Nutrition — Class 10 Biological Science
"Every living thing must EAT. The plant makes its own food from sunlight. The animal must find, ingest, and digest. Nutrition is the FIRST life process."
1. About the Chapter
Nutrition is the process by which organisms OBTAIN and USE food. This chapter covers:
- Autotrophic nutrition: How plants make their own food (Photosynthesis)
- Heterotrophic nutrition: How animals and non-green plants obtain food
- The human digestive system: The complete journey of food through the body
- Enzymes: The biological catalysts that make digestion possible
- Nutritional disorders and their prevention
Why This Chapter Matters
- Photosynthesis produces the OXYGEN we breathe and the FOOD we eat
- Understanding digestion helps you make better FOOD CHOICES
- Nutritional diseases (kwashiorkor, marasmus, anaemia) remain MAJOR health challenges in India
- In Andhra Pradesh, malnutrition and anaemia are significant public health concerns
2. Autotrophic Nutrition — Photosynthesis
Definition
Autotrophic = 'Self-feeding' (auto = self. troph = nourishment). Organisms that can MAKE their own food from simple inorganic substances using an external energy source.
The Photosynthesis Equation
6CO₂ + 12H₂O →(Sunlight, Chlorophyll)→ C₆H₁₂O₆ + 6O₂ + 6H₂O
Where Does Photosynthesis Happen?
Photosynthesis occurs in the CHLOROPLASTS — organelles found in the mesophyll cells of leaves. Chloroplasts contain CHLOROPHYLL, the green pigment that traps SUNLIGHT energy. 'A single leaf cell may contain 40-50 chloroplasts. A square millimetre of leaf surface has about 500,000 chloroplasts.'
Raw Materials — Where Do They Come From?
| Raw Material | Source | How It Reaches the Leaf |
|---|---|---|
| Carbon Dioxide (CO₂) | Atmosphere | Enters through STOMATA — tiny pores on the leaf surface (mainly underside). Each stoma is surrounded by two GUARD CELLS that control its opening and closing. |
| Water (H₂O) | Soil | Absorbed by ROOT HAIRS through osmosis. Travels UP through XYLEM vessels to the leaves. Driven by TRANSPIRATION PULL. |
| Sunlight | Sun | Trapped by CHLOROPHYLL molecules in the thylakoid membranes of chloroplasts. |
| Chlorophyll | Made by the plant | Contains MAGNESIUM (Mg) at its centre. This is why magnesium deficiency causes yellowing of leaves (chlorosis). |
The Two Phases of Photosynthesis
Light Reaction (Light-Dependent Phase)
- Location: THYLAKOID MEMBRANES of chloroplasts
- What happens: Chlorophyll absorbs light energy → Water molecules are SPLIT (photolysis) → OXYGEN is released as a byproduct. The light energy is converted to chemical energy stored in ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
- Key point: This phase REQUIRES light. It produces the energy currency (ATP, NADPH) needed for the next phase.
Dark Reaction (Calvin Cycle / Light-Independent Phase)
- Location: STROMA (fluid-filled space) of chloroplasts
- What happens: CO₂ is FIXED (captured) using the enzyme RuBisCO. Using the ATP and NADPH produced in the light reaction, CO₂ is REDUCED to GLUCOSE (C₆H₁₂O₆).
- Key point: This phase does NOT directly require light. But it DEPENDS on ATP and NADPH from the light reaction — so it STOPS when light is absent.
Products of Photosynthesis
- GLUCOSE (primary product): Immediately converted to STARCH for storage. Starch is INSOLUBLE — it doesn't affect the cell's osmotic balance. Test for starch: IODINE solution turns BLUE-BLACK.
- OXYGEN: Released through stomata. This oxygen is the source of the oxygen we breathe.
Factors Affecting Photosynthesis Rate
| Factor | Effect |
|---|---|
| Light intensity | Rate INCREASES with light — up to a point. Beyond the saturation point: NO further increase (some other factor becomes limiting). |
| Carbon dioxide concentration | Rate increases with CO₂. Atmospheric CO₂ is ~0.04% (420 ppm). In greenhouses, CO₂ is artificially increased to boost crop yields. |
| Temperature | Rate increases up to ~35°C. Beyond this: ENZYMES DENATURE → rate drops sharply. |
| Water availability | Water stress → stomata CLOSE to conserve water → CO₂ cannot enter → photosynthesis STOPS. |
Liebig's Law of the Minimum: 'The rate of photosynthesis is limited by the FACTOR IN SHORTEST SUPPLY — not by the factors that are abundant. If CO₂ is scarce, adding more light will NOT increase photosynthesis.'
3. Heterotrophic Nutrition
Definition
Heterotrophic = 'Other-feeding' (hetero = other). Organisms that CANNOT make their own food. They depend on OTHER organisms for nutrition.
Types of Heterotrophic Nutrition
| Type | Description | Examples | Digestive System |
|---|---|---|---|
| Holozoic | Ingest SOLID food. Digest internally. | Humans, lion, frog, amoeba | Well-developed digestive system |
| Saprophytic (Saprotrophic) | Absorb nutrients from DEAD and DECAYING organic matter. Secrete enzymes OUTSIDE the body. | Fungi (mushroom, bread mould), bacteria | No digestive system — extracellular digestion |
| Parasitic | Live ON or INSIDE a host organism. Derive nutrition at the HOST'S EXPENSE. | Tapeworm (intestine), Cuscuta/dodder (plant parasite), head lice, Plasmodium (malaria) | Simplified or absent — absorb pre-digested nutrients from host |
4. The Human Digestive System — Step by Step
The Complete Path
Mouth → Oesophagus (Food Pipe) → Stomach → Small Intestine → Large Intestine → Rectum → Anus
Step 1 — The Mouth (Buccal Cavity)
- Teeth: MECHANICAL breakdown. Four types — Incisors (cutting, 8), Canines (tearing, 4), Premolars (crushing, 8), Molars (grinding, 12). Dental formula (adult): 2-1-2-3 / 2-1-2-3 = 32 teeth.
- Tongue: Mixes food with saliva. Taste buds detect: sweet, salty, sour, bitter, umami (savoury).
- Salivary Glands (3 pairs — parotid, submandibular, sublingual): Secrete SALIVA (~1-1.5 L/day). Contains SALIVARY AMYLASE (ptyalin) — breaks STARCH into MALTOSE. 'Chew a piece of bread for 2 minutes. It starts tasting SWEET — the amylase is breaking starch into sugar!'
Step 2 — The Oesophagus (Food Pipe)
- A muscular tube (~25 cm long). Food moves by PERISTALSIS — rhythmic contraction and relaxation of muscles, pushing the food bolus DOWNWARD. 'Peristalsis works AGAINST gravity. You can swallow food even while standing on your head!'
Step 3 — The Stomach
- J-shaped, muscular bag. Inner lining has GASTRIC GLANDS.
- Gastric Juice contains:
- Hydrochloric Acid (HCl) : Kills bacteria that entered with food. Provides ACIDIC medium (pH ~1-2) needed for pepsin to work. 'The stomach lining is protected from HCl by a thick MUCUS layer. If this layer breaks → ULCER.'
- Pepsinogen (inactive) → activated by HCl → PEPSIN (active). Pepsin breaks PROTEINS into smaller peptides. 'Pepsin is a PROTEASE — an enzyme that digests proteins.'
- Mucus: Protects the stomach lining from HCl and pepsin.
- Food stays in the stomach for 3-4 hours. The stomach CHURNS and mixes food with gastric juice → semi-liquid paste called CHYME.
Step 4 — The Small Intestine
- LONGEST part (~6-7 metres). Three sections: Duodenum, Jejunum, Ileum.
- Site of COMPLETE DIGESTION and ABSORPTION of nutrients.
Digestive Juices Acting in the Small Intestine:
| Juice | Source | Key Enzymes | Substrate → Product |
|---|---|---|---|
| Bile | LIVER (stored in gall bladder) | NO enzymes | EMULSIFIES fats — breaks large fat globules into tiny droplets. Increases surface area for lipase. 'Bile is a GREENISH-YELLOW fluid. It is NOT an enzyme — it's a MECHANICAL aid.' |
| Pancreatic Juice | PANCREAS | Trypsin. Lipase. Amylase. | Trypsin: Proteins → Peptides. Lipase: Fats → Fatty acids + Glycerol. Amylase: Starch → Maltose. |
| Intestinal Juice (Succus Entericus) | Intestinal glands | Maltase. Lactase. Sucrase. Peptidases. | Maltase: Maltose → Glucose. Peptidases: Peptides → Amino acids. |
Absorption — The Villi The inner wall of the small intestine has millions of tiny finger-like projections called VILLI (singular: villus). 'Villi increase the surface area for absorption by ~600 times — like folds in a towel. Each villus contains a network of BLOOD CAPILLARIES and a central LACTEAL (lymph vessel).' Glucose and amino acids → absorbed into BLOOD. Fatty acids and glycerol → absorbed into LYMPH (through lacteals).
Step 5 — The Large Intestine
- Wider but SHORTER (~1.5 m). No villi. No digestive enzymes.
- Functions: Absorption of WATER and MINERALS (Na⁺, K⁺). Hosts beneficial bacteria (E. coli) that produce VITAMINS (B complex, K). Formation and temporary storage of FAECES.
- Rectum stores faeces. Anus expels it (EGESTION — not excretion).
5. Key Enzymes — Summary Table
| Enzyme | Source | Site of Action | Substrate | Product | Optimal pH |
|---|---|---|---|---|---|
| Salivary Amylase (Ptyalin) | Salivary glands | Mouth | Starch | Maltose | ~7 (neutral) |
| Pepsin | Stomach (as pepsinogen) | Stomach | Proteins | Peptides | ~2 (acidic) |
| Trypsin | Pancreas | Small intestine | Proteins | Peptides | ~8 (alkaline) |
| Lipase | Pancreas | Small intestine | Fats | Fatty acids + Glycerol | ~8 |
| Pancreatic Amylase | Pancreas | Small intestine | Starch | Maltose | ~8 |
| Maltase | Intestinal glands | Small intestine | Maltose | Glucose | ~7.5 |
6. Common Mistakes
-
'Bile contains enzymes for fat digestion' — WRONG. Bile contains NO enzymes. It EMULSIFIES fats — a PHYSICAL process (breaking large globules into smaller ones), not a chemical one. Lipase from the pancreas does the actual CHEMICAL digestion of fats.
-
'Digestion happens ONLY in the stomach' — Digestion BEGINS in the MOUTH (starch → maltose by salivary amylase). It CONTINUES in the stomach (proteins → peptides). It is COMPLETED in the small intestine.
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'Absorption happens in the stomach' — VERY LITTLE absorption occurs in the stomach (only alcohol, some water, and aspirin). The VAST MAJORITY of nutrient absorption occurs in the SMALL INTESTINE through villi.
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'Enzymes work at any pH' — Each enzyme has an OPTIMAL pH. Pepsin works best at pH ~2 (acidic). Trypsin works best at pH ~8 (alkaline). 'If you put pepsin in the alkaline small intestine — it would DENATURE and stop working.'
-
'Plants photosynthesise during the day and respire at night' — Plants photosynthesise ONLY during the day (need light). But they RESPIRE ALL THE TIME — day AND night. 'During the day, photosynthesis produces more O₂ than respiration consumes. At night, only respiration occurs — plants take IN O₂ and release CO₂.'
7. AP SSC Exam Focus
| Topic | Typical Marks | Question Type |
|---|---|---|
| Photosynthesis equation and site | 2-3 | MCQ or Short Answer |
| Digestive system organs and enzymes | 4-5 | Diagram + Table |
| Types of heterotrophic nutrition | 2-3 | MCQ or Short Answer |
| Steps of holozoic nutrition | 3-4 | Short Answer |
| Bile function (trick question!) | 1-2 | MCQ |
Key Diagram to Practice
Draw and label the HUMAN DIGESTIVE SYSTEM. Mark: mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas, gall bladder. 'This diagram appears in almost EVERY AP SSC Biology paper.'
