Subtopics - Photosynthesis in Higher Plants (NEET)
Nine content blocks: historical background, chloroplast structure and pigments, mechanism overview (Van Niel), modern light reactions with photosystems, chemiosmotic ATP synthesis, dark reactions (C3/C4/CAM), photorespiration, bacterial photosynthesis and chemosynthesis, and factors affecting photosynthesis.
1) Historical background
Chronicles the intellectual journey from Van Helmont (1648), who concluded that plant food comes from water, through Stephen Hales (Father of Plant Physiology, 1727) who suggested a role for air and light, to Joseph Priestley (1772) who demonstrated that mint plants purify foul air. Jan Ingen-Housz (1779) showed that only green parts in sunlight produce dephlogisticated air (O2). Jean Senebier (1782) proved CO2 absorption and O2 release are linked, while Nicolus de Saussure (1804) established the importance of water. Julius Robert Mayer (1845) proposed conversion of radiant energy to chemical energy. Julius Von Sachs (1862) demonstrated starch as the first visible product and chlorophyll confined to chloroplasts. Melvin Calvin (1954) traced the path of carbon using radioactive 14C in Chlorella, establishing the C3 cycle (Nobel Prize 1961). Huber, Michel and Deisenhofer (1985) crystallised the photosynthetic reaction centre of Rhodopseudomonas viridis (Nobel Prize 1988). Each discovery builds upon the previous, constructing our modern understanding of photosynthesis as an oxidation-reduction process.
2) Photosynthesis in higher plants
Covers the structural and molecular foundations of photosynthesis. Begins with the chloroplast as the site of photosynthesis in eukaryotic photoautotrophs, with its double membrane, grana (stacked thylakoids), stroma, and DNA. Park and Biggins (1964) defined the photosynthetic unit (quantasome) as 230 chlorophyll molecules required to release one O2. Chloroplast pigments are categorised into chlorophylls (a and b with porphyrin head and phytol tail, Mg at centre), carotenoids (carotenes like beta-carotene and xanthophylls like lutein), and phycobilins (phycocyanin and phycoerythrin in cyanobacteria and red algae). Nature of light covers electromagnetic spectrum, PAR (400-700 nm), and the fact that blue light carries more energy than red. Absorption spectrum (studied by spectrophotometer) reveals chlorophyll a peaks at 430 nm and 662 nm, while action spectrum (first by Engelmann 1882 using Spirogyra) shows maximum photosynthesis in blue and red regions. Leaves appear green because chlorophyll reflects green wavelengths.
3) Mechanism of photosynthesis
Addresses the fundamental question of where the O2 in photosynthesis originates. C.B. Van Niel (1930) studied purple sulphur bacteria using H2S instead of water and proposed, by analogy, that in green plants the O2 must come from H2O rather than CO2. This was definitively confirmed by Ruben and Kamen (1941) using isotope-labelled water H2(18)O in Chlorella. When heavy-oxygen water was supplied, the liberated O2 contained 18O, proving water is the source of oxygen. The overall equation 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O shows that 12 molecules of water are consumed. This discovery was foundational for understanding that photosynthesis is an oxidation-reduction process: water is oxidised to O2 and CO2 is reduced to carbohydrate. The photolysis of water became established as a central event in light reactions, setting the stage for the two-phase model of photosynthesis.
4) Modern concept of photosynthesis
Explains the two-phase architecture of photosynthesis. The light phase (photochemical reactions) occurs in grana thylakoids and produces ATP and NADPH (assimilatory powers). Evidence comes from physical separation of grana and stroma fractions, intermittent light experiments (faster than continuous light because dark reactions consume accumulated assimilatory power), and temperature coefficient studies (Q10 = 1 for light reactions, Q10 > 2 for dark reactions). Robin Hill (1939) demonstrated O2 evolution from isolated chloroplasts using electron acceptors (Hill reaction). Energy transfer involves photoexcitation of chlorophyll electrons through singlet and triplet states (fluorescence and phosphorescence). Quantum yield is 1/8 = 0.125 (8 quanta per O2). The Emerson effect and red drop phenomenon (decreased yield above 680 nm) proved existence of two photosystems. PS I (P700, outer thylakoid surface, cyclic and non-cyclic) generates strong reductant NADPH. PS II (P680, inner surface, non-cyclic only) produces strong oxidant, splits water, evolves O2. Cyclic photophosphorylation involves only PS I producing 2 ATP. Non-cyclic (Z-scheme, Hill and Bendall 1960) involves both PS I and PS II, producing ATP, NADPH, and O2. DCMU inhibits PS II.
5) Chemiosmotic Hypothesis
Explains the mechanism of ATP synthesis in chloroplasts during light reactions. A proton gradient develops across the thylakoid membrane with protons accumulating in the lumen. Three processes build this gradient: (a) photolysis of water on the lumen side releases H+ into the lumen, (b) plastoquinone acts as an H carrier, picking up protons from the stroma and releasing them into the lumen as electrons are transferred, (c) NADP reductase on the stroma side removes protons from the stroma for NADPH formation. This creates a pH decrease in the lumen and a measurable proton gradient. ATP synthase consists of CF0 (transmembrane channel embedded in thylakoid membrane, facilitates proton diffusion) and CF1 (protrudes on stroma surface, catalyses ATP formation through conformational change). The four essentials of chemiosmosis are: a membrane, a proton pump, a proton gradient, and ATP synthase. The resultant ATP and NADPH immediately enter the biosynthetic Calvin cycle reactions in the stroma.
6) Dark phase
Describes the biosynthetic carbon fixation pathways occurring in the stroma of chloroplasts, utilising ATP and NADPH from light reactions. The Calvin cycle (C3 cycle), discovered by Calvin and Benson using 14C in Chlorella and Scenedesmus, has three phases: carboxylation (CO2 + RuBP via Rubisco forming 3-PGA), glycolytic reversal (reduction of 3-PGA to G3P using ATP and NADPH), and regeneration of RuBP. Six turns fix 6 CO2 to produce one hexose, requiring 18 ATP and 12 NADPH. Rubisco constitutes 16% of chloroplast protein and is the most abundant protein on earth. The Hatch-Slack C4 cycle, detailed by Hatch and Slack (1966), first observed by Kortschak and Hart in sugarcane: PEP + CO2 forms OAA (4C) in mesophyll cells via PEPCO, converted to malate, transported to bundle sheath for decarboxylation releasing CO2 for Calvin cycle. C4 plants show Kranz anatomy with dimorphic chloroplasts. C4 requires 30 ATP and 12 NADPH per hexose. CAM pathway (Ting 1971) in succulents: stomata open at night (scotoactive), CO2 stored as malic acid (acidification), decarboxylated during day (deacidification) for Calvin cycle. CO2 compensation point: 0-5 ppm in C4, 25-100 ppm in C3 plants.
7) Photorespiration or CO2 Cycle
Describes the wasteful process of O2 uptake and CO2 release in light, first reported by Decker and Tio (1959) in tobacco and defined by Krotkov (1963). Also called glycolate metabolism or C2 cycle. Biochemically, RuBP reacts with O2 (Rubisco acting as oxygenase) to form phosphoglycolate (2C) and 3-PGA (3C). Phosphoglycolate is dephosphorylated to glycolate, which moves to peroxisomes where it is oxidised to glyoxylate (producing H2O2, decomposed by catalase). Glyoxylate is transaminated to glycine, which enters mitochondria. Two glycine molecules produce one serine + CO2 + NH3. Serine returns to peroxisome and is converted through hydroxypyruvic acid and glyceric acid back to 3-PGA. The process involves three organelles: chloroplasts, peroxisomes, and mitochondria. No ATP or NADH is produced, and up to 50% of photosynthetically fixed carbon may be lost. Enhanced by bright light, high temperature, high O2, and low CO2. Occurs only in C3 plants (high CO2 compensation point); absent in C4 plants because Kranz anatomy maintains high CO2 concentration in bundle sheath cells.
8) Bacterial photosynthesis and Chemosynthesis
Bacterial photosynthesis, first described by Van Niel, occurs in purple and green sulphur bacteria. It is anoxygenic (no O2 evolution), anaerobic, and uses H2S instead of H2O as hydrogen donor. Only one pigment system (PS I) except in cyanobacteria. Photosynthetic pigments include bacteriochlorophyll (differs from Chl a by having one pyrrole ring with two extra hydrogen) and bacterioviridin, housed in chromatophores (coined by Schmitz). Green sulphur bacteria (Chlorobium) absorb 720-750 nm; purple sulphur bacteria (Chromatium); purple non-sulphur bacteria (Rhodospirillum, Rhodopseudomonas). Reaction centre is P890. Photoreductant is NADH2. Cyclic photophosphorylation is dominant. Chemosynthesis, distinct from photosynthesis, uses chemical energy from oxidation of inorganic compounds to fix CO2 without light. Key examples: nitrifying bacteria (Nitrosomonas, Nitrobacter), sulphur bacteria (Beggiatoa, Thiobacillus), iron bacteria (Ferrobacillus, Leptothrix), hydrogen bacteria (Bacillus pentotrophus), and carbon bacteria (Carboxydomonas).
9) Factors affecting photosynthesis
Blackman (1905) proposed the law of limiting factors: when a process depends on multiple factors, the rate is limited by the slowest one. This is a modification of Liebig's law of minimum. Sachs (1860) gave the theory of three cardinal points (minimum, optimum, maximum). External factors include: (1) Light intensity, quality, and duration (80% absorbed, 10% reflected, 10% transmitted; solarization at very high intensity; PAR is blue and red; light saturation point), (2) Temperature (optimum 20-35 degrees C; some conifers at -35 degrees C, hot spring algae at 75 degrees C), (3) CO2 (0.032% in atmosphere, usually limiting; rate increases up to 1% then becomes toxic), (4) Water (rarely limiting, <1% used), (5) O2 (Warburg effect: excess O2 inhibits by competing with CO2 for Rubisco active sites; reported by Warburg 1920 in Chlorella), (6) Pollutants (PAN inhibits Hill reaction; DCMU, CMU block PS II), (7) Minerals (Mn2+ and Cl- for photolysis). Internal factors include protoplasmic factors, chlorophyll content, assimilation number (CO2 fixed per gram chlorophyll per hour), accumulation of end products, and leaf structure.
Photosynthesis in Higher Plants Download Notes & Weightage Plan
For each topic in the Photosynthesis in Higher Plants chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Photosynthesis in higher plants
Chloroplast structure, photosynthetic unit (quantasome of 230 molecules), all pigment types (chlorophyll a and b molecular formulae, carotenoids, phycobilins), nature of light (PAR 400-700 nm), absorption and action spectra with peak wavelengths.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Pigment absorption peaks, molecular formula differences (CH3 vs CHO), quantasome = 230 molecules, PAR range, and Engelmann's experiment details.
- High-risk Area: Confusing chlorophyll a and b absorption peaks. Forgetting that phycobilins are water-soluble while chlorophylls are fat-soluble. Mixing up carotenes (no O) vs xanthophylls (with O).
- Best Practice Style: Diagram-heavy: label chloroplast, then zoom into thylakoid membrane showing pigment arrangement. Colour-code each pigment type.
Modern concept of photosynthesis
Light reactions: Hill reaction (1939), evidence for two phases, energy transfer (fluorescence and phosphorescence), quantum yield (1/8), Emerson effect and red drop, PS I (P700) vs PS II (P680), cyclic vs non-cyclic photophosphorylation, Z-scheme (Hill and Bendall 1960), DCMU as PS II inhibitor.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: P700 vs P680 reaction centres, products of cyclic (only ATP) vs non-cyclic (ATP + NADPH + O2), location of PS I (outer) and PS II (inner), Z-scheme proposers.
- High-risk Area: Swapping P700 and P680 between photosystems. Forgetting that DCMU inhibits PS II specifically. Stating that cyclic produces NADPH (it does not).
- Best Practice Style: Flowchart-based: draw electron flow diagrams with energy levels. Use the Z shape as a visual mnemonic.
Calvin C3 cycle (carboxylation, glycolytic reversal, RuBP regeneration), Rubisco as most abundant protein, Hatch-Slack C4 cycle (PEP + CO2 → OAA in mesophyll, Calvin cycle in bundle sheath), Kranz anatomy and dimorphic chloroplasts, CAM pathway (scotoactive stomata, acidification/deacidification), CO2 compensation point differences.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: First stable product (3-PGA vs OAA), CO2 acceptor (RuBP vs PEP), ATP requirements (18 vs 30), Kranz anatomy definition, dimorphic chloroplasts (agranal in bundle sheath), CAM plant examples.
- High-risk Area: Confusing CO2 acceptor with first stable product. Stating wrong ATP counts. Forgetting that Rubisco is absent in mesophyll chloroplasts of C4 plants but present in bundle sheath.
- Best Practice Style: Table-based comparative learning with diagram support. Practice converting between C3/C4/CAM features using practice MCQs.
Factors affecting photosynthesis
Blackman's law of limiting factors (1905), external factors (light intensity/quality/duration, temperature, CO2, water, O2, pollutants, minerals) and internal factors (chlorophyll content, protoplasmic factors, end product accumulation, leaf structure). Warburg effect (O2 inhibition via competitive binding at Rubisco). Solarization at very high light. CO2 as usual limiting factor. Mn2+ and Cl- essential for photolysis.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Blackman's law exact statement, CO2 as limiting factor, Warburg effect definition and discoverer, role of Mn and Cl in photolysis, light saturation point, solarization mechanism.
- High-risk Area: Confusing Warburg effect (O2 inhibition) with solarization (photo-oxidation at very high light). Forgetting that water is rarely a limiting factor.
- Best Practice Style: Factor table with cause-and-effect reasoning. Practice previous year questions on limiting factors.
Photosynthesis in Higher Plants Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Photosynthesis in Higher Plants chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Swapping reaction centres: Students assign P680 to PS I and P700 to PS II. Remember: PS I = P700, PS II = P680. The numbering of photosystems is historical (PS I was discovered first) and does not correlate with wavelength order.
- Wrong location on thylakoid: PS I lies on the outer surface of thylakoids and PS II on the inner surface. Students reverse this because they assume PS II (discovered later) should be outer.
A NEET question asks which photosystem is involved in photolysis of water. The correct answer is PS II (P680), not PS I. Photolysis occurs on the lumen side where PS II is located.
How NEET Frames The Trap
NEET often frames questions as 'The reaction centre of the photosystem involved in O2 evolution is ___' to test whether you link O2 evolution to PS II (P680) correctly.
Q. Which of the following statements about photosystems in chloroplasts is correct?
A. PS I has reaction centre P680 and is located on the inner surface of thylakoids B. PS II has reaction centre P700 and participates in both cyclic and non-cyclic photophosphorylation C. PS II has reaction centre P680 and is involved in photolysis of water D. PS I has reaction centre P700 and is involved in O2 evolution from water
Trick: Options (a), (b), and (d) deliberately swap properties between PS I and PS II. Option (c) is correct: PS II has reaction centre P680 and is the only photosystem involved in photolysis of water and O2 evolution.
Mistake Snapshot (What Students Do Wrong)
- Confusing CO2 acceptor with first stable product: In C3: CO2 acceptor is RuBP (5C), first stable product is 3-PGA (3C). In C4: CO2 acceptor is PEP (3C), first stable product is OAA (4C). Students often state RuBP is the first stable product in C3.
- Wrong ATP count: C3 cycle requires 18 ATP + 12 NADPH per hexose. C4 cycle requires 30 ATP + 12 NADPH. Students swap these or use 36 ATP (confusion with respiration).
NEET asks: 'The first stable product of C4 pathway is ___'. Students who confuse acceptor with product might choose PEP (the acceptor) instead of OAA (the product).
How NEET Frames The Trap
Questions mix terminology: 'CO2 acceptor', 'first stable product', and 'key enzyme' are three distinct things. NEET exploits this by offering all three as options for a single question about one of them.
Q. In the C4 pathway of photosynthesis, the CO2 acceptor molecule and the first stable product are respectively:
A. RuBP and 3-PGA B. PEP and 3-PGA C. PEP and OAA D. RuBP and OAA
Trick: Option (a) is correct for C3 plants, not C4. Option (b) correctly identifies PEP as acceptor but gives the wrong product (3-PGA is C3 product). Option (d) mixes C3 acceptor with C4 product. Only option (c) is correct: PEP is the CO2 acceptor and OAA (oxaloacetic acid, 4C) is the first stable product in C4 plants.
Mistake Snapshot (What Students Do Wrong)
- O2 comes from CO2: A classic misconception. Students assume since CO2 has oxygen atoms, the O2 released must come from CO2. In reality, Van Niel proposed and Ruben-Kamen (1941) proved using H2(18)O that ALL O2 comes from water.
- Mixing up isotope experiments: Students confuse 14C (used by Calvin for carbon pathway) with 18O (used by Ruben-Kamen for oxygen source). These are two completely different experiments with different purposes.
NEET asks: 'The oxygen released during photosynthesis comes from ___'. The answer is water (H2O), not CO2. This was experimentally proven by using isotope-labelled water H2(18)O.
How NEET Frames The Trap
The question is deceptively simple but tests whether the student knows the historical experiment. Distractors include 'CO2', 'both CO2 and H2O', and 'glucose breakdown'.
Q. Ruben and Kamen (1941) used isotope-labelled water (H218O) in Chlorella and proved that:
A. CO2 is the source of O2 released during photosynthesis B. The first stable product of Calvin cycle is 3-PGA C. O2 released during photosynthesis comes from water D. Starch is the first visible product of photosynthesis
Trick: Option (b) relates to Calvin's experiment with 14C, not Ruben-Kamen. Option (d) is Julius Von Sachs' contribution. Option (a) is the old incorrect belief. The correct answer is (c): Ruben and Kamen proved O2 comes from water using the 18O isotope tracer.
Mistake Snapshot (What Students Do Wrong)
- Claiming cyclic produces NADPH: Cyclic photophosphorylation involves only PS I and the electron returns to P700. It produces ONLY ATP (2 molecules). No NADPH is formed and no O2 is evolved. Students add NADPH because they conflate it with non-cyclic.
- Wrong number of photosystems: Non-cyclic involves BOTH PS I and PS II. Cyclic involves ONLY PS I. Students sometimes state cyclic involves both or non-cyclic involves only one.
NEET asks: 'Which products are formed during cyclic photophosphorylation?' Options list combinations of ATP, NADPH, and O2. The correct answer is ATP only.
How NEET Frames The Trap
Options often include 'ATP and NADPH', 'ATP, NADPH, and O2', 'only NADPH' alongside the correct 'only ATP'. The similarity between the two processes causes confusion.
Q. During cyclic photophosphorylation, which of the following is true?
A. Both ATP and NADPH are produced, but O2 is not evolved B. Only ATP is produced; neither NADPH nor O2 is formed C. ATP, NADPH, and O2 are all produced D. NADPH is produced but ATP is not
Trick: Option (a) is a common mistake: students correctly exclude O2 but wrongly include NADPH. Option (c) describes non-cyclic photophosphorylation. The correct answer is (b): cyclic photophosphorylation produces only ATP because electrons cycle back to P700 via the electron transport chain without reaching NADP+ reductase.
Mistake Snapshot (What Students Do Wrong)
- Photorespiration produces ATP: Unlike normal respiration, photorespiration produces NO ATP and NO NADH. The energy is entirely lost as heat. Up to 50% of photosynthetically fixed carbon can be wasted.
- Photorespiration occurs in all plants: Photorespiration occurs only in C3 plants (high CO2 compensation point). It is absent in C4 plants because Kranz anatomy maintains high CO2 concentration around Rubisco in bundle sheath cells, suppressing its oxygenase activity.
NEET asks: 'Photorespiration does not occur in C4 plants because ___'. The correct reason is Kranz anatomy with bundle sheath cells maintaining high CO2 around Rubisco, not simply 'C4 plants have a different enzyme'.
How NEET Frames The Trap
NEET tests whether students understand the mechanistic reason (Kranz anatomy, high local CO2) rather than just the factual statement (absent in C4). Options may include partially correct but incomplete reasons.
Q. Which of the following is correct about photorespiration?
A. It produces ATP and NADH like normal mitochondrial respiration B. It occurs in both C3 and C4 plants under bright light C. It involves chloroplasts, peroxisomes, and mitochondria and produces no ATP D. It is enhanced by low O2 and high CO2 concentration
Trick: Option (a) confuses photorespiration with normal respiration. Option (b) incorrectly includes C4 plants. Option (d) reverses the conditions (photorespiration is enhanced by high O2 and low CO2). The correct answer is (c): photorespiration involves three organelles and produces no ATP.
Mistake Snapshot (What Students Do Wrong)
- Confusing Warburg effect with solarization: Warburg effect is inhibition of photosynthesis by excess O2 (competitive inhibition of Rubisco by O2 at active sites). Solarization is photo-oxidation of chlorophyll and cellular components at very high light intensity. Both decrease photosynthesis but by completely different mechanisms.
- Wrong scientist for Warburg effect: Students attribute the Warburg effect to different scientists. It was reported by Warburg (1920) in Chlorella algae. The effect is specifically about O2 concentration, not light intensity.
NEET asks: 'Inhibition of photosynthesis by high concentration of O2 is called ___'. The answer is Warburg effect, not solarization (which is caused by very high light).
How NEET Frames The Trap
Both terms cause a decrease in photosynthesis, making them easy to confuse. NEET exploits this by placing both as options alongside other terms like 'Pasteur effect' and 'Emerson effect'.
Q. The phenomenon in which increased oxygen concentration inhibits photosynthesis by competitive inhibition of RuBP carboxylase is called:
A. Emerson effect B. Solarization C. Warburg effect D. Pasteur effect
Trick: Option (a) is the enhancement effect with two wavelengths. Option (b) is chlorophyll destruction at very high light intensity (not O2). Option (d) relates to fermentation inhibition by O2 in respiration, not photosynthesis. The correct answer is (c): Warburg effect, reported in Chlorella by Warburg (1920), is specifically the inhibition of photosynthesis by excess O2 competing with CO2 for Rubisco active sites.