Life Processes

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

Photosynthesis — Simplified Overview

Overall equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Human Digestive System — Organs, Secretions, Functions

Aerobic vs Anaerobic Respiration

Human Heart — Chambers and Blood Flow

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.

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:

1. Diastole: Heart muscle relaxes; ventricles fill with blood from atria
2. Atrial systole: Atria contract; blood pushed into ventricles
3. 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:

1. Bowman's capsule — cup-shaped structure enclosing the glomerulus (knot of capillaries)
2. Glomerulus — filtration under pressure; small molecules (water, glucose, urea, salts) pass through into Bowman's capsule → glomerular filtrate
3. Proximal convoluted tubule (PCT) — reabsorbs glucose, amino acids, water, Na+
4. Loop of Henle — concentrates the filtrate; descending limb is water-permeable; ascending limb is salt-permeable; creates concentration gradient in medulla
5. Distal convoluted tubule (DCT) — further salt and water adjustment; regulated by hormones (ADH, aldosterone)
6. 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)

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:

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

Previous Year 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)