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State Board · Class 11 · Biology

Lesson 12: Photosynthesis

7 concepts · 56 practice questions with answers · Free to read

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Chloroplast and photosynthetic pigments

Link to Std 9–10

Std 8–10 gave the basic idea of photosynthesis; here we study the chloroplast, light reaction, Calvin cycle, C4 and CAM in detail.

From daily life

Like a solar panel, the chloroplast in a leaf traps light and makes food.

Simple definition

Photosynthesis = making carbohydrate (glucose) from CO₂ and H₂O with sunlight trapped by chlorophyll – the only process that turns solar energy into food.

Chloroplast and pigmentsChloroplastOuter + inner membraneGranumStromaThylakoidPigmentsChlorophyll ablue-green; -CH3Chlorophyll byellow-green; -CHOCarotenesorange-red; C40H56Xanthophyllsyellow; contain O₂Double membrane; stroma inside; grana = stacks of thylakoidsPigments sit in thylakoid membrane (fat-soluble)Light reaction = granum; dark reaction = stromaChlorophyll = porphyrin head + phytol tailCarotenoids pass energy to chl a; also protect itChloroplasts in mesophyll; CO₂ via stomata, water via veins

In the figure : Chloroplast structure and pigments.

Rule 1 : Chloroplast structure — Chloroplasts lie mainly in leaf mesophyll cells; discoid; double membrane. Inside is the stroma holding lamellae; stacks of sacs = grana; each sac = thylakoid. Light reaction in the thylakoid membrane; dark reaction in the stroma.

  • Easy : Grana = thylakoids stacked like coins.
  • Medium : Stroma has enzymes, DNA, ribosomes.
  • Exam-type (CET) : Light = granum; dark = stroma.

Rule 2 : Pigments — Chlorophyll a (-CH₃) and b (-CHO); molecule = porphyrin head + phytol tail. Carotenoids: carotenes (C₄₀H₅₆, orange-red) and xanthophylls (with oxygen, yellow). They pass absorbed energy to chlorophyll a and protect chlorophyll from photo-oxidation.

  • Easy : Chl a = -CH₃; chl b = -CHO.
  • Medium : Xanthophyll = contains oxygen.
  • Exam-type (CET) : Pigments are fat-soluble.
Remember

a = -CH₃ (methyl), b = -CHO (aldehyde): ‘a-M, b-A’.

Common mistake and CET trap

‘Dark reaction only in darkness’ is wrong (light-independent). Trap: ‘chlorophyll in stroma’ is wrong (thylakoid).

Shortcut

Memory clue : "Granum = stack of coins; thylakoid = each coin".

Spot the difference

PartWhat?Role
GranumStack of thylakoidsLight reaction
StromaGround substanceDark reaction
CarotenoidAccessory pigmentEnergy + protection

Practice questions

Easy
  1. 1Define photosynthesis.
  2. 2Where are chloroplasts?
  3. 3What is a granum?
Medium
  1. 4Chl a vs b.
  2. 5Role of carotenoids.
  3. 6What is phytol?
Challenging
  1. 7Site of light reaction?
  2. 8Site of dark reaction?
Show answers (try them yourself first)
  1. 1Glucose from CO₂ + H₂O using light.
  2. 2In mesophyll cells.
  3. 3Stack of thylakoids.
  4. 4a: -CH₃; b: -CHO.
  5. 5Pass energy to chl a; protect.
  6. 6Long tail of chlorophyll.
  7. 7Thylakoid membrane.
  8. 8Stroma.

Nature of light and spectra

From daily life

Leaves look green because they reflect green light and absorb blue and red.

Simple definition

Light = electromagnetic radiation; stream of photons / waves. Visible light is 390–730 nm. The absorption spectrum and action spectrum nearly coincide.

Light and spectraVisible spectrum 390–730 nm390730 nmAbsorption spectrum (chl a, b)blueredgreen: low absorptionAction spectrum ≈ absorption spectrum (overlap)Light = electromagnetic radiation; photon (quantum)Absorbed: blue, violet, red most; green reflectedCarotenoids absorb violet–blue regionAbsorption graph = light absorbed per wavelengthAction graph = photosynthesis rate per wavelengthChromatography: green and yellow bands separate

In the figure : Visible spectrum and the absorption curve.

Rule 1 : Light and the visible spectrum — By quantum theory light is a stream of photons; by wave theory, waves of different wavelengths = colours. We see 390–730 nm, between ultraviolet and infra-red. One photon carries one quantum of energy.

  • Easy : Photon = particle of light.
  • Medium : Visible lies between UV and IR.
  • Exam-type (CET) : Shorter wavelength = more energy.

Rule 2 : Absorption and action spectra — Absorption spectrum: graph of light absorbed by pigments per wavelength. Action spectrum: rate of photosynthesis per wavelength. Chl a and b absorb blue, violet and red most, so the two graphs overlap. Carotenoids absorb the violet–blue region.

  • Easy : Absorption = how much taken up; action = how much synthesis.
  • Medium : The two graphs coincide.
  • Exam-type (CET) : Green is reflected, so the leaf looks green.
Remember

Absorbs Blue, Violet, Red (BVR); green bounces back.

Common mistake and CET trap

‘Most photosynthesis in green light’ is wrong (red/blue). Trap: absorption and action are not identical – they overlap but measure different things.

Shortcut

Memory clue : "Absorption = Amount absorbed; Action = Activity (rate)".

Spot the difference

PointAbsorption spectrumAction spectrum
ShowsLight absorbedRate of photosynthesis
Y-axisAbsorptionRate

Practice questions

Easy
  1. 1Visible-light wavelengths?
  2. 2What is a photon?
  3. 3Which colours does chlorophyll absorb?
Medium
  1. 4Why are leaves green?
  2. 5What is absorption spectrum?
  3. 6What is action spectrum?
Challenging
  1. 7Do they coincide?
  2. 8What does chromatography show?
Show answers (try them yourself first)
  1. 1390–730 nm.
  2. 2Quantum particle of light.
  3. 3Blue, violet, red.
  4. 4Green light is reflected.
  5. 5Graph of absorption per wavelength.
  6. 6Graph of rate per wavelength.
  7. 7Yes, nearly.
  8. 8Green and yellow bands.

Experiments and overview of photosynthesis

From daily life

Bubbles rising in a pond by day show that oxygen is being made.

Simple definition

Hill reaction: isolated chloroplasts release O₂ from water. The process has two stages: light reaction and dark reaction (Blackman).

Experiments and overviewOverall equation6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂Olight + chlorophyll (chloroplast)Light reactionDark reactionATP+NADPH₂in granum; needs lightin stroma; Calvin cycleH₂O → O₂↑CO₂ → glucoseVan Niel: bacteria use H₂S → plants use water, O₂ releasedRuben (1941): H₂¹⁸O used → O₂ comes from waterHill (1937): isolated chloroplast + ferric → water splits (photolysis)Arnon: light → ATP + NADPH₂ (photophosphorylation)Blackman: dark reaction follows light reactionHill reaction: O₂ and e⁻ both from water

In the figure : Overall equation and key experiments.

Rule 1 : Source of O₂: experiments — Van Niel: purple bacteria use H₂S and release sulphur → plants use H₂O and release O₂. Ruben (1941): used heavy oxygen ¹⁸O in Chlorella; the released O₂ came from water. Hill (1937): isolated chloroplasts + water + a ferric compound; ferric was reduced and O₂ released in light, proving photolysis of water.

  • Easy : O₂ from water, not from CO₂.
  • Medium : Hill reaction = O₂ without CO₂.
  • Exam-type (CET) : ¹⁸O = tracer.

Rule 2 : Two reactions and the overall equation — Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. Light reaction (granum): light → ATP + NADPH₂, O₂ released. Dark reaction (stroma): CO₂ → glucose using ATP and NADPH₂. Arnon: light energy is stored in ATP = photophosphorylation. The two occur in sequence.

  • Easy : Light → ATP + NADPH₂ → dark.
  • Medium : Water is oxidised; CO₂ is reduced.
  • Exam-type (CET) : NADPH₂ = reducing power.
Remember

O₂ = Origin Of Water; sugar comes from CO₂.

Common mistake and CET trap

‘O₂ from CO₂’ is wrong. Trap: ‘dark reaction needs light’ is wrong (needs ATP/NADPH₂).

Shortcut

Memory clue : "Hill = Hydrogen/oxygen from water".

Spot the difference

PointLight reactionDark reaction
SiteGranumStroma
NeedsLight, H₂OCO₂, ATP, NADPH₂
OutputATP, NADPH₂, O₂Glucose

Practice questions

Easy
  1. 1What did Van Niel propose?
  2. 2Ruben's experiment.
  3. 3What is Hill reaction?
Medium
  1. 4What is photolysis?
  2. 5Write the overall equation.
  3. 6Site of light reaction?
Challenging
  1. 7Role of NADPH₂.
  2. 8Other name of dark reaction.
Show answers (try them yourself first)
  1. 1Plants use H₂O.
  2. 2¹⁸O showed O₂ comes from water.
  3. 3O₂ release by isolated chloroplasts.
  4. 4Splitting of water in light.
  5. 56CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.
  6. 6Granum.
  7. 7Reducing power.
  8. 8Blackman's reaction.

Photosystems and the light reaction

From daily life

Like water falling on a turbine, excited e⁻ run down a chain and make ATP.

Simple definition

Photosystem = light-harvesting pigments + a reaction centre (P680 / P700). Non-cyclic: PS II + PS I; Cyclic: PS I only.

Photosystems and electron flowZ-scheme (non-cyclic)EPS IIP680PS IP700e⁻e⁻ETCNADP⁺ → NADPH₂ATPH₂O → ½O₂ + 2H⁺LightReaction centres: P680 (PS II), P700 (PS I); antenna pigments feed energyPS II: photolysis of water → O₂, H⁺, e⁻; H⁺ gradient → ATPNon-cyclic: PS II + PS I; ATP + NADPH₂ + O₂Cyclic: PS I only; e⁻ return; only ATP; no O₂Photophosphorylation = ATP made in lightATP synthesis: chemiosmosis (H⁺ flow to stroma)

In the figure : Z-scheme: two photosystems and electron flow.

Rule 1 : Photosystems — A photon lifts a chlorophyll electron to a higher level (excited state); without light it returns to the ground state. Antenna / light-harvesting molecules pass energy to the reaction centre. PS II = P680 (680 nm or below), PS I = P700 (700 nm or below). P = Pigment.

  • Easy : PS II = P680; PS I = P700.
  • Medium : Antenna = energy collectors.
  • Exam-type (CET) : PS II acts first despite its name.

Rule 2 : Non-cyclic and cyclic — Non-cyclic: PS II electrons pass through the ETC to PS I; PS I electrons reduce NADP⁺ → NADPH₂. PS II refills electrons by photolysis: 4H₂O → 2H₂O + O₂ + 4H⁺ + 4e⁻. H⁺ builds up inside the thylakoid; flowing out to the stroma it drives ATP synthesis (chemiosmosis). Cyclic: electrons return to PS I; only ATP; no O₂ or NADPH₂.

  • Easy : Non-cyclic = ATP + NADPH₂ + O₂.
  • Medium : Cyclic = only ATP.
  • Exam-type (CET) : H⁺ gradient powers ATP.
Remember

Non-cyclic: 3 products (ATP, NADPH₂, O₂); cyclic: 1 (ATP).

Common mistake and CET trap

‘O₂ released in cyclic’ is wrong. Trap: ‘PS I acts first’ is wrong (PS II takes e⁻ from water).

Shortcut

Memory clue : "Non-cyclic = never returns; cyclic = comes back".

Spot the difference

PointNon-cyclicCyclic
PhotosystemII + II only
O₂ReleasedNo
NADPH₂FormedNo

Practice questions

Easy
  1. 1Where is P680?
  2. 2Where is P700?
  3. 3Role of antenna molecules?
Medium
  1. 4Photolysis equation.
  2. 5How is ATP made?
  3. 6NADPH₂ in cyclic?
Challenging
  1. 7Final electron acceptor.
  2. 8What is photophosphorylation?
Show answers (try them yourself first)
  1. 1PS II.
  2. 2PS I.
  3. 3Pass energy to the reaction centre.
  4. 44H₂O → 2H₂O + O₂ + 4H⁺ + 4e⁻.
  5. 5By H⁺ gradient (chemiosmosis).
  6. 6Not formed.
  7. 7NADP⁺.
  8. 8ATP formation in light.

Dark reaction (Calvin cycle)

From daily life

Give a factory raw material (CO₂) and fuel (ATP, NADPH₂) and it makes sugar.

Simple definition

Calvin cycle (C3): fixation of CO₂ and glucose formation in the stroma; first stable product is 3-PGA (3-carbon).

Calvin cycleCalvin cycle (C3)RuBP (5C)3-PGA (3C)PGAL (3C)6 CO₂RubiscoATPNADPH₂12 PGAL: 2 → glucose; 10 → 6 RuBP1 Carboxylation2 Reduction3 RegenerationStep 1: CO₂ + RuBP → unstable 6C → 2 × 3-PGA (Rubisco)Step 2: PGA + ATP + NADPH₂ → PGALStep 3: 10 PGAL + 6 ATP → 6 RuBP1 glucose = 6 turns; 18 ATP + 12 NADPH₂Calvin, Benson: 14CO₂ + Chlorella; Nobel 1961Rubisco = 16% of chloroplast protein

In the figure : Calvin cycle: carboxylation, reduction, regeneration.

Live figure

Dark reaction (Calvin cycle)

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Rule 1 : Three steps — 1. Carboxylation: CO₂ + RuBP (5C, two phosphates) → unstable 6C → instantly 2 × 3-PGA; enzyme Rubisco. 2. Reduction (glycolytic reversal): PGA + ATP → 1,3-DPGA; with NADPH₂ → 3-PGAL. 3. Regeneration: 10 PGAL + 6 ATP → 6 RuBP.

  • Easy : C3 = first product has 3 carbons.
  • Medium : Rubisco is the most abundant enzyme.
  • Exam-type (CET) : Calvin won the 1961 Nobel Prize.

Rule 2 : Count: one glucose — 6 CO₂ and 6 RuBP → 12 PGAL; 2 of them = one glucose; 10 = regenerate 6 RuBP. Hence 6 turns. Total 18 ATP + 12 NADPH₂. Calvin traced the path with Chlorella and radioactive ¹⁴CO₂. The cycle returns ADP and NADP⁺.

  • Easy : 6 turns = 1 glucose.
  • Medium : 12 PGAL: 2 + 10.
  • Exam-type (CET) : ATP : NADPH₂ = 18 : 12.
Remember

C-R-R: Carboxylation, Reduction, Regeneration; 6 turns, 18 ATP, 12 NADPH₂.

Common mistake and CET trap

‘All PGAL becomes glucose’ is wrong (only 2 of 12). Trap: ‘Calvin in the granum’ is wrong (stroma).

Shortcut

Memory clue : "RCR: Round (fix), Convert (reduce), Reform (regenerate)".

Spot the difference

StepWhat happensATP / NADPH₂
CarboxylationCO₂ + RuBP → PGANone
ReductionPGA → PGALATP + NADPH₂
RegenerationPGAL → RuBPATP

Practice questions

Easy
  1. 1What is RuBP?
  2. 2Role of Rubisco.
  3. 3First stable product?
Medium
  1. 4Turns for one glucose?
  2. 5Fate of 12 PGAL?
  3. 6ATP and NADPH₂ needed?
Challenging
  1. 7Which alga did Calvin use?
  2. 8Where does it occur?
Show answers (try them yourself first)
  1. 15-carbon sugar with 2 phosphates.
  2. 2Joins CO₂ and RuBP.
  3. 33-PGA.
  4. 46.
  5. 52 → glucose; 10 → 6 RuBP.
  6. 618 ATP + 12 NADPH₂.
  7. 7Chlorella.
  8. 8Stroma.

Photorespiration, C4 pathway and CAM

From daily life

On hot afternoons leaves close stomata, CO₂ drops and O₂ rises; plants then use special routes.

Simple definition

Photorespiration: in high O₂ Rubisco acts as an oxygenase; about 25% of fixed carbon is lost. C4: first product 4-carbon OAA; Kranz anatomy. CAM: CO₂ fixed at night; desert plants.

Photorespiration, C4, CAMPhotorespiration | C4 | CAMPhotorespirationRuBP + O₂↓ Rubisco = oxygenasePGA + phosphoglycolate25% carbon lostC4 (Kranz)Mesophyll: PEP + CO₂→ OAA → malateBundle sheath: CO₂ + Calvin30 ATP / glucoseCAMNight: stomata open, malateDay: stomata shut, CO₂ releasedOpuntia, AloePointC3C4CAMFirst productPGA (3C)OAA (4C)OAA (4C)Photoresp.yesnonoATP / glucose183030Plantswheat, ricesugarcane, maizeAloe, OpuntiaStomatadaydaynight

In the figure : Comparison of photorespiration, C4 and CAM.

Rule 1 : Photorespiration — Occurs in high temperature, bright light, high O₂ and low CO₂. RuBP + O₂ → PGA + 2C phosphoglycolate → glycolate → peroxisome (glycine) → mitochondria (serine + CO₂ released). Three organelles: chloroplast, peroxisome, mitochondria (PCO cycle). No ATP gain; a wasteful process.

  • Easy : Rubisco = carboxylase + oxygenase.
  • Medium : Occurs in C3; absent in C4.
  • Exam-type (CET) : 25% carbon lost.

Rule 2 : C4 and CAM — C4 (Hatch-Slack): sugarcane, maize, sorghum. In the mesophyll CO₂ + PEP (PEP carboxylase) → OAA → malate → releases CO₂ in the bundle sheath → Calvin cycle. Kranz anatomy: large chloroplasts around the bundle sheath. C4 needs 30 ATP vs 18 for C3, but no photorespiration. CAM: stomata open at night, malate stored (acidification); CO₂ released by day (deacidification). E.g. Kalanchoe, Opuntia, Aloe.

  • Easy : C4 first product = OAA (4C).
  • Medium : CAM = CO₂ at night, Calvin by day.
  • Exam-type (CET) : Kranz = wreath.
Remember

C3 = 3 (PGA); C4 = 4 (OAA); CAM = Cactus At Midnight.

Common mistake and CET trap

‘C4 needs less ATP’ is wrong (30). Trap: ‘CAM stomata open by day’ is wrong (night).

Shortcut

Memory clue : "C4 = four carbons before Calvin; Kranz = ring".

Spot the difference

PointC3C4CAM
First productPGAOAAOAA
ATP183030
StomataDayDayNight

Practice questions

Easy
  1. 1When does photorespiration occur?
  2. 2Three organelles involved.
  3. 3Carbon lost?
Medium
  1. 4Example of C4.
  2. 5What is Kranz anatomy?
  3. 6Product of PEP carboxylase?
Challenging
  1. 7Example of CAM.
  2. 8Why are C4 plants better?
Show answers (try them yourself first)
  1. 1High O₂, low CO₂, high temperature.
  2. 2Chloroplast, peroxisome, mitochondria.
  3. 3About 25%.
  4. 4Sugarcane, maize, sorghum.
  5. 5Wreath-like bundle sheath arrangement.
  6. 6OAA.
  7. 7Kalanchoe, Opuntia, Aloe.
  8. 8No photorespiration; work at low CO₂.

Factors affecting photosynthesis and significance

From daily life

Seedlings on a dense forest floor grow slowly because little light reaches them.

Simple definition

Blackman's law of limiting factors: the rate is set by the slowest (minimum) factor. Factors: light, CO₂, temperature, water (external); chlorophyll, sugar buildup, leaf structure (internal).

Factors and Blackman's lawFactors and Blackman's lawratelight / CO₂limitedplateauExternalLight: red > blue; bright diffuse bestCO₂: 0.03% in air; up to 1% helpsTemp: optimum 25–30 °C; Opuntia up to 55 °CWater: indirect (turgidity)InternalChlorophyll, sugar buildup, leaf structureBlackman: the slowest (minimum) factor sets the rateRaise one factor and the next becomes limitingSignificance: food, solar → chemical energyO₂ for respiration and ozone layerSource of fossil fuels (coal, petroleum)About 70 billion tons of carbon fixed yearly

In the figure : Factors and the limiting-factor graph.

Live figure

Factors affecting photosynthesis and significance

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Rule 1 : External and internal factors — Light: red best, then blue; bright diffused light is best; very strong or very weak light lowers the rate. CO₂: 0.03% in air; up to 1% helps; more inhibits. Temperature: optimum 25–30 °C; Opuntia up to 55 °C. Water: indirect (turgidity). Internal: amount of chlorophyll; sugar buildup slows the rate; cuticle, stomata, intercellular spaces.

  • Easy : Optimum temperature 25–30 °C.
  • Medium : Water is a raw material but acts indirectly.
  • Exam-type (CET) : Sugar accumulation slows the rate.

Rule 2 : Blackman's law and significance — E.g. at a fixed light a plant can use only 10 mg CO₂ per hour; raising CO₂ raises the rate until it stops – now light is limiting. Raise light and the rate climbs again. The curve shows a sharp bend and plateau. Significance: anabolic; source of food; solar → chemical energy; O₂ for respiration and the ozone layer; coal, petroleum, natural gas.

  • Easy : Only one factor limits at a time.
  • Medium : Plateau = another factor is limiting.
  • Exam-type (CET) : O₂ = respiration + ozone.
Remember

Blackman = ‘weakest link of the chain’.

Common mistake and CET trap

‘All factors together set the rate’ is wrong (the minimum factor). Trap: ‘more CO₂ is always better’ is wrong (inhibits beyond ~1%).

Shortcut

Memory clue : "Weakest link sets the rate".

Spot the difference

FactorOptimum / note
LightRed > blue; bright diffuse
CO₂0.03% → up to 1%
Temperature25–30 °C
WaterIndirect

Practice questions

Easy
  1. 1Blackman's law.
  2. 2Colour with highest rate?
  3. 3Optimum temperature?
Medium
  1. 4CO₂ in air?
  2. 5Why a plateau?
  3. 6Why is water indirect?
Challenging
  1. 7Two internal factors.
  2. 8Two uses of photosynthesis.
Show answers (try them yourself first)
  1. 1Minimum factor sets the rate.
  2. 2Red, then blue.
  3. 325–30 °C.
  4. 40.03%.
  5. 5Another factor is limiting.
  6. 6Maintains turgidity and hydration.
  7. 7Chlorophyll, sugar buildup.
  8. 8Food; O₂; fossil fuels.

Quick revision of the whole lesson

ConceptKey fact
ChloroplastGranum = light; stroma = dark
Light390–730 nm; action ≈ absorption
ExperimentsHill, Ruben: O₂ from water
Light reactionPS II P680, PS I P700; ATP + NADPH₂ + O₂
Calvin6 turns; 18 ATP + 12 NADPH₂
C4 / CAMOAA; Kranz; night stomata
FactorsBlackman: minimum factor

🎯 Most important points for the exam

  • Chloroplast and pigments.
  • Absorption and action spectra.
  • Hill and Ruben experiments.
  • Cyclic and non-cyclic photophosphorylation.
  • Calvin cycle arithmetic.
  • C3 / C4 / CAM comparison.
  • Blackman's law of limiting factors.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.