Nutrition is how organisms obtain and use food. It splits broadly into autotrophic (make own food) and heterotrophic (depend on others). CBSE Class 10 and 11 both touch on it. NEET asks about specific examples like insectivorous plants and parasites.
Core Concepts
Autotrophic nutrition
Make food from inorganic raw materials. Photoautotrophs use sunlight (plants, algae, cyanobacteria). Chemoautotrophs use chemical energy (sulphur bacteria, iron bacteria).
Photosynthesis — the master equation:
This happens in two stages: light reactions (in thylakoid membranes — water is split, ATP and NADPH are produced, O2 is released) and dark reactions or Calvin cycle (in stroma — CO2 is fixed into glucose using ATP and NADPH).
Chemoautotrophs are fascinating: they get energy not from sunlight but from oxidising inorganic chemicals. Nitrifying bacteria oxidise NH3 to NO2- to NO3- and use the energy to fix CO2. They live in soil and are essential for the nitrogen cycle.
Heterotrophic nutrition
Depend on others for food. Saprophytic — decomposers like fungi and bacteria. Parasitic — live on or in a host (Cuscuta, tapeworm). Holozoic — eat, digest and absorb (most animals).
Detailed comparison:
| Mode | Definition | Example | Key Feature |
|---|---|---|---|
| Saprophytic | Feed on dead organic matter | Bread mould (Rhizopus), mushrooms | Secrete enzymes externally, absorb digested food |
| Parasitic | Live on/in a host organism | Cuscuta (plant), tapeworm, Plasmodium | Harm the host, specialised attachment organs |
| Holozoic | Ingest, digest, absorb, assimilate | Amoeba, humans, all vertebrates | Have a digestive system or food vacuole |
| Symbiotic | Both organisms benefit | Lichens (algae + fungus), Rhizobium in legume roots | Mutual dependence |
Nutrition in plants
Photosynthesis in leaves produces glucose from CO2 and water using sunlight. Some plants are insectivorous — Drosera, Utricularia, Nepenthes — for supplementary nitrogen from insects in nitrogen-poor soils.
Insectivorous plants — why they eat insects: These plants grow in nutrient-poor soils (bogs, rocky terrain) where nitrogen and phosphorus are scarce. They photosynthesize normally for carbon, but supplement nitrogen by trapping and digesting insects.
| Plant | Trap Type | Mechanism |
|---|---|---|
| Drosera (sundew) | Sticky tentacles | Glistening droplets attract insects; tentacles curl around them |
| Nepenthes (pitcher) | Pitcher-shaped leaf | Insects slide into the pitcher and drown in digestive fluid |
| Utricularia (bladderwort) | Underwater bladders | Vacuum trap sucks in tiny aquatic organisms |
| Dionaea (Venus flytrap) | Snap trap | Two leaf lobes snap shut in 0.1 seconds when trigger hairs are touched |
NEET frequently asks: “Why are insectivorous plants not fully heterotrophic?” Answer: they photosynthesize for carbon (autotrophic) and only supplement nitrogen from insects. They are partially heterotrophic, not fully.
Nutrition in humans
Balanced diet includes carbohydrates, proteins, fats, vitamins, minerals and water. Deficiencies cause specific diseases — protein-energy malnutrition (kwashiorkor, marasmus), vitamin A (night blindness), vitamin D (rickets), iron (anaemia).
| Nutrient | Deficiency Disease | Key Symptom |
|---|---|---|
| Vitamin A | Night blindness (xerophthalmia) | Cannot see in dim light |
| Vitamin B1 | Beri-beri | Weakness, nerve damage |
| Vitamin C | Scurvy | Bleeding gums, poor wound healing |
| Vitamin D | Rickets (children), Osteomalacia (adults) | Soft, deformed bones |
| Vitamin K | Delayed blood clotting | Excessive bleeding |
| Iron | Anaemia | Fatigue, pale skin |
| Iodine | Goitre | Swollen thyroid gland |
| Protein | Kwashiorkor | Swollen belly, thin limbs |
| Protein + Calories | Marasmus | Severe wasting, skin and bones |
| Calcium | Osteoporosis | Brittle bones |
Five steps of holozoic nutrition
Ingestion (taking in), digestion (breaking down), absorption (crossing into blood), assimilation (using in cells), egestion (removing undigested). All animals follow this sequence.
In Amoeba: food is ingested by phagocytosis (pseudopodia surround the food particle to form a food vacuole). Enzymes are secreted into the vacuole for digestion. Digested molecules diffuse into the cytoplasm (absorption and assimilation). Undigested residue is expelled by exocytosis (egestion).
In humans, the same five steps happen across specialised organs: mouth (ingestion + initial digestion), stomach and small intestine (digestion), small intestine (absorption via villi), cells throughout the body (assimilation), large intestine and rectum (egestion of faeces).
Digestive enzymes and their specificity
| Enzyme | Source | Substrate | Product | Optimum pH |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose | 6.8 (neutral) |
| Pepsin | Stomach | Proteins | Peptides | 1.5-2 (acidic) |
| Trypsin | Pancreas | Proteins | Peptides | 7.5-8 (alkaline) |
| Lipase | Pancreas | Fats | Fatty acids + glycerol | 7-8 |
| Maltase | Small intestine | Maltose | Glucose | 7-8 |
| Sucrase | Small intestine | Sucrose | Glucose + fructose | 7-8 |
Pepsin and trypsin both digest proteins, but at different pH values. Pepsin works in the acidic stomach; trypsin works in the alkaline small intestine. This is why pH matters — each enzyme has an optimum pH that matches its working environment.
Worked Examples
They grow in nitrogen-poor soils like bogs. Photosynthesis gives them carbon, but nitrogen for proteins is scarce, so they supplement by digesting insects.
Cuscuta has no chlorophyll and no proper leaves. It wraps around a host plant and sends haustoria into the host’s vascular tissue to extract water, minerals and sugars.
Bile salts emulsify fats — they break large fat droplets into tiny micelles, increasing the surface area for lipase to act on. Without emulsification, fat digestion would be extremely slow because lipase can only work at the oil-water interface.
Both are protein-energy malnutrition, but kwashiorkor occurs when calorie intake is adequate but protein is deficient — the child has a swollen belly (edema from low blood albumin). Marasmus occurs when both calories and protein are deficient — severe overall wasting, skin-and-bones appearance.
Common Mistakes
Calling fungi photosynthetic. They are heterotrophs — saprophytic or parasitic, with no chlorophyll.
Confusing parasite and predator. Parasite lives on or in host without killing it immediately; predator kills and eats.
Writing that kwashiorkor is caused by lack of carbs. It is caused by protein deficiency despite adequate calories.
Saying insectivorous plants are fully heterotrophic. They photosynthesize for carbon and only supplement nitrogen/minerals from insects.
Confusing absorption and assimilation. Absorption is the passage of digested food from the gut into the blood. Assimilation is the use of absorbed nutrients by cells for energy, growth, and repair.
Exam Weightage and Revision
Nutrition appears in CBSE Class 10 (Life Processes, 4-6 marks) and Class 11 (Digestion and Absorption, 5-8 marks). NEET tests specific examples (insectivorous plants, parasites), enzyme specificity, and deficiency diseases. One question on the deficiency table appears almost every year.
| Question Type | NEET Frequency | Marks |
|---|---|---|
| Deficiency disease matching | Every year | 1-2 |
| Insectivorous plant example | Every 2 years | 1 |
| Enzyme-substrate matching | Most years | 1-2 |
| Autotrophic vs heterotrophic | CBSE focus | 2-3 |
| Digestion process steps | CBSE focus | 3-5 |
The deficiency disease table above is pure marks in NEET. Memorise the 10 rows. One question from this table is almost guaranteed every year.
Practice Questions
Q1. What are the differences between autotrophic and heterotrophic nutrition?
Autotrophic: organisms make their own food from inorganic materials (CO2, H2O) using energy (light or chemical). Examples: plants, algae, some bacteria. Heterotrophic: organisms depend on other organisms for food. They cannot synthesise organic molecules from inorganic ones. Examples: animals, fungi, most bacteria. The key difference is whether the organism can fix carbon independently.
Q2. A child has swollen belly, thin limbs, and retarded growth but is not extremely thin. What is the likely condition and its cause?
Kwashiorkor — caused by protein deficiency with adequate calorie intake. The swollen belly is due to edema: low blood albumin (because protein is needed to make albumin) reduces blood osmotic pressure, so fluid leaks from blood into tissues, especially the abdomen. The child is not extremely thin because calorie intake is sufficient.
Q3. Why does the stomach not digest itself?
The stomach lining secretes a thick layer of mucus that forms a protective barrier between the acid/pepsin and the stomach wall. Pepsin is also secreted as inactive pepsinogen, which is activated only in the stomach lumen by HCl. Additionally, the epithelial cells are rapidly replaced (every 3-5 days). If the mucus layer is damaged (by alcohol, NSAIDs, or H. pylori bacteria), ulcers can form.
FAQs
Why do we need to eat a balanced diet?
No single food provides all nutrients. Carbohydrates provide energy, proteins provide amino acids for growth and repair, fats provide concentrated energy and insulation, vitamins and minerals regulate metabolic processes, and water is the medium for all reactions. Deficiency in any one nutrient causes specific problems.
Can humans digest cellulose?
No. Humans lack cellulase, the enzyme that breaks down cellulose. Cellulose passes through our digestive system as dietary fibre, which helps maintain bowel health. Herbivores like cows have symbiotic bacteria in their rumen that produce cellulase and break down cellulose for them.
Why is vitamin C not stored in the body?
Vitamin C is water-soluble. Water-soluble vitamins dissolve in blood, cannot be stored in fat tissue, and are excreted in urine if in excess. This is why daily intake is necessary. Fat-soluble vitamins (A, D, E, K) are stored in fat and liver, so deficiency takes longer to develop.
Match deficiency to disease in one table — ten rows. PYQs love single-line deficiency questions.
Digestion in the Small Intestine — The Complete Picture
The small intestine is where 90% of chemical digestion and nearly all absorption occurs. Let us walk through what happens to a typical Indian meal (rice, dal, sabzi with oil).
Starch (from rice): Already partially broken down by salivary amylase in the mouth. In the small intestine, pancreatic amylase completes the breakdown: starch → maltose → glucose (by maltase). Glucose is absorbed into blood capillaries of the villi.
Protein (from dal): Partially digested by pepsin in the stomach (proteins → peptides). In the small intestine, trypsin and chymotrypsin from the pancreas break peptides into smaller peptides. Intestinal peptidases (aminopeptidase, dipeptidase) break these into amino acids. Amino acids are absorbed into blood capillaries.
Fat (from oil in sabzi): Bile salts from the liver emulsify fat globules into tiny micelles (increasing surface area). Pancreatic lipase then hydrolyses fats into fatty acids and glycerol. These are absorbed into lacteals (lymphatic capillaries in villi), not blood capillaries. From lacteals, they enter the lymphatic system and eventually reach the bloodstream via the thoracic duct.
Fibre (from vegetable cellulose): Not digested (humans lack cellulase). Passes to the large intestine where bacteria ferment some of it, producing short-chain fatty acids and gases. The remaining fibre adds bulk to faeces, promoting healthy bowel movements.
Absorption Through Villi — Structure and Function
Each villus is about 0.5-1 mm long and contains:
- A dense network of blood capillaries (for absorbing amino acids, sugars, water-soluble vitamins)
- A central lacteal (for absorbing fatty acids and fat-soluble vitamins A, D, E, K)
- Microvilli on the epithelial cells (brush border) that further increase surface area
The total absorptive surface area of the human small intestine (with folds, villi, and microvilli combined) is approximately 250 square metres — roughly the size of a tennis court. This enormous surface area is what makes absorption so efficient that almost 95% of ingested nutrients are absorbed.
Nutrition connects biology to health. Every food choice is a biology question in disguise.