Class 9 Science Chapter 1 Nutrition in Plants: 18 Important Questions with Full Solutions

Nutrition in Plants carries 10–12 marks in the Class 9 Science summative exam. The CBSE rationalized syllabus emphasizes conceptual understanding over memorization. Board examiners ask 'why' and 'how' questions: Why do plants need photosynthesis? How do parasitic relationships differ from mutualistic ones? The question distribution follows a predictable pattern: 1–2 MCQ (1 mark each), 2–3 short-answer questions (2 marks each), 1–2 medium-answer questions (3 marks each), and 1 long-answer or case-study question (5 marks). Understanding the mechanism of photosynthesis (light reactions and dark reactions) and being able to explain the role of chlorophyll, stomata, and chloroplasts separates high-scoring students from average ones. Additionally, the chapter bridges into ecology—examiners expect students to link nutrition types to real ecosystems: lichens on rocks (mutualism), mistletoe on trees (parasitism), and fungi in plant roots (mycorrhizae). Practising varied question formats builds the mental flexibility needed for board exams.

MCQs test quick recall and concept recognition. Each correct answer earns 1 mark; there is no negative marking in most CBSE papers. **Q1: Which organelle is the site of photosynthesis in plant cells?** A) Mitochondrion B) Chloroplast C) Ribosome D) Golgi apparatus **Answer: B) Chloroplast.** Chloroplasts contain chlorophyll pigment and perform light-dependent and light-independent reactions. **Q2: Autotrophic nutrition is found in:** A) Animals B) Fungi C) Green plants D) Bacteria only **Answer: C) Green plants.** Autotrophs synthesize their own food using sunlight, water, and CO₂. Many bacteria and algae are also autotrophic, but green plants are the primary example. **Q3: The dark reaction of photosynthesis occurs in which part of the chloroplast?** A) Thylakoid B) Granum C) Stroma D) Outer membrane **Answer: C) Stroma.** The Calvin cycle (dark reaction) takes place in the stroma and does not require light directly. **Q4: Which organism is a saprotroph?** A) Mushroom B) Pea plant C) Rabbit D) Grasshopper **Answer: A) Mushroom.** Saprotrophs (like fungi and bacteria) feed on dead organic matter and play a vital recycling role in ecosystems. **Q5: Symbiotic relationship between a fungus and plant root is called:** A) Parasitism B) Mutualism C) Mycorrhizae D) Epiphytism **Answer: C) Mycorrhizae.** This is a mutualistic symbiosis where the fungus receives carbohydrates from the plant and helps the plant absorb water and minerals.

Short-answer questions demand concise, focused explanations. Write in 30–50 words and include one key example or equation. **Q1: Define autotrophic and heterotrophic nutrition. Give one example of each.** **Answer:** Autotrophic nutrition is the ability to synthesize food from inorganic substances (CO₂ and H₂O) using light energy. Example: Green plants. Heterotrophic nutrition is the intake of organic food produced by other organisms. Example: Animals, fungi, and most bacteria. (2 marks) **Q2: Write the equation of photosynthesis and name the products formed.** **Answer:** 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Products: Glucose (food) and oxygen (released). (2 marks) **Q3: Distinguish between a parasite and a saprotroph. Name one organism of each type.** **Answer:** A parasite (e.g., tapeworm, mistletoe) lives in or on a host and harms it by extracting nutrients. A saprotroph (e.g., mushroom, bread mold) feeds on dead, decaying matter without harming a living host. (2 marks) **Q4: What is mycorrhizae? State the benefit to both organisms involved.** **Answer:** Mycorrhizae is a mutualistic symbiotic relationship between a fungus and plant root. The fungus absorbs water and minerals from soil (benefiting the plant) while the plant provides carbohydrates to the fungus (benefiting the fungus). (2 marks) **Q5: Name the three raw materials required for photosynthesis and state where each is obtained.** **Answer:** (1) Water—absorbed by roots from soil. (2) Carbon dioxide—entered through stomata on leaves. (3) Light energy—captured by chlorophyll from the sun. (2 marks)

3-mark questions require a multi-part answer: definition or explanation + mechanism + example or application. **Q1: Explain why plants are called producers. How do they obtain and use the energy they capture?** **Answer:** Plants are called producers because they manufacture their own organic food (glucose) from inorganic raw materials using photosynthesis. During photosynthesis, chlorophyll absorbs light energy. In the light reaction (in thylakoids), water is split and light energy is converted into chemical energy (ATP and NADPH). In the dark reaction (in stroma), this chemical energy drives the Calvin cycle, converting CO₂ into glucose. Plants use glucose for respiration (to release energy for growth, movement, and life processes) and store it as starch for future use. This energy ultimately supports all other organisms in the food chain. (3 marks) **Q2: Compare photosynthesis in the light reaction and dark reaction. Where does each occur and what are the main events?** **Answer:** Light reaction: Occurs in thylakoid membranes. Light energy excites chlorophyll, causing photolysis of water (H₂O → 2H⁺ + ½O₂ + 2e⁻). Electrons flow through electron transport chains, generating ATP and NADPH. Oxygen is released. Dark reaction (Calvin cycle): Occurs in stroma. Does not require light directly. Uses ATP and NADPH produced in light reaction. CO₂ is fixed into 3-phosphoglycerate, reduced to G3P (glyceraldehyde-3-phosphate), and regenerated into RuBP (ribulose-1,5-bisphosphate). G3P is used to synthesize glucose. (3 marks) **Q3: A lichen is seen growing on a rock. Explain what type of organism makes up a lichen and describe the nature of the relationship.** **Answer:** A lichen is composed of two organisms: (1) An alga (or cyanobacterium)—autotrophic, performs photosynthesis, produces glucose. (2) A fungus—heterotrophic, absorbs water and minerals from the rock. The relationship is mutualism (symbiosis): the fungus provides structural support, water, and minerals to the alga; the alga provides carbohydrates to the fungus through photosynthesis. Both organisms benefit and cannot survive independently in this harsh rocky environment. Lichens are pioneer species that weather rocks and initiate soil formation. (3 marks) **Q4: Explain how a parasitic plant like mistletoe obtains nutrition. Why is it harmful to its host?** **Answer:** Mistletoe is a parasitic flowering plant that has reduced leaves and lacks a complete root system. It attaches to tree branches using a specialized root-like structure called a haustarium, which penetrates the host's bark and taps into the xylem and phloem vessels. Through the haustorum, mistletoe extracts water, minerals, and organic nutrients (especially sugars) from the host's vascular tissue without photosynthesizing significantly. This parasitism harms the host tree by: (1) Removing vital nutrients and water. (2) Weakening branches, causing dieback. (3) Reducing the host's growth and reproductive output. If heavily infected, the host tree may die. (3 marks)

5-mark questions are essay-style. Structure your answer: introduction + mechanism/process + relevance + conclusion. **Q1: Describe the process of photosynthesis in detail. Explain how light energy is converted into chemical energy and how CO₂ is fixed into glucose.** **Answer:** Photosynthesis is an anabolic (building) process in which green plants synthesize glucose from CO₂ and H₂O using light energy captured by chlorophyll. It occurs in two main stages: **Light Reaction (Light-dependent stage):** Location: Thylakoid membranes of the chloroplast. Process: (1) Chlorophyll absorbs photons of light. (2) Light energy excites electrons in the P680 reaction center of PSII (Photosystem II). (3) These high-energy electrons move through an electron transport chain, pumping H⁺ ions into the thylakoid lumen, creating a proton gradient. (4) Simultaneously, water molecules are photolyzed: 2H₂O → O₂ + 4H⁺ + 4e⁻. (5) The H⁺ gradient drives ATP synthase, phosphorylating ADP to ATP. (6) Electrons eventually reach PSI, where they are re-excited by light and reduce NADP⁺ to NADPH. (7) Net products: ATP, NADPH, and O₂. **Dark Reaction (Light-independent stage / Calvin Cycle):** Location: Stroma of the chloroplast. Process: (1) CO₂ fixation: CO₂ combines with RuBP (ribulose-1,5-bisphosphate), a 5-carbon sugar, catalyzed by the enzyme RuBisCO. This produces an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. (2) Reduction: Using ATP and NADPH from the light reaction, 3-PGA is phosphorylated to 1,3-bisphosphoglycerate and then reduced to G3P (glyceraldehyde-3-phosphate). (3) Regeneration: Most G3P molecules (5 out of 6) are rearranged using ATP to regenerate RuBP, allowing the cycle to continue. (4) One G3P molecule exits the cycle every three turns and is used to synthesize glucose and other organic compounds. **Overall equation:** 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ **Significance:** Photosynthesis is the foundation of nearly all life on Earth. It fixes solar energy into chemical bonds of glucose, which is then used for respiration, growth, and reproduction in plants, and indirectly feeds all heterotrophic organisms. The oxygen produced is essential for aerobic respiration. (5 marks) **Q2: Write a comprehensive account of symbiotic relationships in nature. Classify them and give examples. Explain why they are important for ecosystems.** **Answer:** Symbiosis is a long-term, intimate association between two organisms of different species, where they live in close physical contact. Symbiotic relationships can be classified as: **1. Mutualism:** Both organisms benefit. - **Mycorrhizae:** Fungus helps plant roots absorb water and minerals; plant provides sugars to fungus. - **Nitrogen-fixing bacteria and legume plants:** Bacteria fix atmospheric nitrogen into usable nitrate; plant provides carbohydrates to bacteria. - **Bee and flowering plant:** Bees obtain nectar (food); plants achieve pollination and reproduction. - **Lichen:** Algae photosynthesizes and feeds fungus; fungus provides structure and protection to algae. **2. Parasitism:** One organism (parasite) benefits; the other (host) is harmed. - **Mistletoe on trees:** Extracts water and nutrients from host; weakens or kills branches. - **Tapeworm in intestines:** Absorbs nutrients meant for host; causes malnutrition. - **Cuscuta (dodder plant):** Lacks roots and leaves; taps into host plant's phloem to feed. **3. Commensalism:** One organism benefits; the other is unaffected (neither harmed nor benefited). - **Epiphytic orchids on tree branches:** Orchid gains support and light; tree is neither harmed nor helped significantly. - **Remora fish attached to sharks:** Remora gets food scraps and transport; shark unaffected. **Ecological Importance:** (1) **Nutrient cycling:** Mycorrhizae and nitrogen-fixing bacteria enable nutrient availability to plants, supporting plant growth and the entire food web. (2) **Biodiversity:** Symbiotic relationships increase habitat heterogeneity. Lichens colonize bare rocks, initiating primary succession. (3) **Energy flow:** Mutualistic relationships improve energy transfer efficiency. Plants in mutualistic relationships with fungi/bacteria are healthier and more productive. (4) **Species survival:** Some species (like mistletoe-bearing trees) depend on specific symbiotic partners for survival. (5) **Ecological stability:** Diverse symbiotic networks make ecosystems more resilient to environmental changes. Symbiotic relationships are evidence of coevolution and demonstrate that interdependence, not just competition, shapes nature. (5 marks) **Q3: A farmer notices that his tomato plants in one field are yellowing and growing poorly, while his legume (bean) crop in an adjacent field is thriving and nitrogen-rich. Explain the nutritional differences and suggest how the farmer can improve both crops using biological knowledge.** **Answer:** **Analysis of the two fields:** **Tomato field (yellowing, poor growth):** Tomatoes are not legumes and lack a mutualistic relationship with nitrogen-fixing bacteria. Nitrogen depletion in the soil leads to poor amino acid synthesis, reduced chlorophyll production (yellowing), and stunted growth. **Bean field (thriving, nitrogen-rich):** Beans are legumes that form symbiotic relationships with Rhizobium bacteria in root nodules. The bacteria fix atmospheric N₂ into NH₃/NO₃⁻, a usable form. The plant provides carbohydrates to the bacteria (at least 30% of photosynthetic output). This mutualism allows beans to thrive even in nitrogen-poor soil. **Suggestions to improve both crops:** 1. **For tomatoes:** Apply Rhizobium inoculant or nitrogen-fixing bacteria to the soil (if practical), or rotate tomatoes with legumes (e.g., plant beans one season, tomatoes the next). The legumes will enrich the soil with fixed nitrogen, benefiting subsequent crops. 2. **For sustained bean growth:** Avoid using high-nitrogen synthetic fertilizers, as excess N reduces nodule formation. Instead, use organic compost and allow crop rotation to maintain the symbiotic relationship. 3. **Intercropping:** Plant tomatoes and beans together; nitrogen fixed by beans diffuses into the soil and is absorbed by tomatoes (nitrogen transfer). **Nutritional summary:** Tomatoes: Heterotrophic in the sense they rely entirely on soil nutrients (which farmers must supply). Beans: Semi-autotrophic, as they acquire nitrogen directly from the atmosphere via symbiotic bacteria, reducing external nitrogen input needs. This example illustrates how understanding symbiotic nutrition allows sustainable agriculture and resource efficiency. (5 marks)

**HOTS Question (Higher Order Thinking Skills):** **Case Study: A scientist isolated a single-celled organism from a freshwater pond. After laboratory analysis, the organism was found to contain chlorophyll and was capable of both photosynthesis and heterotrophic feeding (consuming organic particles). The organism also had a flagellum for movement. When cultured in the dark without organic food, it survived. When cultured in light without organic food, it also survived. However, when cultured in darkness with only organic food, it barely survived and eventually died. *Based on this case study, answer the following:* **Q1: Classify the nutritional mode of this organism. Justify your answer using evidence from the experiment.** **Step-by-step solution:** **Step 1—Identify the nutritional capabilities:** The organism possesses chlorophyll → capable of photosynthesis (autotrophic component). It consumes organic particles → capable of heterotrophy (heterotrophic component). Such organisms are called **mixotrophs** or **photoheterotrophs**. **Step 2—Analyze experimental evidence:** - Survived in dark without organic food → partially false (it did survive but may have used stored reserves). - Survived in light without organic food → TRUE; photosynthesis alone sustains it. - Barely survived in darkness with only organic food → It relied on heterotrophy but was inefficient at deriving all energy from organic matter alone. **Step 3—Deduce the primary nutrition mode:** The organism's preference for light and photosynthesis (surviving best in light) indicates **photosynthesis is its primary and preferred mode**. Heterotrophic feeding is supplementary, allowing it to consume dissolved organic matter opportunistically. **Answer:** This organism is a **mixotroph** (or facultative phototroph), primarily autotrophic but capable of heterotrophy. Evidence: (1) Chlorophyll presence and photosynthetic survival in light. (2) Best survival in light, indicating photosynthesis is the principal nutrition source. (3) Heterotrophic capability but poor long-term survival on organic food alone, indicating heterotrophy is supplementary. (4) The combination of flagella and nutritional flexibility suggests an organism like *Euglena* (a protist), adapted to pond environments where light and organic nutrients vary seasonally. (2 marks) **Q2: Why would such a mixed nutritional strategy be advantageous in a pond ecosystem? Explain using ecological reasoning.** **Answer:** A mixotrophic organism has a survival advantage in unstable pond environments: 1. **Seasonal variability:** Ponds experience light fluctuations (seasonal, cloudy weather, algal blooms blocking light). Photosynthesis alone would fail in such periods. Heterotrophy allows survival when light is limited. 2. **Resource diversification:** The organism does not depend solely on one energy source. If photosynthesis is insufficient (shallow water, dense algal cover), it can feed on detritus and dissolved organic matter. 3. **Rapid reproduction:** During nutrient-rich seasons (eutrophication from decomposition), the heterotrophic mode allows faster growth, outcompeting pure autotrophs. 4. **Competitive edge:** While pure autotrophs depend on light and pure heterotrophs depend on external food, mixotrophs thrive across varying conditions, occupying a unique ecological niche. 5. **Energy efficiency:** The organism optimizes energy acquisition—photosynthesis (zero cost for light) when available; heterotrophy (direct uptake of pre-made molecules) when light is scarce. This flexibility is an example of **niche breadth**—a broad ecological strategy that increases survival in unpredictable environments. (3 marks) **Total: 5 marks**

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