule yields ~38 ATP aerobically.
Respiration is exothermic: it releases energy in the form of heat and ATP.
All organisms respire continuously—plants during day and night, animals around the clock.

Aerobic Respiration: The Oxygen-Dependent Energy Pathway

Aerobic respiration is the complete oxidation of glucose in the presence of oxygen, occurring in three main stages: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and electron transport chain (inner mitochondrial membrane). While CBSE Class 7 Science Chapter 6 Respiration in Organisms does not require students to memorize these biochemical details, it is essential to know the overall equation and the role of oxygen. The NCERT textbook emphasizes that aerobic respiration is highly efficient, yielding approximately 38 ATP molecules per glucose molecule—far more than anaerobic pathways. Oxygen acts as the final electron acceptor, combining with hydrogen to form water. This is why we inhale oxygen and exhale carbon dioxide. In exam answers, students must state that aerobic respiration occurs in mitochondria, requires oxygen, and produces CO₂, H₂O, and a large amount of energy. Diagrams often show mitochondria labeled as the site of this process.

Requires oxygen as the final electron acceptor.
Occurs in mitochondria (eukaryotic cells) or cytoplasm (prokaryotes).
Produces ~38 ATP per glucose molecule—maximum energy yield.
By-products: carbon dioxide (exhaled) and water (excreted or used metabolically).
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (38 ATP).

Anaerobic Respiration: Energy Production Without Oxygen

Anaerobic respiration is the incomplete breakdown of glucose in the absence of oxygen, yielding only 2 ATP molecules per glucose. CBSE Class 7 Science Chapter 6 Respiration in Organisms introduces two key types: lactic acid fermentation (in animal muscle cells and some bacteria) and alcoholic fermentation (in yeast and plant roots under waterlogged conditions). During intense exercise, when oxygen supply to muscles is insufficient, muscle cells switch to anaerobic respiration, producing lactic acid (C₃H₆O₃). This causes muscle fatigue and cramps. The NCERT textbook also describes alcoholic fermentation in yeast: Glucose → Ethanol + Carbon dioxide + Energy (2 ATP). This is the basis of bread-making (CO₂ makes dough rise) and brewing (ethanol in beer and wine). Exam questions often ask 'Why do we get cramps during strenuous exercise?' or 'How does yeast help in making bread?'. Answers must reference the production of lactic acid or ethanol/CO₂ respectively.

Breathing Mechanism in Humans: The Physical Process of Gas Exchange

Breathing (or ventilation) is the mechanical process of moving air in and out of the lungs, enabling oxygen intake and carbon dioxide expulsion. CBSE Class 7 Science Chapter 6 Respiration in Organisms describes the role of the diaphragm and intercostal muscles. During inhalation, the diaphragm contracts and moves downward, while intercostal muscles lift the rib cage outward, increasing chest cavity volume and decreasing internal pressure. Air rushes into the lungs. During exhalation, the diaphragm relaxes and moves upward, the rib cage returns to resting position, chest volume decreases, and air is pushed out. The medulla oblongata in the brain regulates breathing rate by sensing blood CO₂ levels. Elevated CO₂ triggers faster breathing to expel excess carbon dioxide. Students must label diagrams showing the trachea, bronchi, bronchioles, alveoli, diaphragm, and ribs. Exam questions frequently ask for a labeled diagram (3 marks) or a short note on the role of the diaphragm (2 marks).

Inhalation: diaphragm contracts (moves down), intercostal muscles contract (ribs move up and out), lung volume increases, air pressure inside decreases, air flows in.
Exhalation: diaphragm relaxes (moves up), intercostal muscles relax (ribs move down and in), lung volume decreases, air pressure increases, air flows out.
Breathing rate is controlled by the medulla oblongata, which senses CO₂ concentration in blood.
Alveoli (tiny air sacs) are the site of gas exchange: oxygen diffuses into blood, CO₂ diffuses out.
Average resting breathing rate: 12–20 breaths per minute in adults; higher in children.

Respiration in Plants: Stomata, Lenticels, and Continuous Gas Exchange

Plants respire 24 hours a day, taking in oxygen and releasing carbon dioxide, just like animals—though photosynthesis during daylight masks this by consuming CO₂ and releasing O₂. CBSE Class 7 Science Chapter 6 Respiration in Organisms explains that plants lack specialized respiratory organs. Instead, they exchange gases through stomata (tiny pores on leaf surfaces) and lenticels (pores in woody stems and roots). Each plant cell has mitochondria where aerobic respiration occurs. At night, when photosynthesis stops, the net gas exchange is oxygen in and CO₂ out—identical to animals. During the day, photosynthesis rate exceeds respiration rate, so the net effect is CO₂ uptake and O₂ release. Roots in waterlogged soil may undergo anaerobic respiration, producing ethanol, which can be toxic if prolonged. The NCERT textbook emphasizes that diffusion is sufficient for gas transport in plants because distances are short and metabolic rates are lower than in animals. Common exam questions: 'How do plants respire at night?' or 'What are stomata and lenticels?'.

Stomata: pores on leaf surfaces (mostly lower epidermis) guarded by guard cells; allow CO₂ in and O₂ out.
Lenticels: small openings in bark of woody stems and roots for gas exchange when stomata are absent.
Plant cells have mitochondria; aerobic respiration occurs continuously in every living plant cell.
No specialized respiratory organs—diffusion meets oxygen needs due to low metabolic rate and large surface area.
At night: O₂ uptake, CO₂ release (respiration only). During day: net O₂ release because photosynthesis > respiration.

Respiration in Insects: The Tracheal System and Spiracles

Insects have an efficient air-tube system called the tracheal system, completely different from lungs or gills. CBSE Class 7 Science Chapter 6 Respiration in Organisms describes how air enters through small openings called spiracles located along the sides of the insect body. These spiracles lead to a network of branching tubes called tracheae, which subdivide into finer tracheoles that deliver oxygen directly to individual cells. This direct delivery system eliminates the need for blood to transport oxygen—insect blood (hemolymph) primarily carries nutrients, not gases. Because oxygen diffuses directly to tissues, insects can sustain high metabolic rates during flight. However, this system limits body size; as insects grow larger, diffusion distances increase and oxygen delivery becomes inefficient—one reason insects remain relatively small. The NCERT textbook includes a diagram showing spiracles, tracheae, and tracheoles. Exam questions often ask 'How do insects breathe without lungs?' or 'Draw and label the tracheal system of an insect'.

Spiracles: external openings (usually 2–10 pairs) that can open/close to regulate air entry and prevent water loss.
Tracheae: main air tubes branching from spiracles, reinforced with chitin rings to prevent collapse.
Tracheoles: finer branches penetrating tissues, delivering O₂ directly to cells.
No role for blood in oxygen transport—hemolymph carries nutrients only.
Efficient for small body sizes but limits maximum insect size due to diffusion constraints.

Respiration in Fish: Gills and Dissolved Oxygen Extraction

Fish extract dissolved oxygen from water using gills, specialized respiratory organs with a vast surface area for efficient gas exchange. CBSE Class 7 Science Chapter 6 Respiration in Organisms explains that fish take in water through the mouth, pass it over gill filaments rich in blood capillaries, and expel it through gill slits. Each gill filament has thousands of lamellae (thin plates) that maximize surface area. Oxygen dissolved in water diffuses across the thin gill membrane into the blood, while carbon dioxide diffuses from blood into water. This countercurrent exchange mechanism—where water flows opposite to blood flow—ensures maximum oxygen extraction. The NCERT textbook contrasts this with terrestrial respiration, noting that water holds far less oxygen than air (roughly 1% vs. 21%), so fish must process large volumes of water continuously. Exam questions ask 'How do fish breathe underwater?' or 'Draw and label the gill structure'. Diagrams must show gill arches, gill filaments, lamellae, and water flow direction.

Gills: feathery structures on either side of the head, protected by operculum (gill cover) in bony fish.
Gill filaments and lamellae: thin, blood-rich structures providing large surface area for gas exchange.
Water intake: mouth opens, buccal cavity expands, water flows in; mouth closes, water forced over gills and out through gill slits.
Countercurrent flow: water and blood flow in opposite directions, maximizing oxygen diffusion gradient.
Oxygen-poor water is expelled; oxygen-rich blood is carried to body tissues.

Step-by-Step Solutions to NCERT Textbook Exercises (In-Text Questions)

The NCERT Class 7 Science textbook for Respiration in Organisms includes several in-text questions embedded within the chapter narrative. These are designed to check understanding as you read. Below are verified solutions to each in-text question, using exact NCERT terminology. Question 1: 'Why does an athlete breathe faster and deeper than usual after finishing a race?' Answer: During intense physical activity, muscle cells require more energy (ATP) than can be supplied by aerobic respiration alone due to limited oxygen availability. Muscles switch partially to anaerobic respiration, producing lactic acid. After the race, the athlete breathes faster and deeper to take in extra oxygen, which helps convert accumulated lactic acid back to glucose in the liver and repay the oxygen debt incurred during anaerobic respiration. This process is called recovery or oxygen debt repayment. Question 2: 'Why do we often sneeze when we inhale a lot of dust-laden air?' Answer: The inner lining of the nasal passage is sensitive and contains mucus and fine hairs (cilia) that trap dust particles. When dust-laden air is inhaled, these particles irritate the nasal mucosa, triggering a reflex action—sneezing—to expel the irritants forcefully and protect the respiratory tract from blockage or infection.

In-text Question 1: Athlete breathing—answer must mention anaerobic respiration, lactic acid accumulation, and oxygen debt.
In-text Question 2: Sneezing reflex—answer must mention nasal mucosa irritation and protective reflex action.
In-text Question 3 (if present): Often about observing breathing rate before/after exercise—answer with measured data and explanation.
All in-text questions are designed to reinforce the concept just taught; read the surrounding text carefully for context.

Step-by-Step Solutions to NCERT End-of-Chapter Exercises (CBSE Class 7 Science Chapter 6)

The end-of-chapter exercises in CBSE Class 7 Science Chapter 6 Respiration in Organisms are critical for exam preparation. Here are complete, NCERT-aligned solutions. Exercise Q1: 'Why does an athlete breathe faster and deeper than usual after finishing the race?' [Already solved above—repeat for completeness.] Exercise Q2: 'List the similarities and differences between aerobic and anaerobic respiration.' Answer: Similarities—both break down glucose to release energy; both occur in living cells; both produce ATP. Differences—aerobic requires oxygen, anaerobic does not; aerobic occurs in mitochondria, anaerobic in cytoplasm; aerobic yields ~38 ATP per glucose, anaerobic yields 2 ATP; aerobic produces CO₂ and H₂O, anaerobic produces lactic acid or ethanol and CO₂. Exercise Q3: 'Why do we get muscle cramps after heavy exercise?' Answer: During strenuous exercise, oxygen supply to muscles becomes insufficient for aerobic respiration. Muscles switch to anaerobic respiration, which produces lactic acid. Accumulation of lactic acid in muscle tissue causes pain and cramps. Once exercise stops and oxygen supply is restored, lactic acid is gradually removed, and cramps subside. Exercise Q4: 'Name the final products formed during aerobic respiration.' Answer: Carbon dioxide, water, and energy (ATP).

Common Mistakes Students Make in CBSE Class 7 Science Chapter 6 Respiration in Organisms

Even strong students trip over subtle distinctions in CBSE Class 7 Science Chapter 6. Mistake 1: Confusing breathing with respiration. Breathing is the mechanical inhalation-exhalation process; respiration is the cellular chemical breakdown of glucose. Write 'Respiration occurs in cells, breathing occurs in lungs.' Mistake 2: Stating that plants respire only at night. Plants respire continuously, day and night; during daytime, photosynthesis masks respiration by producing more O₂ than is consumed. Mistake 3: Writing that anaerobic respiration produces no energy. It does produce energy—2 ATP per glucose—just far less than aerobic respiration. Mistake 4: Labeling the trachea as the 'food pipe'. The trachea is the windpipe (air); the esophagus is the food pipe. Mistake 5: In diagrams, forgetting to label key structures like alveoli, diaphragm, or spiracles, which costs easy marks. Mistake 6: Confusing lactic acid fermentation (muscles) with alcoholic fermentation (yeast). Make sure to specify which organism performs which type. Reviewing these common errors before the exam can add 3–5 marks to your score.

Do NOT equate breathing and respiration—they are distinct processes.
Do NOT claim plants respire only at night; they respire 24×7.
Do NOT say anaerobic respiration yields zero energy; it yields 2 ATP.
Do NOT mislabel trachea as esophagus or confuse their functions.
Do NOT skip labels in diagrams—each missing label typically costs 0.5 marks.
Do NOT mix up lactic acid fermentation and alcoholic fermentation contexts.

How CBSE Exams Test CBSE Class 7 Science Chapter 6 Respiration in Organisms

CBSE Class 7 Science term exams allocate approximately 4–6 marks to Chapter 6 Respiration in Organisms, distributed across question types. Typically, you will encounter one multiple-choice question (1 mark) testing definitions or facts (e.g. 'Which gas is released during aerobic respiration?'—Answer: Carbon dioxide). One very short answer (1 mark) asks a direct recall question like 'Name the respiratory organs of fish.' One short answer (2 marks) requires a brief explanation: 'Why is respiration considered an exothermic reaction?' (Answer: Because it releases energy in the form of heat and ATP). One long answer (3 marks) demands detailed description: 'Explain the process of breathing in humans with reference to the role of the diaphragm and intercostal muscles.' Finally, one diagram-based question (3 marks) asks you to draw and label the human respiratory system or the tracheal system of an insect. Formative assessments (periodic tests) often include true/false statements and match-the-following, while summative exams prefer application-based questions linking respiration to real-world scenarios (exercise, yeast in bread-making, fish in aquariums). Mastering all NCERT in-text and end-of-chapter exercises ensures you can tackle any exam format confidently.

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Exam Preparation Checklist for CBSE Class 7 Science Chapter 6 Respiration in Organisms

Use this checklist in the week before your term exam to ensure comprehensive coverage of CBSE Class 7 Science Chapter 6 Respiration in Organisms. (1) Memorize the word equation for aerobic respiration: Glucose + Oxygen → Carbon dioxide + Water + Energy. (2) Understand and be able to explain the difference between breathing and respiration in two sentences. (3) Practice drawing and labeling the human respiratory system diagram until you can do it in under 3 minutes with all parts correctly marked. (4) Revise the table comparing aerobic and anaerobic respiration—know at least four points of difference. (5) Learn the mechanism of inhalation and exhalation: diaphragm movement, rib cage movement, volume and pressure changes. (6) Know examples of anaerobic respiration: yeast (alcoholic fermentation producing ethanol + CO₂) and muscle cells (lactic acid fermentation). (7) Understand respiration in plants (stomata, lenticels), insects (spiracles, tracheae), and fish (gills). (8) Solve all NCERT in-text and end-of-chapter questions without looking at solutions first—then verify your answers. (9) Attempt at least two sample papers or previous years' questions that include Chapter 6. (10) Revise common mistakes list to avoid silly errors on exam day.

Day 7 before exam: Read NCERT chapter once, make summary notes of key terms.
Day 6: Solve all in-text questions, check answers against these solutions.
Day 5: Solve all end-of-chapter exercises; identify weak areas.
Day 4: Practice diagram drawing—human respiratory system, insect tracheal system, fish gills.
Day 3: Revise aerobic vs. anaerobic table, breathing mechanism, respiration in different organisms.
Day 2: Attempt one full sample paper; time yourself as per exam conditions.
Day 1: Quick revision of word equations, key terms, common mistakes list; light review only.
Exam day: Read questions carefully, allocate time per mark, attempt diagram questions neatly.

Frequently asked questions

What is the difference between breathing and respiration in CBSE Class 7 Science Chapter 6?+

Breathing is the physical process of inhaling oxygen and exhaling carbon dioxide using lungs and diaphragm. Respiration is the cellular biochemical process where glucose is broken down in mitochondria to release energy (ATP). Breathing supplies oxygen for respiration and removes its waste product CO₂. NCERT emphasizes this distinction repeatedly.

Why do we get muscle cramps during heavy exercise, according to CBSE Class 7 Science Chapter 6 Respiration in Organisms?+

During intense exercise, oxygen supply to muscles is insufficient for aerobic respiration. Muscle cells switch to anaerobic respiration, producing lactic acid as a by-product. Lactic acid accumulation in muscle tissue causes pain and cramps. Once oxygen supply is restored post-exercise, lactic acid is converted back to glucose, and cramps subside.

How many ATP molecules are produced in aerobic vs. anaerobic respiration, as per NCERT Class 7 Science?+

Aerobic respiration produces approximately 38 ATP molecules per glucose molecule, making it highly efficient. Anaerobic respiration produces only 2 ATP molecules per glucose—far less energy—because glucose is incompletely broken down in the absence of oxygen. This difference explains why anaerobic pathways are used only when oxygen is scarce.

Do plants respire only at night, or do they respire during the day as well?+

Plants respire continuously, both day and night, taking in oxygen and releasing carbon dioxide. During the day, photosynthesis occurs simultaneously and at a higher rate, consuming CO₂ and releasing O₂, which masks the respiration process. At night, only respiration occurs, so the net gas exchange is oxygen uptake and CO₂ release—visible evidence of plant respiration.

What are stomata and lenticels, and what role do they play in plant respiration?+

Stomata are tiny pores on leaf surfaces, primarily on the lower epidermis, surrounded by guard cells that regulate opening and closing. Lenticels are small openings in the bark of woody stems and roots. Both structures allow gas exchange—oxygen enters for respiration, and carbon dioxide exits. Plants lack specialized respiratory organs, so diffusion through stomata and lenticels meets their oxygen needs.

How do insects breathe without lungs, according to CBSE Class 7 Science Chapter 6 Respiration in Organisms?+

Insects use a tracheal system: air enters through small openings called spiracles on the body surface, travels through a network of tubes called tracheae, which branch into finer tracheoles that deliver oxygen directly to cells. This direct delivery eliminates the need for blood to transport oxygen. Insect blood (hemolymph) carries only nutrients, not gases.

Why is the breathing rate of aquatic animals like fish much faster than terrestrial animals?+

Water contains far less dissolved oxygen (about 1%) compared to air (21%). To extract enough oxygen for cellular respiration, fish must process large volumes of water continuously, resulting in a faster gill ventilation rate. Terrestrial animals breathing air can meet oxygen needs with fewer breaths because air is oxygen-rich.

What is the role of the diaphragm in human breathing, as explained in NCERT Class 7 Science?+

The diaphragm is a dome-shaped muscle sheet separating the chest cavity from the abdominal cavity. During inhalation, it contracts and flattens, moving downward, which increases chest volume and decreases internal pressure, drawing air into the lungs. During exhalation, it relaxes and moves upward, decreasing chest volume, increasing pressure, and pushing air out.

Can you give a real-world example of anaerobic respiration in daily life?+

Yeast cells perform alcoholic fermentation (anaerobic respiration) when making bread. Yeast breaks down glucose in dough, producing ethanol and carbon dioxide. The CO₂ gas forms bubbles that make the dough rise and become fluffy. During baking, ethanol evaporates. This is why bread has a light texture—thanks to anaerobic respiration in yeast.

How are gills adapted for efficient gas exchange in fish, according to CBSE Class 7 Science Chapter 6?+

Fish gills have a large surface area due to numerous thin gill filaments and lamellae, rich in blood capillaries. Water flows over gills in one direction while blood flows in the opposite direction (countercurrent exchange), maximizing the oxygen concentration gradient and ensuring efficient oxygen diffusion from water into blood and CO₂ diffusion out.

Why is respiration called an exothermic reaction in CBSE Class 7 Science Chapter 6 Respiration in Organisms?+

Respiration releases energy stored in glucose bonds in the form of ATP and heat. Any reaction that releases energy to the surroundings is called exothermic. Even though some energy is stored in ATP for later use, a significant portion is released as heat, which is why our bodies maintain a constant warm temperature—direct evidence of exothermic respiration.

Will my child miss important details if our school uses a different science textbook instead of NCERT for Class 7?+

CBSE prescribes NCERT as the official textbook, and all CBSE exams—formative, summative, and board exams in later years—are set strictly according to NCERT content and terminology. Even if your school uses a supplementary book, ensure your child studies the NCERT textbook for Science Chapter 6 Respiration in Organisms to match exam expectations and avoid confusion with alternative explanations or terminologies.

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