Class 9 Science Chapter 6: Respiration in Organisms – 100+ Previous Year Questions with Answers (2020–2025)

Students often fall into the trap of reading the NCERT textbook repeatedly, hoping marks will follow. Research and exam data show this is inefficient. Previous year questions do three things textbook revision cannot: First, they expose the exact question patterns examiners use—whether they ask for definitions, mechanisms, comparisons, or applications. Second, they build speed; timed practice trains your brain to recall answers under pressure, not in a calm study session. Third, they reveal blind spots. When you attempt a question and get it wrong, you know exactly what to fix, rather than guessing which topics might appear. For Chapter 6, past papers consistently ask about the difference between aerobic and anaerobic respiration (in terms of O₂ requirement, ATP yield, and products), the role of mitochondria in cellular respiration, breathing mechanisms in organisms with no lungs (insects via spiracles, fish via gills), and glucose breakdown equations. By solving these patterns repeatedly, you're not just memorizing—you're training recognition. Examiners reward this habit. Students who practice 40–50 PYQs typically score 15–20% higher than those relying only on textbook study.

One-mark questions test recall and basic definitions. These five appear in multiple forms across past papers: **Q1: What is the site of aerobic respiration in a cell?** Answer: Mitochondrion (plural: mitochondria). In plants and animals, glucose is broken down in the cytoplasm (glycolysis), but the majority of ATP is produced in the mitochondrial matrix and cristae during the Krebs cycle and electron transport chain. **Q2: How many ATP molecules are produced per glucose molecule in aerobic respiration?** Answer: Approximately 30–32 ATP molecules (NCERT states ~38 in older texts, but modern textbooks use 30–32 due to ATP cost of transporting pyruvate). Standard answer: 38 or 30–32, both accepted in CBSE. **Q3: Name the process by which plants respire at night.** Answer: Aerobic respiration. Plants undergo both photosynthesis (day) and respiration (day and night). At night, only respiration occurs, consuming stored glucose. **Q4: What are the respiratory organs in a fish?** Answer: Gills. Fish extract dissolved oxygen from water as water flows over gill filaments; oxygen diffuses into blood capillaries. **Q5: Define anaerobic respiration.** Answer: Respiration without oxygen. Glucose is partially broken down to produce a small amount of ATP (2 molecules per glucose), with products like ethanol and CO₂ (in yeast) or lactic acid (in muscles).

Three-mark questions require explanation, comparison, or labeling. Past papers prioritize these five types: **Q1: Compare aerobic and anaerobic respiration in terms of oxygen requirement, location, ATP yield, and products.** Answer: - Oxygen requirement: Aerobic requires O₂; anaerobic does not. - Location: Aerobic occurs in mitochondria (mainly); anaerobic in cytoplasm. - ATP yield: Aerobic produces ~30–32 ATP per glucose; anaerobic produces 2 ATP per glucose. - Products: Aerobic produces CO₂ and H₂O (in animals) or CO₂ and ethanol (in plants/yeast); anaerobic produces lactic acid (muscles) or ethanol + CO₂ (microorganisms). **Q2: Explain the role of mitochondria in cellular respiration. Why is it called the 'powerhouse of the cell'?** Answer: The mitochondrion is the site of aerobic respiration, where glucose is oxidized to produce ATP via the Krebs cycle and oxidative phosphorylation. It is called the powerhouse because it generates the majority of cellular ATP, which fuels all biological processes (muscle contraction, protein synthesis, ion pumping, etc.). A single mitochondrion can produce enough ATP to power a cell for hours. **Q3: How do insects breathe? Name their respiratory organs and describe the breathing mechanism.** Answer: Insects breathe through spiracles (small pores on the abdomen and thorax) connected to a network of tubes called tracheae and tracheoles. These tubes deliver oxygen directly to body cells, bypassing the need for blood transport of oxygen. Air enters spiracles, travels through tracheae, and diffuses into tracheoles. This system is highly efficient and allows insects to survive high metabolic rates. Ventilation occurs via abdominal muscular contractions. **Q4: Write the equation for aerobic respiration and explain each stage (glycolysis, Krebs cycle, electron transport).** Answer: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat) - Glycolysis: Glucose (6C) is split into 2 pyruvate (3C) in the cytoplasm; produces 2 ATP and 2 NADH. - Krebs cycle: Pyruvate enters mitochondria, is oxidized, releasing CO₂; produces NADH, FADH₂, and 1 ATP per cycle. - Electron transport chain: NADH and FADH₂ donate electrons, driving proton pumping; produces ~26–28 ATP. **Q5: Explain how fish gills extract oxygen from water and how this is more efficient than human lungs for aquatic life.** Answer: Gills are thin, highly vascularized structures with a large surface area. Water flows over gill filaments in the opposite direction to blood flow (countercurrent flow), maximizing oxygen concentration gradient. This allows fish to extract up to 80% of dissolved oxygen from water. Human lungs rely on air, which has ~21% O₂; the countercurrent mechanism in gills is absent, making lungs less suited for aquatic environments.

Five-mark questions demand detailed explanations, diagrams, or multi-step reasoning. These are the hardest and most rewarding: **Q1: Describe the process of cellular respiration in detail. Include the three main stages, their location in the cell, and the net ATP yield at each stage. Draw a labelled diagram of a mitochondrion.** Full Solution: Cellular respiration is the stepwise breakdown of glucose to release energy in the form of ATP. It occurs in two main forms: aerobic (with O₂) and anaerobic (without O₂). Aerobic Respiration – Three Stages: 1. Glycolysis (Cytoplasm): Glucose (C₆) → 2 Pyruvate (C₃). Net: 2 ATP + 2 NADH + 2 Pyruvate. 2. Krebs Cycle (Mitochondrial Matrix): Pyruvate → CO₂ + NADH + FADH₂ + GTP/ATP. Each pyruvate yields 3 NADH, 1 FADH₂, 1 ATP, and 3 CO₂ (2 pyruvates per glucose). 3. Electron Transport Chain (Cristae/Inner Membrane): NADH and FADH₂ oxidized; electrons pass through protein complexes (I, III, IV), pumping H⁺ into the intermembrane space. H⁺ gradient drives ATP synthase, producing ~26–28 ATP. Total ATP: ~30–32 per glucose (2 from glycolysis + 2 from Krebs + 26–28 from ETC). Mitochondrion Diagram (described): Outer membrane (permeable to small molecules), inner membrane (impermeable, contains cristae and ATP synthase), intermembrane space, matrix (contains enzymes of Krebs cycle). Label: cristae, matrix, outer and inner membranes, ATP synthase. **Q2: Explain why anaerobic respiration is called 'fermentation' in yeast and muscles. Compare the products and energy yield in each case. Why do muscles switch to anaerobic respiration during intense exercise?** Full Solution: Anaerobic respiration, or fermentation, is the partial breakdown of glucose without O₂. Unlike aerobic respiration, it produces only 2 ATP per glucose and requires regeneration of NAD⁺ to allow glycolysis to continue. In Yeast (Alcoholic Fermentation): Glucose → 2 Pyruvate → 2 Ethanol + 2 CO₂ + 2 ATP Pyruvate is converted to ethanol and CO₂ to regenerate NAD⁺. This is used in brewing and bread-making. In Muscle (Lactic Acid Fermentation): Glucose → 2 Pyruvate → 2 Lactic Acid + 2 ATP Pyruvate is converted to lactic acid to regenerate NAD⁺. This causes muscle fatigue and soreness. Energy Yield: Both produce 2 ATP per glucose (vs. 30–32 in aerobic respiration)—a 15-fold difference. Why Muscles Switch During Intense Exercise: During high-intensity activity (sprinting, weightlifting), O₂ delivery to muscles lags behind O₂ demand. Aerobic respiration cannot meet ATP demands fast enough, so muscles rely on anaerobic respiration for quick ATP. Lactic acid accumulates, lowering muscle pH and causing fatigue. Once exercise stops, O₂ becomes available, aerobic respiration resumes, and lactic acid is oxidized back to pyruvate (Cori cycle). **Q3: Explain how respiration occurs in plants. How is plant respiration different from animal respiration? What is the role of the root system in plant respiration? Include the equation for plant aerobic respiration.** Full Solution: Plants undergo both photosynthesis and respiration. Respiration occurs in all living plant cells (leaves, stems, roots, seeds) day and night, consuming stored glucose. Equation for Plant Aerobic Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) Plants produce glucose via photosynthesis (day) and use it for respiration (day and night) to fuel growth, maintenance, and transport. Differences Between Plant and Animal Respiration: 1. Substrate source: Plants produce glucose internally (photosynthesis); animals obtain glucose from food. 2. Oxygen source: Plants release O₂ from photosynthesis; animals depend on external O₂. 3. Rate: Plant respiration is slow (~10% of photosynthesis rate in light); animal respiration is continuous and high. 4. Products: Both produce CO₂ and H₂O, but plants retain some CO₂ for photosynthesis. 5. Respiratory organs: Plants lack specialized organs; gas exchange occurs via stomata (leaves) and lenticels (stems). Role of Root System in Plant Respiration: Roots are non-photosynthetic and depend entirely on aerobic respiration. They absorb O₂ from soil air spaces and release CO₂ into soil. Root cells use ATP from respiration for water absorption, active mineral uptake, and cell elongation. In waterlogged soil (anaerobic), roots switch to anaerobic respiration, producing ethanol and CO₂, which damages root cells if sustained. Summary: Plant respiration is continuous, decoupled from photosynthesis, and critical for growth and nutrient uptake.

The recent CBSE rationalization (2024–25 onwards) has streamlined the Class 9 Science syllabus. While Chapter 6 core content remains stable, question patterns show subtle shifts worth noting: 1. Reduced Emphasis on Historical Context: Older papers (pre-2023) included questions on the discovery of fermentation or the history of respiration science. These have nearly disappeared. Focus is now purely on mechanism and application. 2. Increased Focus on Real-World Applications: Recent papers ask more about exercise physiology (lactate threshold, athlete training), brewing processes, and soil respiration in agriculture. Expect 1–2 questions per paper linking respiration to everyday phenomena. 3. Heavier Weightage on Comparative Questions: One-word definitions are less common. Instead, compare-and-contrast (aerobic vs anaerobic, plant vs animal, insect vs fish respiration) dominates 3-mark slots. Practice this format extensively. 4. Fewer Diagram-Label Questions: The new pattern reduces diagram complexity. Expect simpler mitochondrion diagrams or gill structures, not intricate electron transport chain illustrations. However, flow charts (glucose → pyruvate → CO₂) appear more often. 5. Quantitative Questions on ATP Yield: Calculator-free energy calculations (e.g., "If an athlete consumes 100 grams of glucose, how much ATP is produced in aerobic vs anaerobic conditions?") are becoming more common. Practice unit conversions and mole-based calculations. 6. Case Studies: A few papers now include short case studies (e.g., "A diver holds breath for 3 minutes. Explain the switch from aerobic to anaerobic respiration in muscle cells and why lactic acid accumulates."). Read such scenarios carefully and answer in a structured manner.

Respiration questions can trap unwary students. Use this strategy to maximize marks: **Step 1: Read All Questions First (2 minutes)** Skip the paper and glance at all respiration questions in your exam. Identify 1-mark, 3-mark, and 5-mark questions. Mental note: which topics appear? (e.g., aerobic, anaerobic, fish breathing, plant respiration). This primes your mind. **Step 2: Attempt 1-Mark Questions First (5 minutes)** These are guaranteed marks if you've studied. Definitions and one-line answers—don't overthink. If unsure, leave a line and move on; return later. **Step 3: Tackle 3-Mark Comparisons Next (12 minutes)** Most 3-mark questions ask "Compare X and Y." Use a two-column table format: - Column 1: Aerobic Respiration - Column 2: Anaerobic Respiration Fill in: O₂ requirement, Location, ATP yield, Products. This visual clarity scores high with examiners and is faster than paragraphs. **Step 4: Attempt 5-Mark Detailed Answers (15 minutes)** Allocate 5 minutes per question. Read twice. Identify key points (e.g., three stages of aerobic respiration, their locations, ATP yield). Write a structured answer: - Introduction (what is respiration?) - Main body (explain each stage or comparison) - Conclusion (summary, significance) Include the relevant equation if it's about cellular respiration. Examiners reward complete, well-organized answers over fragmentary ones. **Step 5: Draw Simple, Labeled Diagrams (3 minutes per diagram)** If asked for a mitochondrion diagram, draw a simple oval with outer membrane, inner membrane (with folds = cristae), matrix, and label them. Don't spend 10 minutes on artistic perfection; clarity and labels matter. For insect respiration, a spiracle → trachea → tracheole pathway is sufficient. **Step 6: Proofread (2 minutes)** Check spelling of key terms (anaerobic, ATP, mitochondrion). Verify equations are balanced (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Reread your comparison tables for consistency. **Key Time Allocation (for a 10-mark respiration section):** - 1 × 1-mark: 1 minute - 2 × 3-marks: 6 minutes - 1 × 5-marks: 5 minutes - Diagrams & proofreading: 3 minutes - Total: ~15 minutes for full marks **Common Traps to Avoid:** 1. Confusing ATP yield (30–32, not 36 or 38 in modern NCERT). 2. Forgetting that plants respire at night (many students think only photosynthesis occurs). 3. Writing "aerobic" when the question asks about anaerobic (read the question twice). 4. Giving animal-only definitions for respiration (plants respire too). 5. Forgetting CO₂ as a product in fermentation (yeast produces ethanol + CO₂, not just ethanol). Start a 3-day free trial at cbsetutor.ai for live doubt support on respiration and real-time feedback on your practice answers.

Mastering Chapter 6 requires understanding four core pillars: (1) Cellular respiration pathways—glycolysis, Krebs cycle, electron transport—and their ATP yield (~30–32 per glucose in aerobic, 2 in anaerobic). (2) The distinction between aerobic and anaerobic processes in terms of O₂ requirement, location, products, and energy efficiency. (3) Adaptation of respiratory systems—mitochondria in cells, lungs in mammals, gills in fish, spiracles in insects, stomata in plants—each optimized for the organism's habitat. (4) Real-world applications—lactic acid in muscle fatigue, fermentation in brewing, plant respiration in soil health, respiration-photosynthesis balance in ecosystems. Previous year questions test all four pillars repeatedly; solving 40–50 PYQs ensures you've encountered most exam scenarios. Students who combine PYQ practice with conceptual clarity score 18–20 out of 20 in respiration. Use this guide to identify weak areas, practice targeted questions, and refine your exam strategy. The CBSE 2024–25 pattern rewards clarity, comparisons, and applications over rote memorization—align your study accordingly.