Photosynthesis in Higher Plants
How green plants capture light energy to synthesise food (glucose) from CO2 and water while releasing oxygen, detailing the photosynthetic machinery and its light and dark reactions.
Photosynthesis is high-yield basic science for Prelims, which regularly tests pigments, photosystems (P680/P700), products of the light reaction, the C3/C4 distinction and the scientists involved. For Mains it feeds GS-III themes in Science & Technology and Environment — food security, the carbon-oxygen cycle, crop productivity, and renewable (solar/artificial photosynthesis) energy research.
Understand the chapter
Photosynthesis: The Basis of Life on Earth
Photosynthesis is a physico-chemical process in which green plants use light energy to synthesise organic compounds from CO2 and water. Green plants are autotrophs that make their own food, while all other organisms are heterotrophs that depend on them. It matters for two reasons: it is the primary source of all food on earth and it releases oxygen into the atmosphere.
- Autotroph: green plant that synthesises its own food via photosynthesis.
- Heterotroph: organism dependent on green plants for food.
- Twin significance: primary food source + source of atmospheric O2.
- Occurs in all green parts (mesophyll cells), not only in leaves.
Early Experiments — How the Story Unfolded
A chain of classic experiments built our understanding. Priestley showed plants restore air damaged by candles and animals; Ingenhousz proved sunlight and the green parts are essential and identified the bubbles as oxygen; Sachs traced glucose and starch to chloroplasts; Engelmann mapped the first action spectrum; van Niel proved the evolved oxygen comes from water, not CO2.
- Priestley (1770): bell-jar experiment — plants restore air; he discovered O2 in 1774.
- Ingenhousz: sunlight is essential and only green parts release O2.
- Sachs (1854): glucose made in green parts, stored as starch in chloroplasts.
- Engelmann: split light + aerobic bacteria gave the first action spectrum (blue & red peaks).
Site of Photosynthesis — The Chloroplast
Photosynthesis occurs in the chloroplasts of mesophyll cells, which show a clear division of labour. The membrane system (grana/thylakoids) traps light and synthesises ATP and NADPH — the light reactions. The stroma carries out enzymatic sugar synthesis — the dark (carbon) reactions — which are not driven directly by light but depend on the products of the light reaction.
- Grana/thylakoids: site of light reactions (ATP + NADPH + O2).
- Stroma: site of dark reactions (Calvin cycle, sugar then starch).
- Dark reaction is a misnomer: it does not occur in darkness; it needs ATP/NADPH.
- Light reactions are photochemical; dark reactions are enzyme-driven.
Photosynthetic Pigments
Leaf colour arises from four pigments separable by paper chromatography: chlorophyll a, chlorophyll b, xanthophylls and carotenoids. Chlorophyll a is the chief pigment forming the reaction centre, while the others are accessory pigments. Maximum absorption and maximum photosynthesis both occur in the blue and red regions of the spectrum.
- Chlorophyll a: bright/blue-green — chief and most abundant pigment.
- Accessory pigments: chlorophyll b (yellow-green), xanthophylls (yellow), carotenoids (yellow-orange).
- Accessory pigments widen usable wavelengths and protect chlorophyll a from photo-oxidation.
- Absorption spectrum of chl a roughly matches the action spectrum of photosynthesis.
Light Reaction & the Two Photosystems
The light (photochemical) phase covers light absorption, water splitting, oxygen release and formation of ATP and NADPH. Pigments are organised into two light-harvesting complexes — Photosystem I and Photosystem II — named in order of discovery, not function. Each photosystem has antennae pigments that funnel energy to a single reaction-centre chlorophyll a.
- PS I reaction centre = P700 (absorption peak 700 nm).
- PS II reaction centre = P680 (absorption peak 680 nm).
- LHC/antennae: hundreds of pigment molecules bound to proteins.
- Outputs of the light reaction: ATP, NADPH and O2.
Electron Transport — The Z Scheme
In PS II, P680 absorbs 680 nm light and excited electrons are picked up by an acceptor and passed downhill through cytochromes to PS I. In PS I, electrons excited by 700 nm light are passed to another acceptor and finally reduce NADP+ to NADPH. Plotted on a redox scale this path forms a Z shape, and the electrons lost by PS II are continuously replaced by the splitting (photolysis) of water, which produces oxygen.
- Z scheme: PS II -> electron transport chain (cytochromes) -> PS I -> NADP+ -> NADPH.
- Photolysis at PS II: 2H2O -> 4H+ + O2 + 4e-.
- Evolved O2 comes from water (proved by radioisotope techniques).
- Non-cyclic: PS I + PS II -> ATP, NADPH, O2; Cyclic: only PS I -> ATP.
Beyond Light Reaction: C3/C4, Photorespiration & Limiting Factors
The dark reaction fixes CO2 into sugar through the Calvin (C3) cycle in the stroma, catalysed by RuBisCO. C4 plants such as maize, sugarcane and sorghum use Kranz anatomy and PEP carboxylase to concentrate CO2, avoiding wasteful photorespiration and performing well in heat. The overall rate is governed by limiting factors (Blackman's Law).
- C3 plants: first stable product is 3-carbon 3-PGA; enzyme RuBisCO.
- C4 plants: first product is 4-carbon OAA; enzyme PEP carboxylase; Kranz anatomy.
- Photorespiration: RuBisCO binds O2, yielding no sugar/ATP/NADPH; negligible in C4.
- Limiting factors: light, CO2, temperature and water (Blackman's Law of Limiting Factors).
Key terms
- Photosynthesis
- Physico-chemical process by which green plants convert light energy into chemical energy, making organic food from CO2 and water.
- Autotroph
- Organism (green plant) that synthesises its own food through photosynthesis.
- Heterotroph
- Organism that cannot make its own food and depends on autotrophs.
- Chlorophyll a
- Chief photosynthetic pigment forming the reaction centre; absorbs mainly blue and red light.
- Accessory pigments
- Chlorophyll b, xanthophylls and carotenoids that absorb extra wavelengths and protect chlorophyll a from photo-oxidation.
- Photosystem (PS I & PS II)
- Light-harvesting pigment-protein complexes with reaction centres P700 and P680 respectively.
- Z scheme
- The zig-zag path of electron flow from PS II through the electron transport chain to PS I and finally to NADP+.
- Photolysis of water
- Light-driven splitting of water at PS II releasing O2, protons and electrons.
- RuBisCO
- Enzyme that fixes CO2 in the Calvin cycle; its affinity for O2 also causes photorespiration in C3 plants.
- Kranz anatomy
- Special wreath-like leaf anatomy of C4 plants with bundle-sheath cells that concentrate CO2.
Must-know facts exam-ready
- Net equation: 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O (in light); the O2 released comes from water.
- Light reactions occur in grana/thylakoids; dark reactions (Calvin cycle) occur in the stroma.
- PS I reaction centre = P700 (700 nm); PS II reaction centre = P680 (680 nm).
- Products of the light reaction are ATP, NADPH and O2.
- Chlorophyll a is the chief and most abundant photosynthetic pigment.
- Maximum photosynthesis occurs in the blue and red regions of the spectrum.
- Photosystems are numbered by order of discovery, not function — PS II acts before PS I.
- Water is split at PS II: 2H2O -> 4H+ + O2 + 4e-.
- van Niel proved the evolved O2 comes from H2O, not CO2; confirmed by radioisotope techniques.
- Melvin Calvin won the Nobel Prize in Chemistry (1961) for mapping carbon assimilation using C14.
- C3 plants' first product is 3-carbon 3-PGA (RuBisCO); C4 plants' first product is 4-carbon OAA (PEP carboxylase).
- Cyclic photophosphorylation uses only PS I and makes only ATP; non-cyclic uses both and makes ATP + NADPH + O2.
Timeline
- 1770Priestley shows plants restore air damaged by burning candles and breathing animals.
- 1774Priestley discovers oxygen.
- 1854Julius von Sachs shows glucose is produced in green parts and stored as starch in chloroplasts.
- 1961Melvin Calvin awarded the Nobel Prize for mapping the carbon-assimilation (Calvin) pathway.
Memory tricks remember it for good
Traps to avoid
- Photosystems are named by order of discovery, NOT function — PS II works before PS I in the Z scheme.
- The dark reaction does not occur in darkness and is not strictly light-independent — it needs ATP and NADPH from the light reaction.
- The O2 released comes from WATER, not from CO2 (van Niel; proved by radioisotopes).
- Light reactions happen in the grana/thylakoid membranes, NOT in the stroma; the Calvin cycle is in the stroma.
- Do not swap P700 = PS I and P680 = PS II, or their wavelengths.
- Cyclic photophosphorylation makes ONLY ATP (PS I alone); non-cyclic makes ATP + NADPH + O2 — do not conflate them.
Exam focus
🧠 Prelims angles
- Products and site of the light reaction (ATP, NADPH, O2 in grana vs Calvin cycle in stroma).
- Identifying the chief/most abundant pigment (chlorophyll a) and the accessory pigments.
- P700/P680 values and the sequence of PS I vs PS II in the Z scheme.
- C3 vs C4: enzymes (RuBisCO vs PEP carboxylase), first products (3-PGA vs OAA), Kranz anatomy.
- Scientists and their contributions (Priestley, Ingenhousz, Sachs, Engelmann, van Niel, Calvin).
- Source of evolved O2 (water) and the balanced overall equation of photosynthesis.
✍️ Mains angles GS-III
- Photosynthesis as the foundation of food security and the carbon-oxygen cycle.Link autotrophy to food chains, atmospheric O2 release and CO2 fixation; connect to crop productivity and climate.
- Artificial photosynthesis and solar-energy research as renewable-energy frontiers.Use Calvin's principle (light to chemical energy) feeding into solar fuels and renewable materials research.
- Raising crop yield via C4 efficiency and curbing photorespiration.Discuss engineering C4 traits into C3 crops like rice to boost efficiency and output.
Last-minute revision tick as you recall
- Photosynthesis: light energy to chemical energy; yields food + O2; basis of life on earth.
- Light reaction in grana (ATP, NADPH, O2); dark/Calvin cycle in stroma (sugar then starch).
- PS II = P680, PS I = P700; numbered by discovery, but function order is II then I.
- Z scheme: PS II -> ETC (cytochromes) -> PS I -> NADP+ -> NADPH.
- Evolved O2 comes from water via photolysis at PS II, not from CO2.
- Chlorophyll a = chief pigment; b, xanthophylls, carotenoids = accessory.
- Maximum photosynthesis occurs in blue and red light.
- C3: RuBisCO, 3-PGA; C4: PEP carboxylase, OAA, Kranz anatomy, negligible photorespiration.
- Calvin mapped carbon path with C14 — Nobel Prize 1961.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.