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Photosynthesis in Higher Plants

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Chapter Notes

Overview
This chapter explains the physico-chemical process by which green plants use light energy to synthesise organic compounds from carbon dioxide and water. It covers the historical experiments that led to our understanding, the structure of the photosynthetic machinery (chloroplasts and pigments), the light-dependent reactions (including the Z scheme and chemiosmotic hypothesis), and the carbon-fixing reactions (Calvin cycle and C4 pathway). The one big idea a student must walk away with is that photosynthesis is a two-stage process: light reactions produce ATP and NADPH, which are then used in the Calvin cycle to fix CO2 into sugars, and that plants have evolved different pathways (C3 and C4) to optimise this process under varying environmental conditions.
Key Concepts (35)
Photosynthesis
A physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds.
e.g. A green leaf making starch in sunlight.
Autotrophs
Organisms that synthesise their own food through photosynthesis.
e.g. Green plants, algae.
Heterotrophs
Organisms that depend on green plants for food.
e.g. All animals including human beings.
Chlorophyll
The green pigment of the leaf, located in chloroplasts, required for photosynthesis.
e.g. The green colour of leaves is due to chlorophyll.
Stomata
Pores on leaves through which carbon dioxide from the atmosphere is taken in.
e.g. Stomata open during the day to allow CO2 entry.
Chloroplast
The organelle within plant cells where photosynthesis takes place, containing grana, stroma lamellae, and stroma.
e.g. Mesophyll cells in leaves have a large number of chloroplasts.
Grana
Membranous system within the chloroplast responsible for trapping light energy and synthesis of ATP and NADPH.
e.g. The grana are stacks of thylakoid membranes.
Stroma
The matrix of the chloroplast where enzymatic reactions synthesise sugar and starch.
e.g. The Calvin cycle occurs in the stroma.
Light reactions
The set of reactions directly driven by light, including light absorption, water splitting, oxygen release, and formation of ATP and NADPH.
e.g. The photochemical phase that occurs in the grana.
Dark reactions (Carbon reactions)
The set of reactions not directly light-driven but dependent on products of light reactions (ATP and NADPH), involving CO2 fixation and sugar synthesis.
e.g. The Calvin cycle is a dark reaction.
Pigments
Substances that have the ability to absorb light at specific wavelengths.
e.g. Chlorophyll a, chlorophyll b, xanthophylls, and carotenoids.
Chlorophyll a
The chief pigment associated with photosynthesis, with absorption peaks in the blue and red regions.
e.g. It is bright or blue green in a chromatogram.
Accessory pigments
Pigments like chlorophyll b, xanthophylls, and carotenoids that absorb light and transfer energy to chlorophyll a, and protect it from photo-oxidation.
e.g. They enable a wider range of wavelengths to be utilised for photosynthesis.
Action spectrum
A graph showing the rate of photosynthesis at different wavelengths of light.
e.g. Engelmann's experiment using Cladophora and aerobic bacteria produced the first action spectrum.
Absorption spectrum
A graph showing the ability of a pigment to absorb light at different wavelengths.
e.g. Chlorophyll a shows maximum absorption in blue and red light.
Photosystem
A discrete photochemical light harvesting complex consisting of a light harvesting system (antennae) and a reaction centre.
e.g. Photosystem I (PS I) and Photosystem II (PS II).
Reaction centre
A single chlorophyll a molecule in a photosystem that is the site of photochemical reaction.
e.g. P700 in PS I and P680 in PS II.
P700
The reaction centre chlorophyll a of Photosystem I with an absorption peak at 700 nm.
e.g. It absorbs red light of wavelength 700 nm.
P680
The reaction centre chlorophyll a of Photosystem II with an absorption peak at 680 nm.
e.g. It absorbs red light of wavelength 680 nm.
Z scheme
The characteristic shape formed when all carriers of the electron transport chain are placed in sequence on a redox potential scale, showing the transfer of electrons from PS II to PS I and finally to NADP+.
e.g. It describes the non-cyclic flow of electrons.
Photophosphorylation
The synthesis of ATP from ADP and inorganic phosphate in the presence of light.
e.g. Non-cyclic and cyclic photophosphorylation.
Non-cyclic photophosphorylation
The process where both PS II and PS I work in series, producing both ATP and NADPH + H+.
e.g. The Z scheme involves non-cyclic photophosphorylation.
Cyclic photophosphorylation
The process where only PS I is functional, and electrons are cycled back to the PS I complex, resulting only in ATP synthesis, not NADPH.
e.g. Occurs when only light of wavelengths beyond 680 nm is available.
Chemiosmotic hypothesis
The hypothesis explaining ATP synthesis in chloroplasts linked to the development of a proton gradient across the thylakoid membrane.
e.g. Protons accumulate in the thylakoid lumen, and their movement through ATP synthase drives ATP formation.
ATP synthase
An enzyme consisting of CF0 (embedded in thylakoid membrane) and CF1 (protruding on stroma side) that synthesises ATP using the proton gradient.
e.g. The breakdown of the proton gradient provides energy for ATP synthesis.
Calvin cycle
The biosynthetic pathway of CO2 fixation discovered by Melvin Calvin, occurring in all photosynthetic plants, consisting of carboxylation, reduction, and regeneration stages.
e.g. The first stable product is 3-phosphoglyceric acid (PGA).
RuBisCO (RuBP carboxylase-oxygenase)
The enzyme that catalyses the carboxylation of RuBP in the Calvin cycle, and also has an oxygenase activity leading to photorespiration.
e.g. It is the most abundant enzyme in the world.
RuBP (Ribulose bisphosphate)
A 5-carbon ketose sugar that acts as the primary CO2 acceptor in the Calvin cycle.
e.g. It is regenerated at the end of the Calvin cycle.
PGA (3-phosphoglyceric acid)
The first stable 3-carbon product of CO2 fixation in the Calvin cycle.
e.g. Two molecules of PGA are formed from one molecule of RuBP after carboxylation.
C4 pathway (Hatch and Slack Pathway)
A pathway in which the first stable product of CO2 fixation is a 4-carbon organic acid (oxaloacetic acid), occurring in plants adapted to dry tropical regions.
e.g. Maize and sorghum are C4 plants.
Kranz anatomy
A special leaf anatomy in C4 plants characterised by large bundle sheath cells around vascular bundles, with thick walls and no intercellular spaces.
e.g. The bundle sheath cells have a large number of chloroplasts.
PEP carboxylase (PEPcase)
The enzyme in mesophyll cells of C4 plants that fixes CO2 into phosphoenol pyruvate (PEP) to form oxaloacetic acid (OAA).
e.g. It is present in mesophyll cells, not in bundle sheath cells.
Photorespiration
A wasteful process in C3 plants where RuBisCO binds O2 instead of CO2, leading to the formation of phosphoglycolate, with no synthesis of ATP or sugars, and release of CO2.
e.g. It decreases the efficiency of photosynthesis in C3 plants.
Blackman's Law of Limiting Factors
If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value.
e.g. Even with optimal light and CO2, a plant may not photosynthesise if temperature is very low.
Light saturation
The point beyond which increasing light intensity does not increase the rate of photosynthesis, occurring at about 10% of full sunlight.
e.g. Light is rarely a limiting factor in nature except for plants in shade.
Formulas & Equations (8)
CO2 + H2O → [CH2O] + O2 (Light)
Reads as: Carbon dioxide plus water, in the presence of light, yields carbohydrate and oxygen.
Variables: [CH2O] represents a carbohydrate (e.g., glucose).
Use when: Writing the overall empirical equation for photosynthesis.
⚠ Students forget to write 'Light' above the arrow or omit the square brackets around CH2O.
2H2A + CO2 → 2A + CH2O + H2O (Light)
Reads as: Two molecules of a hydrogen donor plus carbon dioxide, in the presence of light, yields two molecules of oxidised donor, carbohydrate, and water.
Variables: H2A is the hydrogen donor (e.g., H2O in green plants, H2S in purple and green sulphur bacteria).
Use when: Explaining van Niel's generalised equation for photosynthesis.
⚠ Students confuse H2A with water only, forgetting that in some bacteria H2S is the donor.
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2 (Light)
Reads as: Six molecules of carbon dioxide plus twelve molecules of water, in the presence of light, yields one molecule of glucose, six molecules of water, and six molecules of oxygen.
Variables: C6H12O6 is glucose; O2 released comes from water.
Use when: Writing the correct overall balanced equation for photosynthesis in green plants.
⚠ Students write 6H2O on the left instead of 12H2O, or forget that water appears on both sides.
2H2O → 4H+ + O2 + 4e-
Reads as: Two molecules of water are split into four protons, one molecule of oxygen, and four electrons.
Variables: H+ = proton; e- = electron; O2 = oxygen.
Use when: Explaining the splitting of water associated with PS II.
⚠ Students write 2H2O → 2H2 + O2, which is incorrect; the correct products are protons, oxygen, and electrons.
RuBP + CO2 → 2 × 3PGA (catalysed by RuBisCO)
Reads as: Ribulose bisphosphate plus carbon dioxide yields two molecules of 3-phosphoglyceric acid, catalysed by RuBisCO.
Variables: RuBP = ribulose-1,5-bisphosphate; 3PGA = 3-phosphoglyceric acid.
Use when: Describing the carboxylation step of the Calvin cycle.
⚠ Students forget that two molecules of PGA are produced, not one.
RuBP + O2 → phosphoglycerate + phosphoglycolate (catalysed by RuBisCO)
Reads as: Ribulose bisphosphate plus oxygen yields one molecule of phosphoglycerate and one molecule of phosphoglycolate, catalysed by RuBisCO.
Variables: Phosphoglycolate is a 2-carbon compound.
Use when: Explaining the first step of photorespiration.
⚠ Students think photorespiration produces ATP or NADPH; it does not.
For every CO2 molecule entering Calvin cycle: 3 ATP + 2 NADPH are required.
Reads as: For every molecule of carbon dioxide that enters the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.
Variables: ATP = adenosine triphosphate; NADPH = nicotinamide adenine dinucleotide phosphate (reduced).
Use when: Calculating energy requirements for glucose synthesis (6 turns: 18 ATP, 12 NADPH).
⚠ Students confuse the numbers: 3 ATP and 2 NADPH per CO2, not per glucose.
In: 6 CO2, 18 ATP, 12 NADPH → Out: 1 glucose, 18 ADP, 12 NADP
Reads as: Inputs: six carbon dioxide, eighteen ATP, twelve NADPH; outputs: one glucose, eighteen ADP, twelve NADP.
Variables: ADP = adenosine diphosphate; NADP = nicotinamide adenine dinucleotide phosphate (oxidised).
Use when: Summarising the stoichiometry of the Calvin cycle for one glucose molecule.
⚠ Students forget that 18 ATP and 12 NADPH are used, not 6 ATP and 6 NADPH.
How It Works
Priestley's Experiment
  1. A candle burning in a closed bell jar soon gets extinguished.
  2. A mouse placed in a closed bell jar soon suffocates.
  3. When a mint plant is placed in the same bell jar, the mouse stays alive and the candle continues to burn.
  4. Priestley concluded that plants restore to the air whatever breathing animals and burning candles remove.
💡 Priestley did not know about oxygen; he discovered it in 1774. The experiment shows plants 'purify' air.
Ingenhousz's Experiment
  1. Using a setup similar to Priestley's, he placed it once in the dark and once in sunlight.
  2. He showed that sunlight is essential for the plant process that purifies air.
  3. With an aquatic plant, he observed small bubbles formed around green parts in bright sunlight but not in the dark.
  4. He identified the bubbles as oxygen and concluded that only green parts of plants release oxygen.
💡 Ingenhousz proved sunlight is essential; only green parts release oxygen.
Engelmann's Experiment (Action Spectrum)
  1. Using a prism, he split light into its spectral components.
  2. He illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria.
  3. The bacteria accumulated mainly in the region of blue and red light of the split spectrum.
  4. This produced the first action spectrum of photosynthesis, resembling the absorption spectra of chlorophyll a and b.
💡 Bacteria were used to detect sites of O2 evolution; they accumulated where O2 was released most.
Light Reactions (Z Scheme)
  1. Photosystem II (P680) absorbs red light of 680 nm, exciting electrons.
  2. Excited electrons are picked up by an electron acceptor and passed through an electron transport chain (cytochromes).
  3. Electrons move downhill in terms of redox potential, and are passed to Photosystem I.
  4. Simultaneously, electrons in PS I (P700) are excited by red light of 700 nm and transferred to another acceptor.
  5. These electrons then move downhill to NADP+, reducing it to NADPH + H+.
  6. Water is split on the inner side of the thylakoid membrane, providing electrons to replace those lost from PS II, releasing O2 and protons.
  7. The overall shape of the electron flow on a redox potential scale is called the Z scheme.
💡 The Z scheme involves both PS II and PS I; water splitting is associated with PS II; NADPH is produced at the end.
Chemiosmotic Hypothesis for ATP Synthesis
  1. Splitting of water on the inner side of the thylakoid membrane releases protons (H+) into the lumen.
  2. As electrons move through the photosystems, protons are transported across the membrane from the stroma to the lumen via an H carrier.
  3. The NADP reductase enzyme on the stroma side removes protons from the stroma to reduce NADP+ to NADPH.
  4. This creates a high concentration of protons in the thylakoid lumen and a low concentration in the stroma, forming a proton gradient.
  5. Protons diffuse back to the stroma through the CF0 channel of ATP synthase.
  6. The energy released from the breakdown of the gradient causes a conformational change in CF1, leading to ATP synthesis.
💡 Chemiosmosis requires a membrane, a proton pump, a proton gradient, and ATP synthase. Protons accumulate in the lumen, not the stroma.
Calvin Cycle (C3 Pathway)
  1. Carboxylation: RuBP (5C) combines with CO2, catalysed by RuBisCO, forming two molecules of 3-PGA (3C).
  2. Reduction: 3-PGA is phosphorylated by ATP and reduced by NADPH to form triose phosphate (sugar).
  3. Regeneration: Triose phosphate is used to regenerate RuBP, requiring one ATP per CO2 fixed.
  4. For every CO2 fixed, 3 ATP and 2 NADPH are used.
  5. Six turns of the cycle fix six CO2 molecules to produce one molecule of glucose.
💡 The Calvin cycle occurs in the stroma; it is common to all photosynthetic plants; RuBP is regenerated.
C4 Pathway (Hatch and Slack Pathway)
  1. In mesophyll cells: CO2 is fixed by PEP carboxylase using PEP (3C) as the primary acceptor, forming oxaloacetic acid (OAA, 4C).
  2. OAA is converted to other 4-carbon compounds (malic acid or aspartic acid) in mesophyll cells.
  3. These 4-carbon compounds are transported to bundle sheath cells.
  4. In bundle sheath cells, the 4-carbon compounds are broken down to release CO2 and a 3-carbon molecule.
  5. The released CO2 enters the Calvin cycle (C3 pathway) in bundle sheath cells.
  6. The 3-carbon molecule is transported back to mesophyll cells and converted to PEP again, completing the cycle.
💡 C4 plants have Kranz anatomy; mesophyll cells have PEPcase but lack RuBisCO; bundle sheath cells have RuBisCO but lack PEPcase; photorespiration is negligible.
Photorespiration
  1. In C3 plants, RuBisCO binds O2 instead of CO2 when O2 concentration is relatively high.
  2. RuBP reacts with O2 to form one molecule of phosphoglycerate (3C) and one molecule of phosphoglycolate (2C).
  3. This pathway does not synthesise ATP or NADPH.
  4. It results in the release of CO2 and utilisation of ATP.
  5. No sugar is produced; it is a wasteful process.
💡 Photorespiration decreases CO2 fixation efficiency; it is absent or negligible in C4 plants due to CO2 concentration mechanism.
Exam Traps — Don't Make These Mistakes
⚠ Confusing the source of oxygen released
✗ The oxygen released during photosynthesis comes from carbon dioxide.
→ Van Niel's experiments with sulphur bacteria and later radioisotope studies proved that O2 comes from water (H2O), not CO2.
✓ The oxygen released during photosynthesis comes from water (H2O), as demonstrated by van Niel and confirmed using radioisotopic techniques.
⚠ Calling dark reactions 'light-independent' in a misleading way
✗ Dark reactions occur only in the dark.
→ Dark reactions do not directly depend on light, but they depend on the products of light reactions (ATP and NADPH). They can occur in the light as well; the term 'dark reaction' is a misnomer.
✓ Dark reactions (carbon reactions) are not directly light-driven but are dependent on ATP and NADPH produced in light reactions; they occur in the stroma and can proceed in the presence or absence of light as long as ATP and NADPH are available.
⚠ Mixing up the roles of PS I and PS II
✗ Photosystem I comes first in the light reaction, then Photosystem II.
→ The photosystems are named in the order of their discovery, not in the sequence of function. In the Z scheme, PS II functions first, then PS I.
✓ In the light reactions, Photosystem II (P680) functions first, followed by Photosystem I (P700). The naming is based on the order of discovery, not the sequence of electron flow.
⚠ Confusing the primary CO2 acceptor in C3 and C4 plants
✗ The primary CO2 acceptor in C4 plants is RuBP.
→ In C4 plants, the primary CO2 acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule, not RuBP. RuBP is the primary acceptor in C3 plants (Calvin cycle).
✓ In C4 plants, the primary CO2 acceptor is PEP (phosphoenol pyruvate), catalysed by PEP carboxylase, forming OAA. In C3 plants, the primary CO2 acceptor is RuBP, catalysed by RuBisCO.
⚠ Forgetting that the Calvin cycle occurs in bundle sheath cells of C4 plants
✗ In C4 plants, the Calvin cycle occurs in mesophyll cells.
→ In C4 plants, the initial carboxylation occurs in mesophyll cells, but the Calvin cycle occurs only in bundle sheath cells. Mesophyll cells lack RuBisCO.
✓ In C4 plants, the Calvin cycle takes place in bundle sheath cells, not in mesophyll cells. Mesophyll cells carry out the initial fixation of CO2 into C4 acids.
⚠ Misunderstanding photorespiration products
✗ Photorespiration produces ATP and NADPH.
→ Photorespiration does not produce ATP or NADPH; it consumes ATP and releases CO2 without any sugar synthesis. It is a wasteful process.
✓ Photorespiration is a wasteful process where RuBisCO binds O2 instead of CO2, leading to the formation of phosphoglycolate, with no synthesis of ATP or NADPH, and release of CO2.
Compare & Contrast
First stable product of CO2 fixation
C3 Plants
C4 Plants
In C3 plants, the first stable product is a 3-carbon compound, 3-phosphoglyceric acid (PGA). In C4 plants, the first stable product is a 4-carbon compound, oxaloacetic acid (OAA).
Products formed
Cyclic Photophosphorylation
Non-cyclic Photophosphorylation
Cyclic photophosphorylation produces only ATP, while non-cyclic photophosphorylation produces both ATP and NADPH + H+.
Reaction centre and absorption peak
Photosystem I (PS I)
Photosystem II (PS II)
PS I has reaction centre P700 with absorption peak at 700 nm; PS II has reaction centre P680 with absorption peak at 680 nm.
Dependence on light
Light Reactions
Dark Reactions (Carbon Reactions)
Light reactions are directly light-driven and occur in the grana; dark reactions are not directly light-driven but depend on ATP and NADPH from light reactions, and occur in the stroma.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (2-3 marks), 1 Long Answer (5 marks). Total approximately 10-12 marks.
Most asked: Z scheme of light reaction · Chemiosmotic hypothesis · Calvin cycle (C3 pathway) · C4 pathway (Hatch and Slack Pathway) · Photorespiration · Comparison between C3 and C4 plants · Factors affecting photosynthesis (Blackman's Law of Limiting Factors)
Don't skip: Draw and explain the Z scheme of light reaction (5 marks).; Describe the Calvin cycle with a diagram (5 marks).; Differentiate between C3 and C4 plants in a table (3 marks).; Explain the chemiosmotic hypothesis of ATP synthesis (3 marks).
One-Line Revision (read aloud before the exam)
  1. Photosynthesis is a physico-chemical process by which green plants use light energy to synthesise organic compounds from CO2 and water.
  2. Priestley showed that plants restore to the air whatever breathing animals and burning candles remove.
  3. Ingenhousz proved that sunlight is essential for photosynthesis and that only green parts of plants release oxygen.
  4. Engelmann used a prism and aerobic bacteria to produce the first action spectrum of photosynthesis.
  5. Van Niel demonstrated that oxygen released in photosynthesis comes from water, not CO2.
  6. Chlorophyll a is the chief pigment; accessory pigments (chlorophyll b, xanthophylls, carotenoids) absorb light and transfer energy to chlorophyll a.
  7. The light reactions occur in the grana and produce ATP, NADPH, and O2.
  8. The Z scheme describes the non-cyclic electron flow from PS II to PS I to NADP+.
  9. Water splitting is associated with PS II: 2H2O → 4H+ + O2 + 4e-.
  10. Chemiosmotic hypothesis: ATP synthesis is driven by a proton gradient across the thylakoid membrane.
  11. The Calvin cycle occurs in the stroma and has three stages: carboxylation, reduction, and regeneration.
  12. RuBisCO catalyses both carboxylation and oxygenation; photorespiration is wasteful and occurs in C3 plants.
  13. C4 plants have Kranz anatomy and use PEP carboxylase for initial CO2 fixation, minimising photorespiration.
  14. For every CO2 fixed in the Calvin cycle, 3 ATP and 2 NADPH are required.
  15. Blackman's Law of Limiting Factors: the rate of a process is determined by the factor nearest its minimal value.
  16. Light saturation occurs at about 10% of full sunlight; CO2 is a major limiting factor for C3 plants.
  17. C4 plants have higher productivity and temperature tolerance due to absence of photorespiration.
  18. The primary CO2 acceptor in C3 plants is RuBP; in C4 plants it is PEP.
  19. Cyclic photophosphorylation produces only ATP; non-cyclic produces ATP and NADPH.
  20. The Calvin cycle is common to both C3 and C4 plants, but in C4 plants it occurs in bundle sheath cells.

MCQs — 10 Questions with Answers

Q1. Which of the following is essential for photosynthesis to occur?

  • A. Chlorophyll, light energy, and carbon dioxide ✓
  • B. Chlorophyll, glucose, and oxygen
  • C. Light energy, water, and starch
  • D. Carbon dioxide, starch, and heat

Answer: A — Chlorophyll absorbs light energy, CO₂ is the carbon source, and water is the hydrogen source; all three are essential for photosynthesis.

Q2. Priestley's experiment with a mouse and a candle in a sealed bell jar demonstrated that:

  • A. Animals cannot survive without plants
  • B. Plants restore something to the air that is damaged by breathing animals and burning candles ✓
  • C. Oxygen is harmful to living organisms
  • D. Light is not required for plant growth

Answer: B — Priestley hypothesised that plants restore to the air whatever breathing animals and burning candles remove, laying groundwork for understanding oxygen production.

Q3. In Ingenhousz's experiment using an aquatic plant, oxygen bubbles were observed:

  • A. Only in bright sunlight around green parts ✓
  • B. In both bright sunlight and dark around all parts
  • C. Only in darkness around non-green parts
  • D. Continuously regardless of light exposure

Answer: A — Ingenhousz showed bubbles formed only in bright sunlight and only around green parts, proving sunlight and chlorophyll are necessary for oxygen evolution.

Q4. Which experimental observation proved that carbon dioxide is necessary for photosynthesis?

  • A. Starch formation in variegated leaves only in green parts
  • B. No starch formation in the leaf portion enclosed in KOH-soaked tube despite light exposure ✓
  • C. Bacteria accumulation in red and blue light regions
  • D. Continuous oxygen evolution in aquatic plants under sunlight

Answer: B — KOH absorbs CO₂; the absence of starch in the enclosed portion proved CO₂ is essential, while the exposed part formed starch normally.

Q5. Engelmann's prism experiment using Cladophora algae and aerobic bacteria showed that:

  • A. All wavelengths of light are equally effective for photosynthesis
  • B. Bacteria are necessary for photosynthesis to occur
  • C. Blue and red wavelengths are most effective for photosynthesis ✓
  • D. Green light is the most important wavelength for photosynthesis

Answer: C — Bacteria accumulated in blue and red regions where oxygen evolution was greatest, establishing the action spectrum resembling chlorophyll absorption.

Q6. Which is NOT a correct conclusion from the early photosynthesis experiments?

  • A. Only green parts of plants can perform photosynthesis
  • B. Sunlight is essential for the process that purifies air
  • C. Chlorophyll is located in special bodies called chloroplasts
  • D. Photosynthesis occurs primarily in roots and non-green tissues ✓

Answer: D — All experiments consistently showed photosynthesis occurs only in green parts containing chlorophyll and chloroplasts, never in roots or non-green tissues.

Q7. Both statements below relate to early photosynthesis experiments: (I) Priestley's work showed oxygen is produced by plants. (II) Ingenhousz proved that light is essential for the oxygen-producing process. Which is true?

  • A. Both statements are correct ✓
  • B. Only statement I is correct
  • C. Only statement II is correct
  • D. Neither statement is correct

Answer: A — Priestley inferred oxygen production when the candle continued burning, and Ingenhousz directly identified oxygen bubbles in light, proving light necessity.

Q8. When a variegated leaf is tested for starch after exposure to sunlight, starch is found:

  • A. Only in the green portions because only chlorophyll-containing cells perform photosynthesis ✓
  • B. In both green and white portions equally throughout the leaf
  • C. Only in the white portions where starch accumulates for storage
  • D. Nowhere in the leaf because chlorophyll prevents starch formation

Answer: A — Chlorophyll is required to absorb light energy; non-green (white) parts lack chlorophyll and cannot photosynthesise, so no starch forms there.

Q9. In the KOH absorption experiment, if the enclosed leaf portion still formed starch despite KOH presence, this would suggest: (Assume KOH did not interfere with the leaf's metabolism)

  • A. CO₂ is not necessary for photosynthesis
  • B. The leaf obtained CO₂ from another source besides air ✓
  • C. Light intensity was the limiting factor instead of CO₂
  • D. Chlorophyll functions without CO₂

Answer: B — If starch formed despite KOH absorption, the leaf must have obtained CO₂ internally (from respiration or stored compounds), not from the air.

Q10. HOTS: Melvin Calvin used C¹⁴-labelled CO₂ to trace photosynthesis pathways and won the Nobel Prize. This approach was significant because it: (A) Provided direct evidence that CO₂ enters the Calvin cycle. (B) Showed that light reactions are independent of dark reactions. (C) Proved that starch is the final product of photosynthesis. (D) Demonstrated that chlorophyll directly converts light to chemical energy.

  • A. Provided direct evidence that CO₂ enters the Calvin cycle ✓
  • B. Showed that light reactions are independent of dark reactions
  • C. Proved that starch is the final product of photosynthesis
  • D. Demonstrated that chlorophyll directly converts light to chemical energy

Answer: A — C¹⁴ tracing revealed the exact carbon assimilation pathway, establishing how CO₂ is fixed and incorporated into organic compounds step-by-step.

Flashcards

Define photosynthesis in simple terms.

Photosynthesis is a physico-chemical process by which green plants use light energy to synthesise organic compounds (glucose) from CO₂ and water.

What was Priestley's key observation about plants?

Priestley observed that a plant in a sealed bell jar restored the air fouled by a burning candle or breathing animal, suggesting plants restore something to the air.

How did Ingenhousz improve Priestley's experiment?

Ingenhousz showed that sunlight is essential for the plant process and only green parts release oxygen (identified as bubbles in aquatic plants).

What did Engelmann's prism experiment demonstrate?

Engelmann showed that bacteria accumulated in blue and red regions of the light spectrum, establishing that these wavelengths are most effective for photosynthesis.

Why is chlorophyll location important for photosynthesis?

Chlorophyll is located in chloroplasts, special organelles where glucose is synthesised and usually stored as starch.

What three substances are essential for photosynthesis to occur?

Chlorophyll (green pigment), light energy, and carbon dioxide (CO₂) are the three essential requirements for photosynthesis.

Why do variegated leaves show photosynthesis only in green parts?

Only green parts contain chlorophyll, the pigment necessary to absorb light energy and drive photosynthesis.

What does KOH do in the leaf photosynthesis experiment?

KOH-soaked cotton absorbs CO₂ from the air, allowing demonstration that carbon dioxide is required for starch formation during photosynthesis.

Who discovered oxygen and in what year?

Joseph Priestley discovered oxygen in 1774, a discovery that emerged from his experiments on plant respiration and photosynthesis.

What did Julius von Sachs prove about green plants?

Sachs provided evidence that glucose is produced during plant growth and that chlorophyll in chloroplasts is the site of glucose synthesis.

Important Board Questions

Define photosynthesis and state any two reasons why it is important for life on Earth. [2 marks]

Define as light-driven synthesis of organic compounds from CO₂ and water in chloroplasts. Two reasons: (1) primary food source for all organisms, (2) oxygen release essential for respiration.

Describe Priestley's experiment with a plant, mouse, and burning candle in a sealed bell jar. What conclusion did he draw, and how did Ingenhousz improve this experiment using aquatic plants? [5 marks]

Priestley: sealed jar setup, candle extinguished, mouse suffocated, but plant restored air — hypothesis that plants restore breathing-damaged air. Ingenhousz: used light vs. dark conditions, identified oxygen bubbles in light only, proved sunlight essential for oxygen production. Use action spectrum concept.

Using evidence from early experiments (variegated leaf, KOH absorption, Engelmann's prism setup, and Julius von Sachs' observations), explain what three essential factors are required for photosynthesis and where it occurs in the plant cell. Why did these experiments form the foundation for modern understanding of photosynthesis? [6 marks]

Three factors: chlorophyll (variegated leaf proves this), light (Ingenhousz and KOH experiment), CO₂ (KOH test). Location: chloroplasts in green cells (Sachs). Importance: established that light + CO₂ + chlorophyll → glucose; action spectrum showed effective wavelengths; direct evidence pathway for modern molecular studies by Calvin using C¹⁴. Explain how each experiment isolated one variable.

Assertion-Reason Questions for Photosynthesis in Higher Plants

Q1. Assertion (A):

Chlorophyll is the essential pigment for photosynthesis in green plants.

Reason (R):

Chlorophyll is the only pigment present in plant leaves and absorbs all wavelengths of light equally.

Answer: C — A is true but R is false — The assertion is true — chlorophyll is essential for photosynthesis. However, the reason is false because chlorophyll is not the only pigment (accessory pigments like carotenoids exist) and does not absorb all wavelengths equally (it absorbs blue and red maximally, reflects green).

Q2. Assertion (A):

Photosynthesis is a light-dependent process that converts light energy into chemical energy stored in glucose.

Reason (R):

Priestley's bell jar experiment and Ingenhousz's sunlight experiment together demonstrated that light energy is converted into chemical products that organisms depend on for survival.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. Priestley showed plants produce O₂ (chemical product), and Ingenhousz showed this requires sunlight. Together they established light-dependent conversion of energy, supporting the assertion that light energy is converted to chemical energy.

Q3. Assertion (A):

Oxygen bubbles observed by Ingenhousz in aquatic plants under sunlight are direct evidence that photosynthesis occurs only in green parts.

Reason (R):

Oxygen is produced only during the dark reaction of photosynthesis and therefore appears as bubbles only near chlorophyll-containing regions.

Answer: C — A is true but R is false — The assertion is true — Ingenhousz observed O₂ bubbles in green parts under light, proving photosynthesis occurs there. However, the reason is false because oxygen is produced during light reactions (photolysis of water in photosystem II), not dark reactions. The assertion is supported by correct reasoning about light reactions, not dark reactions.

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Case-Based Questions (CBQ) for Photosynthesis in Higher Plants

Case Study:

A biology class conducted an experiment to understand photosynthesis using aquatic plants (Hydrilla). They placed identical plants in three different setups: Setup A under white light, Setup B under red light (600 nm), and Setup C under green light (550 nm). After 4 hours of exposure, they counted the number of oxygen bubbles released from each plant. The results showed: Setup A produced 120 bubbles, Setup B produced 95 bubbles, and Setup C produced 15 bubbles. The class then reviewed Engelmann's historical experiment and compared their results with his action spectrum findings.

Q1. Based on the experimental data, which light wavelength was most effective for photosynthesis?

  • A. Green light (550 nm) as it produced the most bubbles
  • B. White light as it produced the maximum number of bubbles (120) ✓
  • C. Red light (600 nm) as it is closer to the infrared spectrum
  • D. All wavelengths are equally effective for photosynthesis

Answer: B

Q2. Why did green light produce the fewest oxygen bubbles compared to white and red light?

  • A. Green light has insufficient energy to drive photosynthesis
  • B. Chlorophyll absorbs green light least effectively and reflects most of it ✓
  • C. Green light is absorbed by water and does not reach the plant
  • D. Plants have evolved to reject green light in all conditions

Answer: B

Q3. How do these experimental results align with Engelmann's findings about the action spectrum?

  • A. They contradict Engelmann because red light produced more bubbles than in his experiment
  • B. They prove that bacterial accumulation in Engelmann's experiment was unrelated to photosynthesis
  • C. They show that white light is always superior, unlike Engelmann's predictions
  • D. They support Engelmann's conclusion that blue and red light wavelengths are most effective for photosynthesis ✓

Answer: D

Q4. If the experimental setup in Setup B (red light) was modified to include additional red light intensity (doubling the illumination), what would likely happen to oxygen bubble production?

  • A. Bubble production would decrease by half due to reduced chlorophyll efficiency
  • B. Bubble production would remain unchanged because wavelength, not intensity, determines photosynthetic rate
  • C. Bubble production would approximately double because more photons are available for photosynthesis ✓
  • D. Bubble production would exceed Setup A because red light is superior to white light

Answer: C

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