Why this chapter matters for UPSC: Photosynthesis is the foundation of all food chains and the global carbon cycle — both core UPSC environment topics. Plant life processes connect to forest conservation, climate change, ecosystem services, and India's biodiversity policy. The role of forests as carbon sinks is a major climate negotiation issue.

PART 1 — Quick Reference Tables

Life Process Definition Organ/Structure
Photosynthesis Synthesis of food using CO₂, water, and sunlight Leaves (chloroplasts)
Respiration Breaking down glucose to release energy All living cells (mitochondria)
Transpiration Loss of water vapour through stomata Leaves (stomata)
Transport (water) Absorption and movement of water and minerals Roots → xylem → leaves
Transport (food) Movement of synthesised food to all parts Phloem (leaves → all parts)
Reproduction Sexual (seed) and asexual (vegetative) Flowers, bulbs, runners, rhizomes
Photosynthesis — Key Facts Detail
Raw materials Carbon dioxide (CO₂) + Water (H₂O)
Energy source Sunlight (absorbed by chlorophyll)
Products Glucose (C₆H₁₂O₆) + Oxygen (O₂)
Overall equation 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Location Chloroplasts (in mesophyll cells of leaves)
Pigment Chlorophyll (green; absorbs red and blue light best; reflects green)
Conditions needed Light, chlorophyll, CO₂, water, suitable temperature
Plant Transport System Structure Function
Xylem Hollow tubes; dead cells Transports water and minerals from roots to leaves (unidirectional, upward)
Phloem Living cells with sieve tubes and companion cells Transports food (glucose/sucrose) from leaves to all plant parts (bidirectional)
Root hairs Thin extensions of root cells Increase surface area for water and mineral absorption
Stomata Tiny pores in leaves controlled by guard cells Gas exchange (CO₂ in, O₂ and water vapour out); transpiration

PART 2 — Detailed Notes

Key Term

Photosynthesis: The process by which green plants (and some bacteria) convert light energy into chemical energy (glucose), using carbon dioxide and water.

6CO₂ + 6H₂O → (light energy, chlorophyll) → C₆H₁₂O₆ + 6O₂

Two stages:

  1. Light reactions (Photo stage): Occur in thylakoid membranes; water is split (photolysis), releasing O₂; ATP and NADPH produced
  2. Calvin Cycle (Synthesis stage): Occur in stroma; CO₂ is fixed into glucose using ATP and NADPH

Chlorophyll: The green pigment that absorbs sunlight. Found in chloroplasts. Contains magnesium at its centre. Absorbs red and blue light; reflects green light (hence leaves appear green).

Stomata (singular: Stoma): Tiny pores on the surface of leaves, mainly on the underside. Surrounded by two guard cells that regulate opening and closing. Open during day for CO₂ intake and close at night. Also allow water vapour loss (transpiration).

Transpiration: Loss of water in the form of vapour from plant surfaces (mainly through stomata). Creates a pull (transpiration pull) that draws water up the xylem from roots. Contributes to local humidity and the water cycle.

Nitrogen Fixation: Plants need nitrogen (for proteins, DNA). Most cannot use atmospheric N₂ directly. Nitrogen-fixing bacteria (Rhizobium — in root nodules of legumes; Azotobacter, Cyanobacteria — free-living) convert N₂ into ammonia (NH₃), which plants can absorb. This is why legumes (pulses) are important for soil fertility and crop rotation.

Saprophytes: Plants that feed on dead and decaying organic matter. Example: mushrooms, bracket fungi, Monotropa. Unlike green plants, they have no chlorophyll.

UPSC Connect

Forests as Carbon Sinks — Climate Change

Forests store carbon in their biomass (wood, roots, leaves) and in soil. Global forests absorb approximately 2.6 billion tonnes of CO₂ per year (about 25% of annual human emissions).

India's Forest Cover:

  • India's total forest and tree cover: 25.17% of geographic area (ISFR 2023)
  • Target under National Forest Policy 1988: 33% geographic area under forest/tree cover
  • India sequestered 2.29 billion tonnes of CO₂ equivalent through its forest and tree cover as of 2021 (BUR submission to UNFCCC)
  • India's NDC (2022): Create additional carbon sink of 2.5–3 billion tonnes of CO₂ equivalent through forests and tree cover by 2030

Green India Mission (GIM): One of the 8 missions under National Action Plan on Climate Change (NAPCC); aims to increase forest cover on 5 million ha and improve quality of forest cover on another 5 million ha by 2030.

Deforestation and Carbon Release:

  • Deforestation converts forests from carbon sinks to carbon sources
  • Amazon deforestation, South Asian forest fires, and Southeast Asian palm oil plantations are the largest sources of land-use change emissions globally
  • REDD+ (Reducing Emissions from Deforestation and Forest Degradation) — UN mechanism to compensate developing countries for preserving forests

Nitrogen Cycle and Agriculture:

  • Synthetic nitrogen fertilisers (urea, DAP) supply nitrogen to crops but cause problems: runoff leads to eutrophication; excess use causes soil acidification; production is energy-intensive (uses natural gas)
  • Biofertilisers (Rhizobium, Azospirillum, PSB — Phosphate Solubilising Bacteria) promote natural nitrogen fixation — promoted under PM Pranam (Programme for Restoration, Awareness, Nourishment and Amelioration of Mother Earth)
  • Pulse crops in crop rotation fix atmospheric nitrogen, reducing fertiliser requirement
Explainer

Ecosystem Services from Plants — UPSC GS3 Mains

Plants provide essential ecosystem services that have economic value:

  1. Provisioning services: Food (crops, fruits), timber, fibre, medicinal plants, rubber, resins
  2. Regulating services: Carbon sequestration, oxygen production, water cycle regulation, flood control, soil stabilisation, pollination, pest control
  3. Cultural services: Aesthetic value, spiritual significance (sacred groves — devaravana in Karnataka, orans in Rajasthan)
  4. Supporting services: Nutrient cycling (nitrogen, phosphorus, carbon), habitat for biodiversity, primary production

Economic Valuation of Ecosystem Services: The TEEB (The Economics of Ecosystems and Biodiversity) framework and India's National Biodiversity Action Plan attempt to value these services. India's Compensatory Afforestation Fund Act (CAMPA), 2016 collects funds from industries diverting forest land and uses them for afforestation — recognising the economic value of forests.

Photosynthesis and Food Security: All food ultimately derives from photosynthesis. Net Primary Productivity (NPP) — the rate of photosynthetic production minus plant respiration — determines the maximum food available to all other organisms in an ecosystem. Threats to plant productivity (climate change, drought, soil degradation, air pollution with tropospheric ozone) threaten global food security.

CAM and C4 Plants — Adaptation to Stress:

  • C3 plants (rice, wheat, soybeans — most crops): Standard photosynthesis; less efficient in hot, dry conditions
  • C4 plants (maize, sugarcane, sorghum): More efficient in hot, sunny conditions; concentrate CO₂ around RuBisCO enzyme
  • CAM plants (cacti, succulents, pineapple): Open stomata only at night to conserve water; store CO₂ for daytime photosynthesis Climate change may advantage C4 crops in warmer regions — relevant to crop adaptation strategies.

Exam Strategy

  • Photosynthesis equation: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Know the reactants, products, and conditions. Plants release O₂ as a by-product — common Prelims trap: "what gas is released by plants during photosynthesis" (during day: O₂; during both day and night: CO₂ from respiration).
  • Xylem transports water and minerals (upward, unidirectional, from roots to leaves). Phloem transports food/sugar (bidirectional, from source to sink — leaves to roots or growing regions). Classic MCQ distinction.
  • India's forest cover target is 33% (National Forest Policy 1988). Current coverage is ~25.17% (ISFR 2023). The gap is a key policy concern.
  • Nitrogen-fixing bacteria: Rhizobium (symbiotic, in root nodules of legumes — peas, beans, lentils, groundnut); Azotobacter and Cyanobacteria (free-living in soil). This connects to biofertilisers and sustainable agriculture.
  • REDD+ (Reducing Emissions from Deforestation and Forest Degradation) is the UN climate mechanism to compensate developing countries for conserving forests. India is a recipient country. Administered under UNFCCC.
  • Green India Mission: One of 8 NAPCC missions. Target: add 5 million ha + improve quality of 5 million ha forest cover. Ministry of Environment, Forest and Climate Change.

Previous Year Questions

Q1. Consider the following statements about photosynthesis:

  1. Photosynthesis occurs only during daytime.
  2. Oxygen released during photosynthesis comes from the splitting of water molecules.
  3. Chlorophyll absorbs mainly yellow and green light for photosynthesis.

Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

(b) 1 and 2 only

Q2. Rhizobium bacteria are associated with nitrogen fixation in:
(a) Root nodules of cereal crops like wheat and rice
(b) Root nodules of leguminous plants like peas and groundnut
(c) Stems of aquatic plants
(d) Leaves of tropical trees

(b) Root nodules of leguminous plants like peas and groundnut

Q3. Which of the following is/are correct about REDD+?

  1. It stands for Reducing Emissions from Deforestation and Forest Degradation.
  2. It is a mechanism under the UNFCCC to compensate developing countries for forest conservation.
  3. India does not participate in REDD+ as it is a developed economy.

Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

(b) 1 and 2 only