Class 9 Science Chapter 2 Nutrition in Animals: 18+ Important Questions with Complete Solutions

Nutrition in Animals (Chapter 2) represents 15–20% of the typical CBSE Science final exam. The 2024-25 rationalized syllabus emphasizes conceptual clarity over memorization, meaning examiners now focus on application-based questions. Key areas tested include: (1) distinguishing autotrophic and heterotrophic nutrition with examples (prokaryotes, animals, fungi); (2) tracing the complete human digestive pathway—ingestion, digestion, propulsion, absorption, and egestion; (3) explaining enzymatic digestion of carbohydrates, proteins, and fats in the mouth, stomach, and small intestine; (4) comparing ruminant digestion (four-chambered stomach, cud chewing, symbiotic bacteria) with simple stomachs; and (5) relating tooth structure (incisors, canines, premolars, molars) to diet and mechanical digestion. Board examiners routinely ask 2-mark questions on peristalsis, 3-mark questions on enzyme action in different pH environments, and 5-mark questions requiring you to compare digestive systems of herbivores, carnivores, and omnivores. This guide drills all predictable patterns.

**Q1. Which of the following is a heterotrophic organism?** A) Cyanobacteria B) Algae C) Amoeba D) Photosynthetic bacteria **Answer: C) Amoeba** Explanation: Amoeba is a protozoan that engulfs food particles (holozoic nutrition). Cyanobacteria, algae, and photosynthetic bacteria are all autotrophs. --- **Q2. Saliva contains an enzyme that begins the breakdown of ___.** A) Proteins B) Fats C) Carbohydrates D) Minerals **Answer: C) Carbohydrates** Explanation: Salivary amylase (ptyalin) breaks down starch into maltose in the mouth. --- **Q3. The stomach secretes HCl to ___.** A) Activate pepsin and kill bacteria B) Emulsify fats C) Absorb minerals D) Buffer stomach acid **Answer: A) Activate pepsin and kill bacteria** Explanation: Gastric juice (HCl + pepsin) creates an acidic environment (pH ≈ 2) where pepsin denatures proteins and pathogens are destroyed. --- **Q4. Ruminant animals have a ___ chambered stomach.** A) Two B) Three C) Four D) Five **Answer: C) Four** Explanation: Ruminants (cow, sheep, goat) have rumen, reticulum, omasum, and abomasum. This allows them to digest cellulose from plant cell walls. --- **Q5. Which tooth type is absent in herbivores?** A) Incisors B) Canines C) Molars D) Premolars **Answer: B) Canines** Explanation: Herbivores have reduced or no canines because they do not tear flesh. They have sharp incisors (cutting grass) and large molars (grinding).

**Q1. Define autotrophic and heterotrophic nutrition. Give one example of each.** **Answer:** Autotrophic nutrition: Organisms synthesize their own organic food from inorganic substances (CO₂ and H₂O) using energy from sunlight or chemical reactions. Example: Green plants, cyanobacteria. Heterotrophic nutrition: Organisms cannot synthesize their own food; they depend on other organisms. Example: Humans, animals, fungi. --- **Q2. Name the region where (a) starch digestion is completed, (b) fat digestion begins.** **Answer:** (a) Small intestine (duodenum) — pancreatic amylase completes starch breakdown into glucose. (b) Small intestine (duodenum) — bile from the liver emulsifies fats; pancreatic lipase breaks down triglycerides into fatty acids and glycerol. Note: While gastric lipase initiates 10–30% of fat digestion in the stomach, the majority occurs in the small intestine. --- **Q3. What is peristalsis? Why is it important?** **Answer:** Peristalsis is the rhythmic, wave-like contraction and relaxation of smooth muscles in the oesophagus, stomach, and intestines that propels food downward. Importance: • Moves food through the digestive tract even against gravity. • Mixes food with digestive juices (in stomach). • Brings food into contact with absorptive surfaces (small intestine). --- **Q4. Why is the small intestine the principal site of nutrient absorption?** **Answer:** • Presence of villi and microvilli increases surface area (~6 m²) for maximum absorption. • Thin epithelial lining (one cell layer) allows rapid diffusion and active transport of nutrients. • Rich blood supply transports absorbed nutrients to the liver and body. • Optimal pH (7–8) and intestinal enzymes ensure complete digestion of all macronutrients. --- **Q5. What is the role of gastric juice in the stomach?** **Answer:** Gastric juice (HCl + pepsin + mucus) performs three functions: 1. HCl denatures proteins and activates pepsinogen → pepsin (protein-digesting enzyme). 2. Pepsin begins protein hydrolysis into smaller polypeptides. 3. Mucus protects the stomach lining from acid attack. Result: Stomach converts food into a semi-liquid chyme, which is released into the small intestine.

**Q1. Explain the complete pathway of starch digestion from the mouth to absorption. Name the enzymes involved.** **Answer:** 1. **Mouth:** Salivary amylase breaks starch → maltose. Action stops in the oesophagus (enzyme deactivated by stomach acid). 2. **Stomach:** Starch remains largely undigested (acidic pH denatures salivary amylase). 3. **Small intestine (duodenum):** Pancreatic amylase breaks starch → maltose. Maltase (on intestinal brush border) converts maltose → glucose. 4. **Absorption:** Glucose enters bloodstream via active transport (glucose-galactose transporter) in duodenum and jejunum. **Summary table:** Location | Enzyme | End Product Mouth | Salivary amylase | Maltose Small intestine | Pancreatic amylase | Maltose Small intestine (brush border) | Maltase | Glucose --- **Q2. Compare the digestive adaptations of herbivores and carnivores.** **Answer:** | Feature | Herbivores (e.g., cow) | Carnivores (e.g., lion) | |---------|------------------------|------------------------| | **Teeth** | No canines; sharp incisors & large molars | Sharp canines & premolars; reduced molars | | **Stomach** | Four-chambered (rumen, reticulum, omasum, abomasum) | Simple, one chamber | | **Intestine** | Long (20–30 m in cattle) | Short (5–8 m) | | **Saliva** | Contains amylase (starch digestion) | No amylase | | **Digestion time** | 2–3 days (cellulose breakdown requires fermentation) | 12–24 hours (easy protein digestion) | | **Diet reason** | Plant cellulose is hard to digest; requires symbiotic bacteria | Protein & fat are readily digestible | --- **Q3. Describe the role of the liver and pancreas in digestion.** **Answer:** **Liver:** • Produces bile (contains bile salts and cholesterol). • Bile emulsifies fats → smaller fat droplets, increasing surface area for lipase action. • Bile does NOT contain digestive enzymes. • Stores glucose as glycogen; regulates blood glucose. **Pancreas:** • Secretes pancreatic juice (pH 8–8.5, alkaline) containing three enzymes: 1. **Pancreatic amylase:** breaks starch → maltose. 2. **Pancreatic protease (trypsin):** breaks proteins → amino acids. 3. **Pancreatic lipase:** breaks fats → fatty acids & glycerol. • Secretes bicarbonate to neutralize stomach acid (HCl) in the duodenum. • Endocrine function: produces insulin and glucagon (blood glucose regulation). --- **Q4. Explain ruminant digestion with reference to the four-chambered stomach.** **Answer:** Ruminants (cows, sheep, goats) possess a specialized four-chambered stomach to digest cellulose from plant material: **Pathway:** 1. **Rumen:** Food (grass, hay) enters and is fermented by symbiotic bacteria and protozoa. Bacteria produce cellulase enzyme (absent in animal cells), breaking down cellulose into glucose. 2. **Reticulum:** Food is mixed; partially digested food (cud) is regurgitated for "rumination" (rechewing), further grinding plant fibers. 3. **Omasum:** Water and mineral absorption occur. 4. **Abomasum:** True stomach; HCl and pepsin digest proteins and remaining organic matter (similar to monogastric stomach). **Advantages:** • Symbiotic bacteria enable cellulose digestion (unavailable to carnivores). • Two passages through the stomach ensure maximum nutrient extraction. • Slow fermentation provides sustained energy from plant material. **Time taken:** 2–3 days for complete digestion, contrasting with 12–24 hours in carnivores.

**Q1. Explain the structure and functions of the human digestive system. Trace the journey of a piece of bread from ingestion to egestion.** **Answer:** **Structure of digestive system:** The human digestive system comprises the alimentary canal and accessory organs: *Alimentary canal:* Mouth → Oesophagus → Stomach → Small intestine (duodenum, jejunum, ileum) → Large intestine (colon, rectum) → Anus. *Accessory organs:* Salivary glands, liver, pancreas, gallbladder. **Journey of bread from mouth to anus:** **Step 1: Ingestion & Mechanical Digestion (Mouth)** • Teeth cut, tear, and grind bread → smaller particles. • Saliva (from salivary glands) moistens and contains salivary amylase. • **Chemical digestion:** Salivary amylase breaks starch (bread carbohydrate) into maltose. • Bolus (food ball) formed and swallowed. **Step 2: Propulsion (Oesophagus)** • **Peristalsis:** Rhythmic muscular contractions push bolus downward. • Gravity aids movement. • Stomach acid inactivates salivary amylase; no further starch digestion occurs in oesophagus. **Step 3: Gastric Digestion (Stomach)** • Bread reaches stomach; mixes with gastric juice (HCl + pepsin + mucus). • **Mechanical digestion:** Stomach muscles churn, breaking bread into smaller pieces. • **Chemical digestion:** HCl activates pepsinogen → pepsin. Pepsin begins protein digestion (if bread contains protein/gluten). Carbohydrate digestion ceases (acidic pH). • Converts food to **chyme** (semi-liquid paste). • Stays in stomach 3–4 hours. **Step 4: Small Intestinal Digestion & Absorption (Duodenum, Jejunum, Ileum)** • Chyme enters duodenum; triggers release of cholecystokinin (CCK) & secretin hormones. • **Bile from liver:** Emulsifies lipids (if bread contains fat). • **Pancreatic juice:** Bicarbonate neutralizes HCl. Enzymes break down starch (pancreatic amylase → glucose), proteins (trypsin → amino acids), and fats (lipase → fatty acids & glycerol). • **Intestinal enzymes (brush border):** Disaccharidases (e.g., maltase) convert maltose → glucose. • **Absorption:** Glucose, amino acids, fatty acids enter epithelial cells via active transport and diffusion. Blood vessels carry nutrients to liver and cells. • Stays in small intestine 3–4 hours. **Step 5: Large Intestine (Colon & Rectum)** • Undigested cellulose, water, and electrolytes enter. • **Absorption:** Water and mineral ions (Na⁺, K⁺, Cl⁻) reabsorbed; faeces formed. • **Bacteria:** Symbiotic colonic bacteria ferment remaining carbohydrates, produce vitamins (K, B₁₂). • Stays 12–24 hours. **Step 6: Egestion (Anus)** • Faeces (indigestible matter, water, dead bacteria, bile pigments) expelled via defecation. **Timeline:** Bread takes approximately 24–72 hours for complete transit. --- **Q2. Explain the differences between photosynthesis and digestion. How is digestion essential for life?** **Answer:** | Aspect | Photosynthesis | Digestion | |--------|---|---| | **Definition** | Synthesis of organic food from CO₂ and H₂O using light energy | Breakdown of complex organic food into simple, absorbable molecules | | **Energy requirement** | Requires light energy (endergonic) | Releases energy (exergonic) | | **Location** | Chloroplasts of photosynthetic organisms | Alimentary canal | | **Organisms** | Only autotrophs (plants, algae, some bacteria) | All organisms require digestion | | **Process type** | Anabolic (building up) | Catabolic (breaking down) | | **Raw materials** | Inorganic (CO₂, H₂O, light) | Organic polymers (starch, proteins, lipids) | | **Products** | Glucose, oxygen | Monosaccharides, amino acids, fatty acids, glycerol | | **Purpose** | Store solar energy in chemical bonds | Release stored chemical energy for cellular use | **Why digestion is essential:** 1. **Nutrient absorption:** Polymers (starch, proteins, lipids) are too large to cross intestinal epithelium. Digestion reduces them to monomers (glucose, amino acids, fatty acids) that are absorbable via active/passive transport. 2. **Energy provision:** Glucose from digested carbohydrates enters glycolysis and aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2870 kJ energy (ATP production). 3. **Tissue synthesis:** Amino acids from protein digestion are required for synthesis of enzymes, antibodies, structural proteins (collagen, elastin), and muscle fibers. 4. **Metabolic regulation:** Digestive hormones (insulin, glucagon, gastrin, secretin) regulate blood glucose, enzyme secretion, and intestinal motility. 5. **Elimination of waste:** Digestion separates nutrients from indigestible matter (fibre, cellulose), which is expelled as faeces, preventing toxic accumulation. 6. **Pathogen defence:** Stomach HCl and proteases destroy ingested bacteria and viruses, providing non-specific immunity. **Conclusion:** Without digestion, even abundant food cannot be utilized. A person may starve despite eating, if digestion is impaired (e.g., pancreatitis, coeliac disease, cystic fibrosis). --- **Q3. Describe the structure of a tooth and explain how tooth structure reflects diet (herbivore vs. carnivore vs. omnivore).** **Answer:** **Structure of a tooth:** A typical molar has three main parts: 1. **Crown:** Exposed portion above the gum line. • *Enamel:* Hardest substance in the body (96% hydroxyapatite, Ca₅(PO₄)₃OH). Covers crown; lacks sensory nerves; regeneration is limited. • *Dentine:* Softer than enamel; contains micro-tubules connecting to pulp; exposed dentine is sensitive. 2. **Neck:** Junction between crown and root; covered by gum. 3. **Root:** Embedded in jawbone; anchored by periodontal ligament. • *Pulp:* Central cavity containing blood vessels, nerves, and connective tissue; provides nutrition and sensation. • *Cementum:* Bone-like material covering root; allows ligament attachment. **Types of teeth:** • **Incisors (8):** Chisel-shaped; for cutting and biting. • **Canines (4):** Pointed; for tearing flesh. • **Premolars (8):** Two cusps; for grinding. • **Molars (8–12):** Multiple cusps; for crushing and grinding. **Dental formula (humans):** 2.1.2.3 (per half jaw) = 32 teeth total. **Diet-based tooth structure:** **Herbivores (e.g., cow, horse):** • **Incisors:** Sharp, strong; cut grass and leaves. • **Canines:** Absent or greatly reduced (no predatory need). • **Molars:** Large, flat, with ridged surfaces (grinding cellulose). • **Jaw motion:** Side-to-side grinding action (lateral jaw movement). • **Dental formula example (cow):** 0.0.3.3 (per half jaw); 32 total. • **Justification:** Plant material (cellulose) requires prolonged grinding to expose cell contents and increase surface area for microbial fermentation in the rumen. **Carnivores (e.g., lion, dog):** • **Incisors:** Small, sharp; minimally used. • **Canines:** Long, pointed, sharp; pierce prey and tear flesh. (Considered "fangs" in predators.) • **Premolars (carnassials):** Sharp, blade-like; shear meat and bone. • **Molars:** Reduced or absent; not needed for grinding. • **Jaw motion:** Vertical (up-and-down) biting force; powerful masseter and temporalis muscles. • **Dental formula example (lion):** 3.1.3.1 (per half jaw); 30 total. • **Justification:** Protein and fat (meat) are easily digestible and require minimal mechanical processing. Canines are optimized for capturing and tearing prey. **Omnivores (e.g., humans, pigs):** • **Incisors:** Moderate size; bite and cut vegetables and meat. • **Canines:** Present but not elongated; tear tough foods. • **Premolars & molars:** Mix of sharp and flat surfaces; grind mixed diet. • **Jaw motion:** Both up-and-down (biting) and side-to-side (grinding). • **Dental formula:** 2.1.2.3 (per half jaw); 32 total. • **Justification:** Mixed diet (plants + meat) requires both cutting (carnassial function) and grinding (molar function) to efficiently process diverse foods. **Tooth adaptations summary table:** | Feature | Herbivores | Carnivores | Omnivores | |---------|---|---|---| | Canines | Reduced/absent | Large, pointed (fangs) | Moderate | | Molars | Large, flat, ridged | Reduced/absent | Medium, mixed cusps | | Jaw strength | Moderate (grinding) | Very powerful (tearing) | Moderate (mixed) | | Saliva amylase | High (starch digestion) | Low/absent (protein diet) | Moderate | | Digestive tract length | Long (slow fermentation) | Short (fast protein metabolism) | Medium | **Evolutionary significance:** Tooth structure is a primary indicator of trophic level and is used by palaeontologists to classify extinct animals.

**Case-Study: A Patient with Pancreatic Insufficiency** Rajeev, a 45-year-old patient, visited a gastroenterologist complaining of chronic diarrhoea, weight loss, and fatty, foul-smelling stools (steatorrhoea). Blood tests revealed low pancreatic enzyme levels and elevated blood glucose (120 mg/dL, normal = 70–100 mg/dL). **Questions:** 1. **Identify the problem and explain why steatorrhoea (fatty stools) occurs.** **Answer:** Rajeev likely has pancreatic insufficiency (chronic pancreatitis, cystic fibrosis, or pancreatic cancer). His pancreas cannot produce adequate digestive enzymes (pancreatic lipase, amylase, protease). Steatorrhoea mechanism: • Without pancreatic lipase, dietary fats (triglycerides) cannot be broken down into fatty acids and glycerol. • Although liver bile emulsifies fats, emulsified fats require lipase for hydrolysis. Without lipase, emulsified fat droplets pass unabsorbed into the large intestine. • Colonic bacteria ferment unabsorbed fat → gas (bloating), organic acids (osmotic diarrhoea). • Fatty stools are also malodorous because undigested fat and protein are fermented by bacteria, producing sulphur compounds (H₂S) and ammonia. --- 2. **Explain the elevated blood glucose and predict consequences of chronic malabsorption.** **Answer:** Elevated blood glucose (120 mg/dL > 100 mg/dL fasting): • Pancreatic insufficiency impairs both endocrine (insulin production) and exocrine (enzyme production) functions. • Without adequate insulin, glucose cannot enter cells efficiently → hyperglycemia (high blood glucose). • Glucose is lost in urine (if ≥ 180 mg/dL). • Risk of Type 2 diabetes. Consequences of chronic malabsorption: • **Protein malnutrition:** Reduced amino acid absorption → low albumin, weak immune response, muscle wasting, slow wound healing. • **Fat-soluble vitamin deficiency:** Impaired absorption of vitamins A, D, E, K → night blindness (vitamin A), weak bones (vitamin D), clotting disorders (vitamin K). • **Carbohydrate malabsorption:** Despite elevated blood glucose, cells cannot efficiently use glucose → fatigue, weight loss paradox. • **Iron and B₁₂ deficiency:** Without gastric intrinsic factor and adequate stomach acid, B₁₂ and iron absorption decline → anaemia, neuropathy. • **Electrolyte loss:** Chronic diarrhoea → dehydration, hypokalemia (low potassium), hyponatremia. --- 3. **Suggest interventions and explain how enzyme replacement therapy works.** **Answer:** **Interventions:** 1. **Pancreatic Enzyme Replacement Therapy (PERT):** • Oral pancreatin (derived from porcine pancreas) contains lipase, amylase, and protease in standardized units. • Dosing: Typically 25,000–40,000 lipase units per meal. • Taken with meals; enteric-coated capsules protect enzymes from stomach acid (HCl) and release them in the duodenum (pH > 6). • **Mechanism:** Exogenous enzymes replace missing pancreatic enzymes, enabling fat, protein, and carbohydrate digestion in the small intestine. 2. **Proton Pump Inhibitors (PPIs):** • Reduce stomach acid to prevent premature enzyme inactivation. • Example: Omeprazole. • Benefit: Increases pancreatin efficacy. 3. **Dietary modifications:** • Low-fat diet (< 50 g/day initially); increase gradually as tolerance improves. • Small, frequent meals (5–6 per day) reduce digestive burden. • Avoid alcohol and smoking (exacerbate pancreatitis). • Supplement fat-soluble vitamins (A, D, E, K) and B₁₂. 4. **Management of hyperglycemia:** • Antidiabetic medications (metformin, GLP-1 agonists) if glucose remains elevated after enzyme replacement. • Monitor blood glucose and HbA1c. **Expected outcomes:** • Resolution of diarrhoea within 2–4 weeks. • Weight gain and improved nutritional status. • Normalization of blood glucose with insulin secretion improvement. --- 4. **Why would a similar patient with gastric bypass surgery (weight-loss surgery) also develop malabsorption despite a functioning pancreas?** **Answer:** Gastric bypass (Roux-en-Y) surgery reduces stomach size and bypasses part of the small intestine (duodenum and proximal jejunum). Consequences: • **Reduced digestion time:** Food moves rapidly through the shortened small intestine → inadequate enzyme exposure. • **Reduced mixing:** Bile and pancreatic juice may not adequately mix with food. • **Reduced nutrient contact:** Smaller absorptive surface area (proximal small intestine bypassed). • **Selective deficiency:** Fat-soluble vitamins (A, D, K, E), B₁₂, iron, and calcium absorption particularly impaired because these require specialized transporters in the duodenum and proximal jejunum. • **Iron-deficiency anaemia:** Common post-bypass complication (requires lifelong supplementation). **Difference from pancreatic insufficiency:** In gastric bypass, pancreatic enzymes are normal and present, but the anatomical pathway for nutrient absorption is shortened. In pancreatic insufficiency, enzymes are absent. Both result in malabsorption but via different mechanisms.

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