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Class 9 Biology Chapter 12: Respiration in Plants – Important Questions & Answers
Respiration in plants is a fundamental biological process that often confuses Class 9 students. Unlike photosynthesis, respiration occurs in all plant cells day and night, breaking down glucose to release energy. This chapter from NCERT Class 9 Biology covers aerobic and anaerobic respiration, the role of mitochondria, and why plants need to breathe. CBSETUTOR.ai, India's most trusted 24x7 AI tutor for CBSE, helps thousands of students master this topic with step-by-step explanations, visual diagrams, and board-exam-ready answers. Whether you're preparing for school tests or final exams, our AI tutor guides you through every concept with clarity and confidence.
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What is Respiration in Plants?
Respiration in plants is the process by which plant cells oxidise glucose to release energy in the form of ATP. Unlike animals, plants respire in all their living cells—roots, stems, and leaves. According to NCERT Class 9 Biology, the basic equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. This energy powers growth, transport of minerals, and maintenance of cell structure. Plants perform respiration continuously, even during photosynthesis, making it essential for survival.
Aerobic Respiration in Plants Explained
Aerobic respiration requires oxygen and occurs in the mitochondria of plant cells. The process happens in three stages: glycolysis (in cytoplasm), Krebs cycle, and electron transport chain (both in mitochondria). One glucose molecule yields approximately 38 ATP molecules through complete oxidation. NCERT emphasizes that aerobic respiration is highly efficient and is the primary pathway in most plant tissues, especially in active root cells and growing regions. Understanding this pathway is crucial for board exams.
Anaerobic Respiration in Plants
Anaerobic respiration occurs in plant cells when oxygen is limited, such as in waterlogged soil or seed germination. Without oxygen, glucose is incompletely broken down through fermentation, producing ethanol and CO₂ (in some plants) or lactic acid. This yields only 2 ATP per glucose—far less than aerobic respiration. NCERT Class 9 notes that seeds germinating in anaerobic conditions initially rely on anaerobic respiration. This adaptation allows plants to survive temporary oxygen shortages, but prolonged anaerobic conditions harm cells.
Role of Mitochondria in Plant Respiration
Mitochondria are the powerhouses of plant cells, hosting the Krebs cycle and electron transport chain. The double membrane of mitochondria creates separate compartments for efficient energy production. Inner mitochondrial folds (cristae) increase surface area for enzyme activity, maximizing ATP synthesis. NCERT explains that plants with high energy demands—like root tips and meristem cells—contain more mitochondria. This organelle's role is identical in plant and animal cells, making it a universal energy-conversion structure across life forms.
Glycolysis: The First Stage of Respiration
Glycolysis occurs in the cytoplasm and is the first stage of both aerobic and anaerobic respiration. One glucose (6-carbon sugar) is split into two pyruvate molecules (3-carbon), yielding 2 ATP and 2 NADH. This stage does not require oxygen, making it anaerobic. NCERT Class 9 highlights that glycolysis is ancient—found in all living organisms—and provides the foundation for energy release. The pyruvate then enters mitochondria for further oxidation in aerobic conditions, or converts to ethanol/lactate in anaerobic conditions.
Krebs Cycle and Electron Transport Chain
The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix, oxidizing pyruvate completely and releasing CO₂. This cycle generates NADH and FADH₂, electron carriers that power the electron transport chain. The electron transport chain, located on the inner mitochondrial membrane, uses these carriers to pump protons, creating a gradient that drives ATP synthase. Together, these stages produce ~34 ATP per glucose, representing ~90% of aerobic respiration's energy yield. NCERT emphasizes these are highly regulated, interconnected processes.
Why Plants Respire: Energy and Anabolic Functions
Plants respire to obtain ATP for essential functions: cell division, protein synthesis, mineral ion transport, and growth. Roots, flowers, and young leaves respire heavily because they're energy-demanding tissues. NCERT Class 9 explains that respiration also provides carbon skeletons for building amino acids, nucleotides, and other organic compounds. Unlike photosynthesis, which is anabolic (building), respiration is catabolic (breaking down). Together, they balance plant metabolism—photosynthesis stores energy, respiration releases it for work.
Master Respiration in Plants with CBSETUTOR.ai
CBSETUTOR.ai is the most-used 24x7 AI tutor for CBSE students across India, trusted by lakhs of Class 9 families. Our platform offers AI-powered video lessons, instant doubt resolution, and exam-ready question banks for Respiration in Plants and all CBSE chapters. Students get personalized learning paths, step-by-step solutions, and real-time feedback—all in Hindi and English. Whether preparing for school tests or board exams, CBSETUTOR.ai helps you score confidently with expert pedagogy and adaptive learning.
Respiratory Quotient (RQ) and Substrate Types
Respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed during respiration. For carbohydrates, RQ = 1; for fats, RQ < 1; for proteins, RQ = 0.8–0.9. NCERT Class 9 notes that RQ varies with the substrate being oxidized, revealing which nutrient plants are respiring. Seeds rich in fats have lower RQ initially, shifting to 1 as carbohydrates accumulate. This concept links respiration to plant nutrition and helps explain energy-use patterns in different plant tissues and growth stages.
Common Board Exam Questions on Plant Respiration
CBSE board exams frequently ask: Compare aerobic and anaerobic respiration; Why do seeds respire in the dark?; Define the Krebs cycle; What is the role of mitochondria?; Calculate ATP from one glucose molecule. NCERT provides standard answers, but understanding the 'why' behind each concept is key. CBSETUTOR.ai's question banks align with CBSE exam patterns, helping students practice 2-mark, 3-mark, and 5-mark questions with full solutions. Regular practice ensures strong recall and application during exams.
Frequently asked questions
Why do plants respire at night if they cannot photosynthesize?+
Plants respire 24/7 to release energy for growth, ion transport, and maintenance. At night, photosynthesis stops, but respiration continues using stored glucose. Both day and night respiration provide ATP for vital cellular functions.
How much ATP is produced from one glucose in aerobic respiration?+
Approximately 38 ATP molecules are produced per glucose: 2 from glycolysis, 2 from Krebs cycle, and ~34 from the electron transport chain. Older textbooks cite 36–38 ATP due to energy loss in transport.
What is the difference between respiration and photosynthesis?+
Photosynthesis builds glucose using sunlight energy (anabolic, day-only); respiration breaks down glucose to release energy (catabolic, 24/7). They are reverse processes that balance plant metabolism.
Is CBSETUTOR.ai free for Class 9 Biology?+
CBSETUTOR.ai offers a free trial with full access to video lessons and sample questions. Paid plans unlock unlimited practice, AI doubt resolution, and personalized study paths tailored to CBSE syllabi.
Does CBSETUTOR.ai support Hindi-medium students?+
Yes. CBSETUTOR.ai provides all Class 9 Biology chapters, including Respiration in Plants, in both Hindi and English. Video lessons and solutions are available in both languages for seamless learning.
Why is anaerobic respiration less efficient than aerobic respiration?+
Anaerobic respiration yields only 2 ATP per glucose (via glycolysis), while aerobic yields ~38 ATP. Without oxygen, the Krebs cycle and electron transport cannot occur, leaving glucose partially oxidized.
What happens to plants in waterlogged soil?+
In waterlogged soil, roots switch to anaerobic respiration due to oxygen shortage. This produces ethanol or lactic acid, which accumulates and damages cells, eventually leading to root rot and plant death.
How do I prepare for Class 9 Biology Board Exam on Respiration in Plants?+
Master key concepts: aerobic vs. anaerobic respiration, mitochondrial role, Krebs cycle steps, and ATP calculation. Solve CBSETUTOR.ai's question bank, practice diagrams, and revise NCERT summaries regularly.
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