Why this chapter matters for UPSC: Plants — their classification, functions, and role in ecosystems — are foundational for ecology, forest policy, and biodiversity topics in GS3. Photosynthesis is the basis of all food chains and understanding carbon sequestration (climate change mitigation).

PART 1 — Quick Reference Tables

Types of Plants

Type Height Stem Examples Examples in India
Herb Short (< 1m) Soft, green, tender Wheat, rice, coriander, mint, grass Most food crops
Shrub Medium (1–3m) Woody at base; branches from base Rose, tulsi, cotton, lemon Tulsi, hibiscus
Tree Tall (> 3m) Hard, thick, woody trunk Mango, banyan, teak, neem, peepal All major forest trees
Climber Variable Weak; needs support; climbs using tendrils/hooks Grapevine, bitter gourd, pea, passion fruit —
Creeper Low Weak; spreads along ground Pumpkin, watermelon, sweet potato —

Parts of a Plant and Their Functions

Part Function Notes
Root Anchors plant; absorbs water and minerals Taproot (carrot) vs fibrous root (grass)
Stem Transports water and nutrients; supports leaves Xylem (water up) + Phloem (food down)
Leaf Photosynthesis (food making); transpiration (water loss); gas exchange Chlorophyll in leaves; stomata for gas exchange
Flower Reproduction; attracts pollinators Contains male (stamen) and female (pistil) parts
Fruit Protects seed; aids seed dispersal Develops from ovary after fertilisation
Seed Dispersal + germination → new plant Contains embryo + stored food (endosperm)

PART 2 — Detailed Notes

Photosynthesis — The Foundation of Life

Key Term

Photosynthesis:

6CO₂ + 6H₂O + sunlight energy → C₆H₁₂O₆ (glucose) + 6O₂

Plants use sunlight (absorbed by chlorophyll — green pigment in chloroplasts), carbon dioxide (from air, through stomata), and water (absorbed by roots) to make glucose (food) and release oxygen.

Why photosynthesis matters:

  1. Primary producer: Plants are at the base of all food chains; all food ultimately comes from photosynthesis
  2. Oxygen: All the oxygen in Earth's atmosphere comes from photosynthesis (primarily by ancient cyanobacteria, then plants)
  3. Carbon sequestration: Photosynthesis removes CO₂ from the atmosphere; forests are major carbon sinks (critical for climate change mitigation)
  4. Fossil fuels: Coal and petroleum are fossilised plants/organisms — ancient stored photosynthetic energy
UPSC Connect

UPSC GS3 — Forests as carbon sinks:

India's forest cover is ~21.76% of geographical area (State of Forest Report 2023). India's forest and tree cover stores ~7,204 million tonnes of carbon — the equivalent of removing decades of CO₂ emissions.

India's NDC (Nationally Determined Contribution) to the Paris Agreement includes a target to create an additional carbon sink of 2.5–3 billion tonnes of CO₂ equivalent through additional forest and tree cover by 2030.

Green India Mission (one of 8 National Missions under NAPCC): Aims to increase forest cover on degraded forests and expand forest cover on non-forest land — directly building photosynthetic carbon sink capacity.

REDD+ (Reducing Emissions from Deforestation and Forest Degradation): UN Framework under UNFCCC; India participates; compensates countries for preserving forests (thus preserving carbon sinks).

Plant Adaptations — Biodiversity Connection

Plants have adapted to every habitat on Earth:

  • Desert plants (xerophytes): Cactus — thick stem stores water; leaves reduced to spines (reduce transpiration); deep roots
  • Water plants (hydrophytes): Lotus — flat leaves float; air spaces for buoyancy; roots in mud
  • Mangroves: Salt-tolerant; aerial roots (pneumatophores) for oxygen in waterlogged soil; found in Sundarbans (world's largest mangrove forest), Bhitarkanika, Pichavaram
  • Alpine plants: Low-growing; thick leaves; adapted to cold, thin air; found in Himalayan zone

Leaves — Stomata and Transpiration

Explainer

Stomata (singular: stoma): Tiny pores on leaf surface (mostly undersurface) through which:

  • CO₂ enters for photosynthesis
  • O₂ exits
  • Water vapour exits (transpiration)

Transpiration: Water loss from plants through leaves. A large tree can transpire hundreds of litres of water per day — contributing to local humidity and rainfall. This is why deforestation affects regional rainfall patterns (critical for India's monsoon).

Guard cells: Each stoma is surrounded by two guard cells that open/close the pore based on water availability and light.

Exam Strategy

Prelims traps:

  • Chlorophyll is in the chloroplast (organelle in plant cells) — NOT in the nucleus
  • Xylem transports water upward (root → stem → leaf); Phloem transports food (sugar) downward (leaf → stem → root) — and upward to growing tips
  • Stomata are mostly on the underside (lower surface) of leaves — reduces water loss from direct sunlight
  • Transpiration affects local rainfall — major ecological reason against deforestation
  • India's forest cover: ~21.76% of geographical area (SFR 2023) — needs to reach 33% as per Forest Policy 1988

Previous Year Questions

Prelims:

  1. The process by which plants make food using sunlight, CO₂, and water is called:
    (a) Photosynthesis
    (b) Transpiration
    (c) Respiration
    (d) Germination

  2. Xylem tissue in plants is responsible for:
    (a) Transport of water from roots to leaves
    (b) Transport of food from leaves to roots
    (c) Gas exchange
    (d) Photosynthesis

  3. India's NDC target for additional carbon sink through forests by 2030 is:
    (a) 1 billion tonnes CO₂ equivalent
    (b) 2.5–3 billion tonnes CO₂ equivalent
    (c) 5 billion tonnes CO₂ equivalent
    (d) 500 million tonnes CO₂ equivalent