BharatNotes Life processes — nutrition, respiration, transportation, and excretion — are the fundamental biological activities that keep organisms alive. For UPSC, this chapter bridges biology and health policy: understanding how the heart works underpins cardiovascular disease (India's leading cause of death), how kidneys function is essential for understanding dialysis access (a major health equity issue), and how the digestive system works explains malabsorption and malnutrition. GS2 (Health) and GS3 (Science & Tech) questions on NCDs, PMJAY, and healthcare infrastructure all connect back to the physiology in this chapter.
PART 1 — Quick Reference Tables
Nutrition: Autotrophic vs Heterotrophic
| Type | Definition | Mode | Examples |
|---|---|---|---|
| Autotrophic | Makes own food from inorganic sources | Photosynthesis (mainly), Chemosynthesis | Green plants, Algae, Cyanobacteria, Nitrifying bacteria |
| Holozoic | Ingests solid food; internal digestion | Ingestion → Digestion → Absorption → Assimilation → Egestion | Humans, Most animals |
| Saprophytic | Absorbs nutrients from dead organic matter | Secretes enzymes externally; absorbs products | Fungi, some bacteria |
| Parasitic | Obtains nutrition from living host | Attachment to host | Tapeworm, Plasmodium, Mistletoe |
Photosynthesis — Simplified Overview
| Stage | Location | Inputs | Outputs |
|---|---|---|---|
| Light reactions (photolysis) | Thylakoid membranes | Light, H2O, ADP, NADP+ | ATP, NADPH, O2 (released), electrons |
| Dark reactions (Calvin cycle) | Stroma | CO2, ATP, NADPH | Glucose (C6H12O6), ADP, NADP+ |
Overall equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Human Digestive System — Organs, Secretions, Functions
| Organ | Secretion/Enzyme | Action |
|---|---|---|
| Mouth | Salivary amylase (ptyalin) | Breaks starch → maltose |
| Stomach | HCl, pepsin, mucus | HCl kills bacteria, activates pepsin; pepsin digests proteins → peptides; mucus protects stomach wall |
| Small intestine — duodenum | Bile (from liver), pancreatic juice (amylase, trypsin, lipase) | Bile emulsifies fats; pancreatic enzymes digest carbs, proteins, fats |
| Small intestine — ileum | Intestinal juice (maltase, sucrase, lactase, peptidase) | Final digestion; villi absorb nutrients |
| Large intestine | Mucus, water absorption | Absorbs water; forms faeces; houses gut bacteria |
Aerobic vs Anaerobic Respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required | Yes | No |
| Location | Cytoplasm (glycolysis) + Mitochondria | Cytoplasm only |
| End products | CO2 + H2O | Lactic acid (animals/muscle) OR Ethanol + CO2 (yeast) |
| ATP yield | 36–38 ATP per glucose | 2 ATP per glucose |
| Duration | Sustained | Short bursts only |
| Example | Normal cellular respiration | Muscle during intense exercise; fermentation |
Human Heart — Chambers and Blood Flow
| Structure | Side | Function |
|---|---|---|
| Right atrium | Right | Receives deoxygenated blood from body (via superior/inferior vena cava) |
| Right ventricle | Right | Pumps deoxygenated blood to lungs (via pulmonary artery) |
| Left atrium | Left | Receives oxygenated blood from lungs (via pulmonary veins) |
| Left ventricle | Left | Pumps oxygenated blood to body (via aorta) — thickest walls |
| Tricuspid valve | Between right atrium & right ventricle | Prevents backflow |
| Bicuspid (mitral) valve | Between left atrium & left ventricle | Prevents backflow |
| Pulmonary semilunar valve | Right ventricle outlet | Prevents backflow into right ventricle |
| Aortic semilunar valve | Left ventricle outlet | Prevents backflow into left ventricle |
PART 2 — Detailed Notes
1. Nutrition — Getting Energy and Raw Materials
Photosynthesis is the process by which chlorophyll-containing organisms convert light energy to chemical energy (glucose):
The process occurs in two stages:
- Light-dependent reactions: In the thylakoid membranes. Light energy splits water molecules (photolysis) → releases oxygen, generates ATP and NADPH. Photosystems I and II capture light.
- Light-independent reactions (Calvin cycle): In the stroma. ATP and NADPH are used to fix CO2 into glucose. Key enzyme: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) — the most abundant enzyme on Earth.
Factors affecting photosynthesis: light intensity, CO2 concentration, temperature, water availability, chlorophyll content.
[Additional] C3 vs C4 Plants — Climate Change and Food Security (GS3):
The standard Calvin cycle (C3 pathway) fixes CO2 into a 3-carbon compound (3-phosphoglycerate). A problem: RuBisCO can also bind O2 instead of CO2 (photorespiration) — wasting energy, especially in hot conditions.
C4 pathway (evolved independently in ~60 plant lineages): CO2 is first fixed into a 4-carbon compound (oxaloacetate) in mesophyll cells, then concentrated and released around RuBisCO in bundle sheath cells. This concentrating mechanism suppresses photorespiration.
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First product of CO2 fixation | 3-phosphoglycerate (3C) | Oxaloacetate (4C) |
| Key enzyme | RuBisCO (in mesophyll) | PEP carboxylase (+ RuBisCO in bundle sheath) |
| Water use efficiency | Lower | ~40% higher |
| Temperature optimum | Cooler (15–25°C) | Warmer (30–40°C) |
| Photorespiration | Significant (reduces yield up to 40%) | Negligible |
| Crop examples | Wheat, rice, potato, soybean, most vegetables | Maize (corn), sugarcane, sorghum, millets (bajra, jowar), amaranth |
UPSC GS3 relevance:
- Climate change impact on agriculture: Rising temperatures favour C4 crops (maize, millets, sugarcane) but disadvantage C3 crops (wheat, rice) — a key concern for India's food security since wheat and rice are the staple crops (PDS, MSP system)
- Millets promotion (Shree Anna): India declared 2023 the International Year of Millets (with UN support); millets are C4 crops — drought-tolerant, heat-tolerant, less water-intensive — promoted as climate-smart crops
- Genetic engineering: Attempts to introduce C4 pathway into rice (C4 Rice Project, IRRI) to increase yield and water use efficiency — one of the most ambitious crop biotechnology projects
- CAM plants (a third pathway: Crassulacean Acid Metabolism): Open stomata at night, fix CO2 at night, photosynthesize in daytime — maximally water-efficient; cacti, succulents, agave (used for ethanol). Relevant to arid zone agriculture.
Photorespiration and its agricultural cost: In C3 crops (wheat, rice), photorespiration under warm conditions can reduce potential yield by 20–40%. This is why wheat production declines in India during unusually hot Rabi seasons (as observed in 2022 when a March heatwave cut wheat output, triggering an export ban).
Human Digestion:
The digestive system converts complex food molecules into absorbable nutrients. The alimentary canal runs from mouth to anus (~9 metres in adults).
Absorption in the small intestine: The inner surface of the small intestine is folded into finger-like projections called villi (singular: villus). Each villus is covered with microvilli (brush border), enormously increasing surface area. Inside each villus is a capillary network (absorbs glucose and amino acids → bloodstream) and a lacteal (lymph vessel — absorbs fatty acids and glycerol as chylomicrons).
Digestion summary:
- Carbohydrates → glucose, fructose, galactose (monosaccharides)
- Proteins → amino acids
- Fats → fatty acids + glycerol
- Nucleic acids → nucleotides
2. Respiration — Releasing Energy
Glycolysis (occurs in cytoplasm, common to both aerobic and anaerobic): Glucose (6C) → 2 Pyruvate (3C) → net gain of 2 ATP, 2 NADH
Aerobic continuation (in mitochondria):
- Pyruvate → Acetyl CoA (link reaction)
- Krebs cycle (citric acid cycle) — 8 steps in mitochondrial matrix; produces CO2, NADH, FADH2
- Oxidative phosphorylation (electron transport chain in inner mitochondrial membrane) — uses NADH and FADH2 to drive ATP synthase; produces ~34 ATP and water
Anaerobic pathways:
- In yeast (fermentation): Pyruvate → Ethanol + CO2 (basis of brewing, bread-making)
- In human muscle during intense exercise: Pyruvate → Lactic acid (causes muscle fatigue and burning sensation)
Human Respiratory System:
- Nasal cavity: Filters, warms, and humidifies air; olfactory receptors
- Pharynx → Larynx (voice box) → Trachea → Bronchi → Bronchioles → Alveoli
- Alveoli: Tiny air sacs with extremely thin walls (one cell layer), surrounded by capillaries. Site of gas exchange — O2 diffuses in; CO2 diffuses out. About 700 million alveoli in human lungs — surface area ~70 m2 (half a tennis court).
- Diaphragm and intercostal muscles contract during inhalation (increases thoracic volume, decreases pressure, air rushes in) and relax during exhalation.
Lung volumes:
- Tidal volume: ~500 mL (normal breath)
- Vital capacity: ~4,500 mL (maximum in+out)
- Residual volume: ~1,200 mL (always remains in lungs)
3. Transportation — Moving Substances Around
Human Circulatory System:
The heart is a double pump. The pulmonary circulation (right side) sends deoxygenated blood to lungs; the systemic circulation (left side) sends oxygenated blood to the body — this is double circulation.
Cardiac cycle:
- Diastole: Heart muscle relaxes; ventricles fill with blood from atria
- Atrial systole: Atria contract; blood pushed into ventricles
- Ventricular systole: Ventricles contract; blood pumped to lungs (right) and body (left)
Blood pressure: Force exerted by blood on vessel walls. Normal: 120/80 mmHg (systolic/diastolic). Hypertension (>140/90 mmHg) is a major risk factor for stroke, heart attack, kidney failure. India has an estimated 220 million hypertensives — one of the highest burdens globally.
Blood composition:
- Plasma (55%): Mostly water; carries dissolved proteins (albumin, fibrinogen, immunoglobulins), nutrients, hormones, CO2
- RBCs (erythrocytes): Contain haemoglobin (iron-containing protein); carry O2. No nucleus in mature human RBCs. Life span ~120 days.
- WBCs (leukocytes): Immune cells. Five types: neutrophils, eosinophils, basophils, lymphocytes, monocytes.
- Platelets (thrombocytes): Cell fragments; involved in blood clotting (coagulation cascade)
Lymphatic system: Network of vessels carrying lymph (tissue fluid that has entered lymph vessels). Lymph nodes filter pathogens. Lymph eventually returns to blood circulation via the thoracic duct.
Plant Transport:
- Xylem transports water and minerals upward. Driving force: transpiration pull — water evaporates from leaf stomata, creating tension that pulls water up.
- Phloem transports food (mainly sucrose) from leaves to other parts. Process: translocation driven by active loading of sugar at source (leaves) and unloading at sink (roots, fruits, seeds). Requires ATP.
4. Excretion — Removing Waste
Excretion is the removal of metabolic waste products. In humans, the main excretory organs are:
- Kidneys — excrete nitrogenous waste (urea), regulate water and salt balance
- Lungs — excrete CO2 and water vapour
- Skin — excretes small amounts of salts, urea, and water via sweat
- Liver — converts amino acids to urea (urea cycle); excretes bile pigments (bilirubin from broken-down haemoglobin)
Kidney structure and function:
The kidney contains about 1 million nephrons — the functional unit.
Nephron structure:
- Bowman's capsule — cup-shaped structure enclosing the glomerulus (knot of capillaries)
- Glomerulus — filtration under pressure; small molecules (water, glucose, urea, salts) pass through into Bowman's capsule → glomerular filtrate
- Proximal convoluted tubule (PCT) — reabsorbs glucose, amino acids, water, Na+
- Loop of Henle — concentrates the filtrate; descending limb is water-permeable; ascending limb is salt-permeable; creates concentration gradient in medulla
- Distal convoluted tubule (DCT) — further salt and water adjustment; regulated by hormones (ADH, aldosterone)
- Collecting duct — final concentration of urine; ADH (antidiuretic hormone) controls water reabsorption here
The filtrate becomes progressively concentrated as it passes through the nephron. Final urine is about 60 times more concentrated than blood plasma.
Dialysis: When kidneys fail, dialysis removes waste from blood artificially. Haemodialysis: Blood is pumped out, passed over a dialysis membrane, and returned. Peritoneal dialysis: Uses the peritoneum (abdominal membrane) as a natural filter.
🎯 UPSC Connect: Dialysis and Healthcare Access
India has approximately 220,000 dialysis patients, but only about 35% have access to regular dialysis — due to cost, distance, and infrastructure gaps. One haemodialysis session costs Rs 1,500–3,000; patients typically need 3 sessions per week. PM National Dialysis Programme under NHM provides free dialysis at district hospitals. PMJAY (Ayushman Bharat) covers dialysis-related costs.
Kidney transplantation (preferred over dialysis — better quality of life, less costly long-term) is governed by THOTA 1994. India has a severe shortage of donor kidneys. NOTTO (National Organ and Tissue Transplant Organisation) manages the national organ donation registry.
Excretion in plants:
- CO2 released during respiration, O2 released during photosynthesis — both escape through stomata and lenticels
- Excess water eliminated through transpiration (stomata) and guttation (special pores called hydathodes at leaf margins — occurs when water pressure is high and transpiration is low, typically at night)
- Some plants excrete waste into old leaves that are then shed (autumn leaf fall)
- Others deposit waste in vacuoles, bark, or as plant-insoluble crystals (calcium oxalate in taro leaves)
5. [Additional] Blood Groups and Transfusion — ABO System
Blood groups are determined by antigens (glycoproteins) on the surface of red blood cells and corresponding antibodies in plasma. The ABO system (discovered by Karl Landsteiner, 1901; Nobel Prize 1930):
| Blood Group | Antigen on RBC | Antibody in Plasma | Can donate to | Can receive from |
|---|---|---|---|---|
| A | A antigen | Anti-B antibody | A, AB | A, O |
| B | B antigen | Anti-A antibody | B, AB | B, O |
| AB | A + B antigens | None | AB only | All groups (Universal Recipient) |
| O | None | Anti-A + Anti-B | All groups (Universal Donor) | O only |
Rh factor: A separate antigen (D antigen). Rh+ = present; Rh− = absent. Critical in pregnancy: Rh− mother carrying Rh+ foetus may develop anti-Rh antibodies that attack foetal RBCs in subsequent pregnancies (haemolytic disease of the newborn / erythroblastosis foetalis). Prevented by anti-D immunoglobulin within 72 hours of first delivery.
Blood transfusion compatibility: Mismatched transfusion triggers agglutination (clumping of RBCs by antibodies) — a life-threatening haemolytic reaction; cross-matching before transfusion is mandatory.
[Additional] India's Blood Transfusion System (GS2 — Health):
India's annual blood requirement: ~14.6 million units. Collection in 2024-25: ~14.6 million units (15% rise over previous year). 70% from voluntary, non-remunerated donors — target is 100% voluntary (WHO standard).
National Blood Transfusion Council (NBTC): Constituted 1996 under MoHFW/NACO; apex body for national standards. Blood safety is linked to HIV prevention — HIV transmitted through unscreened blood is a historical crisis that drove blood bank regulation.
Key gaps: A 2024 geospatial analysis found ~60 districts in 8 northern states with no licensed blood bank; 80% of PHCs have no blood storage facility. Rural trauma victims and women with postpartum haemorrhage (PPH — leading cause of maternal death) face critical shortages.
Blood components: Whole blood is separated into packed RBCs, platelets (dengue/chemotherapy), fresh frozen plasma (FFP, clotting disorders), cryoprecipitate (haemophilia A). Component therapy maximises one unit's benefit for multiple patients.
Prelims trap: O negative (O−) is the true universal donor (no ABO or Rh antigen) — used in emergencies before blood type is known. O positive can donate to all Rh+ groups but NOT to Rh− patients.
6. [Additional] Insulin, Diabetes, and the Pancreas
The pancreas is both an exocrine gland (secretes digestive enzymes into the duodenum — amylase, lipase, trypsinogen) and an endocrine gland (secretes hormones into blood).
Islets of Langerhans — clusters of endocrine cells within the pancreas:
- Beta cells (β-cells): Secrete insulin — the only hormone that lowers blood glucose (promotes glucose uptake by cells; glycogen synthesis in liver/muscle; fat storage)
- Alpha cells (α-cells): Secrete glucagon — raises blood glucose (promotes glycogen breakdown in liver; gluconeogenesis)
- Delta cells: Secrete somatostatin — inhibits both insulin and glucagon (a regulatory brake)
Mechanism of insulin action: Blood glucose rises after a meal → beta cells release insulin → insulin binds receptors on muscle/liver/fat cells → cells take up glucose → blood glucose falls back to ~80–120 mg/dL → liver stores excess as glycogen.
Diabetes mellitus — failure of glucose regulation:
| Type | Mechanism | Onset | Treatment |
|---|---|---|---|
| Type 1 (T1DM) | Autoimmune destruction of beta cells — no insulin produced | Childhood/young adult | Insulin injections (lifelong) |
| Type 2 (T2DM) | Insulin resistance — cells don't respond; beta cells eventually exhaust | Adult (increasingly in youth) | Lifestyle; metformin; insulin (later) |
| Gestational diabetes | Insulin resistance during pregnancy | Pregnancy | Diet; insulin; resolves post-delivery |
[Additional] India's Diabetes Crisis (GS2 — Health):
IDF Diabetes Atlas, 11th Edition (April 2025): India has 89.8 million adults (aged 20–79) with diabetes — 2nd highest globally after China. Prevalence: ~10.5%. Projected: 134 million by 2045. Deaths in 2024: ~3.35 lakh (IDF).
South Asian paradox: Indians develop T2DM at lower BMI and younger ages than Europeans (“thin-fat” phenotype — lower muscle mass, higher visceral fat). ICMR-INDIAB study: Indians derive 65–75% of calories from carbohydrates (polished rice, refined wheat) — driving insulin resistance.
Policy:
- National Programme for Prevention and Control of NCDs (NP-NCD): Covers diabetes, hypertension, CVD, cancer; screening at Ayushman Arogya Mandirs (HWCs)
- Biocon recombinant insulin (Bengaluru): India produces recombinant human insulin domestically — reduces import dependency for this life-saving drug
Prelims trap: Insulin is produced in beta (β) cells of islets of Langerhans — NOT alpha cells, NOT the liver, NOT the entire pancreas. Glucagon (alpha cells) does the opposite of insulin.
PART 3 — Frameworks & Analysis
Framework: Life Processes and Non-Communicable Disease (NCD) Burden
Every life process discussed in this chapter corresponds to a major NCD challenge in India:
| Life Process | Disruption | Disease | India's Burden |
|---|---|---|---|
| Nutrition/Digestion | Malabsorption, overconsumption | Malnutrition vs obesity | 35.5% children under 5 stunted (NFHS-5, 2019-21); rising obesity |
| Respiration | Lung damage | COPD, asthma, lung cancer | 55 million COPD patients; tobacco use |
| Transportation | Blocked arteries, hypertension | CVD (heart disease, stroke) | India's #1 cause of death |
| Excretion (kidneys) | Renal failure | Chronic Kidney Disease | 17% of population has CKD risk |
Framework: Double Circulation and Blood Pressure Policy
Understanding double circulation explains why:
- Blood pressure varies between systemic (higher) and pulmonary circulation (lower)
- Left ventricular hypertrophy occurs in untreated hypertension (heart works harder)
- India's National Programme for Prevention and Control of NCDs targets hypertension, diabetes, and CVD through screening at Health and Wellness Centres
Exam Strategy
Prelims traps:
- Pulmonary artery carries deoxygenated blood (from right ventricle to lungs); pulmonary veins carry oxygenated blood (from lungs to left atrium) — the only vessels where artery/vein labels seem reversed.
- Guttation is NOT the same as transpiration — guttation is liquid water release through hydathodes; transpiration is water vapour loss through stomata.
- The left ventricle has the thickest wall (pumps to entire body; highest pressure).
- RBCs in humans have no nucleus at maturity (but do in birds, reptiles, fish).
- Anaerobic respiration in yeast produces ethanol + CO2; in muscles produces lactic acid (not CO2).
Mains frameworks:
- Dialysis access: kidney physiology → renal failure → cost burden → PM National Dialysis Programme → PMJAY → equity in healthcare
- NCDs: cardiovascular disease → risk factors (hypertension, diabetes, tobacco) → NCD programme → SDG 3.4
- Nutrition and malnutrition: digestion physiology → nutrient absorption → POSHAN Abhiyaan → SDG 2
Practice Questions
Q1 (Prelims 2022): With reference to the human body, which of the following statements about the lymphatic system is correct? (Tests: lymph composition, immune function, relation to blood circulation)
Q2 (Prelims 2018): With reference to "Pradhan Mantri National Dialysis Programme", which of the following statements is/are correct? (Direct policy application of kidney excretion physiology)
Q3 (Mains GS2 2021): "Non-communicable diseases are the new challenge facing India's health system." Critically examine with specific reference to cardiovascular diseases. Connects heart physiology → hypertension → CVD burden → NCD programme
Q4 (Prelims 2016): Consider the following statements with respect to photosynthesis… (Tests light vs dark reactions, role of chlorophyll, products of each stage)
