Photosynthesis in Higher Plants Class 11 Biology: Light Reaction, Calvin Cycle and C4 Pathway

What Photosynthesis in Higher Plants is about

Weightage: This chapter is part of Unit IV, Plant Physiology, which carries 12 of the 70 theory marks in the school annual exam together with Respiration in Plants and Plant Growth and Development.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit IV Plant Physiology

Key concepts

What we know and the early experiments

NCERT section 11.1 What Do We Know?; 11.2 Early Experiments

Basic facts (recalled from earlier classes): photosynthesis needs chlorophyll, light and CO₂. A variegated leaf (green and white parts) or a leaf partly covered with black paper, kept in light and then tested for starch, shows starch only in the green, exposed parts. In another set-up, half a leaf inside a test tube containing KOH-soaked cotton (which absorbs CO₂) makes no starch, proving CO₂ is needed.

Milestones:

• Joseph Priestley (1770): a candle or a mouse in a closed bell jar soon dies or goes out, but a mint plant kept inside lets the mouse live and the candle burn. He concluded that plants restore to the air what breathing animals and burning candles remove (later known to be oxygen; Priestley discovered oxygen in 1774).
• Jan Ingenhousz (1730-1799): using Priestley's set-up in light and dark, showed that sunlight is needed for plants to purify air. With an aquatic plant, he showed small bubbles form around green parts in bright light, but not in the dark; the bubbles were oxygen. So only green parts release oxygen.
• Julius von Sachs (1854): plants produce glucose, usually stored as starch, and the green substance (chlorophyll) is located in special bodies (later called chloroplasts) inside cells.
• T.W. Engelmann (1843-1909): split light into its colours with a prism and shone it on the green alga *Cladophora* placed in a suspension of aerobic bacteria. The bacteria gathered mainly in the blue and red regions, where most oxygen was released. This gave the first action spectrum, which roughly matches the absorption spectra of chlorophyll a and b.
• Cornelius van Niel (1897-1985): worked on purple and green sulphur bacteria, which use H₂S instead of H₂O and release sulphur instead of O₂. He showed photosynthesis is a light-dependent reaction in which hydrogen from a suitable donor reduces CO₂ to carbohydrate. He inferred that the O₂ released by green plants comes from H₂O, not CO₂. This was later confirmed using radioisotopic techniques.

General equation: 2H₂A + CO₂ → 2A + CH₂O + H₂O (light). For green plants H₂A = H₂O; for sulphur bacteria H₂A = H₂S.

Worked example

What did Engelmann's experiment with Cladophora and aerobic bacteria show, and how did van Niel's work change our idea of where oxygen comes from? (3 marks)

Try it first, then show the solutionSolution

1. Aerobic bacteria gathered most in blue and red light, showing these wavelengths drive the most oxygen release (first action spectrum). (1)
2. This action spectrum resembles the absorption spectra of chlorophyll a and b, linking chlorophyll to photosynthesis. (1)
3. Van Niel, studying sulphur bacteria that use H₂S and release sulphur, inferred that the hydrogen donor is oxidised; in green plants the donor is water, so the O₂ released comes from H₂O, not CO₂. (1)

Answer: Blue and red light are most effective; O₂ comes from water.

Site of photosynthesis and pigments (elementary idea)

NCERT section 11.3 Where Does Photosynthesis Take Place?; 11.4 How Many Types of Pigments Are Involved in Photosynthesis?

Site: photosynthesis occurs in the green parts, mainly leaves. Within a leaf, the mesophyll cells have many chloroplasts. Chloroplasts usually line up along the cell walls so they receive the best light; they turn their flat surfaces towards the light when it is moderate and their edges when it is very strong.

Inside the chloroplast:

• Grana (stacks of thylakoids) and stroma lamellae: membrane system that traps light and makes ATP and NADPH: the light reactions.
• Stroma: contains enzymes that use ATP and NADPH to fix CO₂ into sugar: the biosynthetic (carbon) reactions. These do not directly need light but depend on the products of the light reactions; calling them 'dark reactions' does not mean they happen in the dark.

Pigments (separated by paper chromatography of a leaf extract):

• Chlorophyll a: bright or blue-green; the chief pigment. Most photosynthesis occurs in the blue and red regions.
• Chlorophyll b: yellow-green.
• Xanthophylls: yellow.
• Carotenoids: yellow to yellow-orange.

Absorption spectrum: how much light of each wavelength a pigment absorbs. Chlorophyll a absorbs most in blue and red.
Action spectrum: the rate of photosynthesis at each wavelength. It closely matches the absorption spectrum of chlorophyll a, but some photosynthesis also occurs at other wavelengths.

Accessory pigments (chlorophyll b, xanthophylls, carotenoids):
1. Absorb light of other wavelengths and transfer the energy to chlorophyll a, so a wider range of light is used.
2. Protect chlorophyll a from photo-oxidation.

Worked example

Give two roles of accessory pigments and name the colour of each pigment band seen in chromatography of a leaf extract. (3 marks)

Try it first, then show the solutionSolution

1. They absorb wavelengths that chlorophyll a cannot and pass the energy to chlorophyll a, widening the usable spectrum. (1)
2. They protect chlorophyll a from photo-oxidation. (1)
3. Chlorophyll a blue-green; chlorophyll b yellow-green; xanthophylls yellow; carotenoids yellow to yellow-orange. (1)

Answer: Broaden light capture and protect chlorophyll a; colours as listed.

Light reaction, photosystems and the Z scheme

NCERT section 11.5 What Is Light Reaction?; 11.6 The Electron Transport (11.6.1 Splitting of Water)

Light reactions (photochemical phase) include: light absorption, water splitting, oxygen release, and formation of ATP and NADPH.

Light harvesting complexes (LHC): pigments are organised into two photosystems, PS I and PS II, named in the order of their discovery, not the order in which they work. Each has many pigment molecules (hundreds of chlorophyll molecules and accessory pigments) forming an antenna, which absorbs light of different wavelengths and passes it to a single reaction centre: one chlorophyll a molecule.

• PS I: reaction centre absorbs maximally at 700 nm: P700.
• PS II: reaction centre absorbs maximally at 680 nm: P680.

Electron transport (non-cyclic, Z scheme):
1. PS II absorbs 680 nm red light; electrons in its reaction centre are excited and boosted to a higher energy level.
2. They are taken up by a primary electron acceptor, which passes them along an electron transport system of cytochromes. Movement is downhill in redox potential.
3. The electrons reach PS I. Meanwhile PS I electrons, excited by 700 nm light, have been passed to another acceptor with a higher redox potential.
4. These are then moved downhill to NADP⁺, which is reduced to NADPH + H⁺.
5. The whole path from PS II, uphill to the acceptor, down to PS I, excitation, uphill again, then down to NADP⁺, gives a Z shape when drawn against redox potential: the Z scheme.

Splitting of water (photolysis): PS II keeps losing electrons, so it needs a supply. These come from water, which is split, a process linked to PS II:
2H₂O → 4H⁺ + O₂ + 4e⁻

• Protons stay in the lumen; O₂ is released as a by-product.
• The water-splitting complex is on the inner side of the thylakoid membrane, so protons and O₂ form in the lumen.

Worked example

Trace the path of electrons in the Z scheme from water to NADP⁺. Why is it called the Z scheme? (3 marks)

Try it first, then show the solutionSolution

1. Water is split near PS II (2H₂O → 4H⁺ + O₂ + 4e⁻); electrons replace those lost by P680. (1)
2. Light excites P680; electrons pass via a primary acceptor and cytochrome chain downhill to PS I; light excites P700, and electrons pass via another acceptor to NADP⁺, forming NADPH + H⁺. (1)
3. Plotted on a redox-potential scale, the two uphill boosts and two downhill runs form a Z shape. (1)

Answer: H₂O → PS II → acceptor → cytochromes → PS I → acceptor → NADP⁺; zigzag path gives the name.

Cyclic and non-cyclic photophosphorylation; chemiosmotic hypothesis

NCERT section 11.6.2 Cyclic and Non-cyclic Photo-phosphorylation; 11.6.3 Chemiosmotic Hypothesis

Photophosphorylation: making ATP from ADP and inorganic phosphate (Pi) in the presence of light.

Non-cyclic photophosphorylation:

• Both PS II and PS I work in series (Z scheme).
• Electrons flow from water to NADP⁺ and do not return.
• Products: ATP, NADPH and O₂.
• Occurs in the grana thylakoids.

Cyclic photophosphorylation:

• Only PS I works. The excited electron passes along the transport chain and returns to PS I (cyclic flow).
• Products: ATP only (no NADPH, no O₂, since water is not split).
• Likely site: the stroma lamellae, whose membranes lack PS II and NADP reductase.
• Also occurs when only light of wavelengths beyond 680 nm is available.

Chemiosmotic hypothesis (how ATP is actually made):
ATP synthesis is linked to the formation of a proton gradient across the thylakoid membrane. Protons build up inside the lumen because:
1. Water splitting occurs on the inner (lumen) side, releasing H⁺ into the lumen.
2. As electrons move through the photosystems, the primary acceptor on the outer (stroma) side passes electrons to an H carrier, which picks up a proton from the stroma and releases it into the lumen.
3. NADP reductase is on the stroma side; reducing NADP⁺ to NADPH + H⁺ removes protons from the stroma.

Result: high H⁺ in the lumen, low H⁺ in the stroma, so lower pH in the lumen.

ATP synthase: protons can only flow back to the stroma through the CF₀ part of ATP synthase.

• CF₀: embedded in the thylakoid membrane; forms a transmembrane channel for facilitated diffusion of protons.
• CF₁: projects into the stroma (on the outer surface of the thylakoid facing the stroma). The energy of the proton flow causes a conformational change in CF₁, which makes ATP.

The energy from the electron flow is thus used to pump protons, and the proton gradient drives ATP synthesis. ATP and NADPH are then used in the stroma for CO₂ fixation.

Worked example

Give three reasons why protons accumulate inside the thylakoid lumen during the light reaction. (3 marks)

Try it first, then show the solutionSolution

1. Water is split on the lumen side of the membrane, releasing H⁺ into the lumen. (1)
2. An H carrier picks up protons from the stroma and releases them into the lumen as electrons pass through the transport chain. (1)
3. NADP reductase on the stroma side uses H⁺ from the stroma to form NADPH, lowering stromal H⁺. (1)

Answer: Water splitting inside, proton pumping by carriers, and NADPH formation in the stroma.

Biosynthetic phase: the Calvin cycle (C3 pathway)

NCERT section 11.7 Where are the ATP and NADPH Used? (11.7.1 The Primary Acceptor of CO₂; 11.7.2 The Calvin Cycle)

The products of the light reaction (ATP, NADPH) are used to fix CO₂ into sugar in the stroma. Once light stops, the biosynthesis continues for a short while and then stops; it starts again when light is available. So 'dark reaction' is a misnomer.

Discovery: Melvin Calvin used radioactive ¹⁴C in algal photosynthesis and found the first stable product is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound. Hence plants using this pathway are C3 plants.

Primary CO₂ acceptor: RuBP (ribulose-1,5-bisphosphate), a 5-carbon ketose sugar. (Scientists first searched for a 2-carbon acceptor and failed.)

Three stages of the Calvin cycle:
1. Carboxylation: CO₂ is fixed with RuBP to form two molecules of 3-PGA, catalysed by RuBisCO (RuBP carboxylase-oxygenase). This is the most crucial step.
2. Reduction: 3-PGA is reduced to carbohydrate (triose phosphate) using ATP for phosphorylation and NADPH for reduction. Fixing 6 CO₂ and removing one glucose needs 6 turns of the cycle.
3. Regeneration: the CO₂ acceptor RuBP is re-formed, which needs one ATP per turn (phosphorylation), so the cycle continues.

Energy accounting:

• Per CO₂ fixed: 3 ATP + 2 NADPH (2 ATP in reduction + 1 ATP in regeneration).
• Per glucose (6 CO₂): 18 ATP + 12 NADPH.
• Ins and outs for one glucose: In: 6 CO₂, 18 ATP, 12 NADPH. Out: 1 glucose, 18 ADP, 12 NADP⁺.
• This is probably why cyclic phosphorylation is needed: the Z scheme alone may not supply the extra ATP relative to NADPH.

The Calvin pathway occurs in all photosynthetic plants, whether C3 or C4.

Worked example

Name the three stages of the Calvin cycle and state the role of RuBisCO and of ATP in each stage where it is used. (3 marks)

Try it first, then show the solutionSolution

1. Carboxylation: RuBisCO combines CO₂ with RuBP to form two 3-PGA. (1)
2. Reduction: ATP and NADPH convert 3-PGA to triose phosphate/carbohydrate. (1)
3. Regeneration: ATP is used to re-form RuBP so the cycle continues. (1)

Answer: Carboxylation (RuBisCO), reduction (ATP + NADPH), regeneration (ATP).

The C4 pathway and photorespiration

NCERT section 11.8 The C4 Pathway; 11.9 Photorespiration

C4 plants are adapted to dry tropical regions. Their first CO₂ fixation product is a 4-carbon acid, oxaloacetic acid (OAA), but they still use the Calvin cycle for making sugar. Examples: maize, sorghum. Their features:

• Kranz anatomy ('kranz' = wreath): large bundle sheath cells form a ring around the vascular bundles. These cells have many chloroplasts, thick walls impervious to gas exchange, and no intercellular spaces.
• Tolerate higher temperatures and show a response to high light intensity.
• Lack photorespiration and have greater biomass productivity.

Hatch and Slack pathway (cyclic):
1. In mesophyll cells, the primary acceptor phosphoenolpyruvate (PEP, 3C) fixes CO₂ by PEP carboxylase (PEPcase) to form OAA (4C). Mesophyll cells lack RuBisCO.
2. OAA is converted to other 4C acids such as malic acid or aspartic acid, which move to the bundle sheath cells.
3. In bundle sheath cells, the 4C acid is broken down to release CO₂ and a 3C molecule.
4. The 3C molecule returns to the mesophyll and is converted back to PEP.
5. The released CO₂ enters the Calvin cycle in the bundle sheath cells, which are rich in RuBisCO but lack PEPcase.

Photorespiration (in C3 plants):

• RuBisCO can bind both CO₂ and O₂, but it has a much greater affinity for CO₂ when both are at equal concentration. The relative concentration decides which one binds.
• In C3 plants, some O₂ binds RuBisCO, so less CO₂ is fixed. RuBP + O₂ gives one 3-PGA and one phosphoglycolate.
• In this pathway there is no synthesis of sugars, ATP or NADPH; instead CO₂ is released with the use of ATP. So photorespiration is a wasteful process.
• C4 plants avoid it: they increase CO₂ concentration at the enzyme site in the bundle sheath, so RuBisCO works mainly as a carboxylase. This is why C4 plants are more productive even though only bundle sheath cells run the Calvin cycle.

Worked example

In a C4 plant, name the CO₂ acceptor and enzyme in mesophyll cells and in bundle sheath cells. Why does photorespiration not occur in C4 plants? (3 marks)

Try it first, then show the solutionSolution

1. Mesophyll: PEP is the acceptor, PEP carboxylase the enzyme (forms OAA). (1)
2. Bundle sheath: RuBP is the acceptor, RuBisCO the enzyme (Calvin cycle). (1)
3. The C4 cycle concentrates CO₂ in bundle sheath cells, so RuBisCO binds CO₂ rather than O₂ and acts as a carboxylase; the oxygenase reaction is minimised. (1)

Answer: PEP/PEPcase in mesophyll; RuBP/RuBisCO in bundle sheath; high CO₂ around RuBisCO prevents photorespiration.

Factors affecting photosynthesis and the law of limiting factors

NCERT section 11.10 Factors Affecting Photosynthesis

Internal (plant) factors: number, size, age and orientation of leaves; mesophyll cells and chloroplasts; internal CO₂ concentration; amount of chlorophyll. These depend on the plant's genetics and growth.
External factors: sunlight, temperature, CO₂ concentration and water.

Blackman's Law of Limiting Factors (1905): if a chemical process is affected by more than one factor, its rate is determined by the factor that is nearest its minimal value; it is the factor that directly affects the process if its quantity is changed.
Example: a green leaf with optimal light and CO₂ may still not photosynthesise if the temperature is very low; giving it the optimal temperature starts photosynthesis.

Light:

• At low intensities, the rate rises linearly with light.
• At higher intensities the rate levels off because other factors become limiting.
• Light saturation occurs at about 10% of full sunlight, so except for plants in shade or dense forests, light is rarely limiting in nature.
• Very high light causes breakdown of chlorophyll and a fall in photosynthesis.

Carbon dioxide (the major limiting factor):

• Atmospheric CO₂ is very low: 0.03-0.04%. Raising it up to 0.05% increases fixation; higher levels can be damaging over longer periods.
• At low light, neither C3 nor C4 plants respond to high CO₂; at high light both do.
• C4 plants saturate at about 360 µL L⁻¹; C3 plants keep responding and saturate only beyond 450 µL L⁻¹. So present CO₂ levels limit C3 plants.
• Use: greenhouse crops such as tomatoes and bell pepper are grown in CO₂-enriched air for higher yields.

Temperature: the dark (biosynthetic) reactions are enzymatic and so temperature-controlled; light reactions are much less affected. C4 plants have a higher temperature optimum; tropical plants have higher optima than temperate plants.

Water: its effect is mostly indirect. Water stress closes stomata, cutting CO₂ supply, and makes leaves wilt, reducing leaf surface area and metabolic activity.

Worked example

State Blackman's law of limiting factors. Explain why supplying more light to a C3 crop growing in bright midday sun may not increase photosynthesis, while adding CO₂ might. (3 marks)

Try it first, then show the solutionSolution

1. When several factors affect a process, its rate is set by the factor closest to its minimal value; changing that factor changes the rate. (1)
2. Light saturation occurs at about 10% of full sunlight, so in bright sun light is no longer limiting. (1)
3. CO₂ (0.03-0.04%) is the major limiting factor; C3 plants keep responding to CO₂ up to beyond 450 µL L⁻¹, so adding CO₂ increases the rate. (1)

Answer: The limiting factor is CO₂, not light, in bright sun.

Photosynthesis in Higher Plants in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Calvin Cycle, C4 Pathway and Photorespiration
• Light Reaction and the Z Scheme

Photosynthesis in Higher Plants: key facts

Overall photosynthesis

$6{\text{CO}}_2+12{\text{H}}_2\text{O}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{H}}_2\text{O}+6{\text{O}}_2$

Light and chlorophyll needed; all O₂ comes from water.

Photolysis of water

$2{\text{H}}_2\text{O}\to 4{\text{H}}^{+}+{\text{O}}_2+4e^{-}$

Linked to PS II, on the lumen side of the thylakoid.

Van Niel's general equation

$2{\text{H}}_2\text{A}+{\text{CO}}_2\to 2\text{A}+{\text{CH}}_2\text{O}+{\text{H}}_2\text{O}$

A = O for green plants, S for sulphur bacteria.

Calvin cycle cost

${\text{per CO}}_2:3\text{ATP}+2\text{NADPH};\text{per glucose (6 turns)}:18\text{ATP}+12\text{NADPH}$

2 ATP in reduction + 1 ATP in regeneration per CO₂.

Carboxylation

$\text{RuBP (5C)}+{\text{CO}}_2\stackrel{\text{RuBisCO}}{\to }2\times \text{3-PGA (3C)}$

First stable product of C3 plants.

C4 fixation

$\text{PEP (3C)}+{\text{CO}}_2\stackrel{\text{PEPcase}}{\to }\text{OAA (4C)}$

In mesophyll cells; OAA → malate/aspartate → bundle sheath.

Photosynthesis in Higher Plants: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Photosynthesis in Higher Plants

The question patterns to expect, and the method that works for each.

Tricks to remember Photosynthesis in Higher Plants

Short memory aids. Say each one aloud once, then use it on a question the same day.

• PS II comes first, but was found second

  Photosystems are numbered by discovery; electrons go II → I. P680 is 'smaller number, works first'.

  Use it when: Order in the Z scheme.

• Cyclic = Car-wash loop

  Only PS I, electron goes round and comes back, only ATP made (no NADPH, no O₂).

  Use it when: Cyclic photophosphorylation.

• 3-2 per CO₂, 18-12 per sugar

  Three ATP and two NADPH per CO₂; multiply by 6 for glucose.

  Use it when: Calvin cycle numericals.

• C-R-R

  Carboxylation, Reduction, Regeneration: 'Calvin Rides Round'.

  Use it when: Stages of the Calvin cycle.

• PEP in the Mesophyll, RuBisCO in the Sheath

  'M' for Mesophyll and the 'M' in PEP-Malate; bundle Sheath does the Sugar (Calvin).

  Use it when: Cell types in C4 plants.

• Priestley-Ingenhousz-Sachs-Engelmann-van Niel

  'Plants Inhale Sunlight, Expel Vapour-oxygen': air restored, light needed, starch made, red-blue works, water gives O₂.

  Use it when: Order and findings of early experiments.

Photosynthesis in Higher Plants: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Photosynthesis in Higher Plants

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying oxygen released in photosynthesis comes from CO₂

  CO₂ contains oxygen and is the visible reactant.

  ✓Do this: All O₂ comes from water (van Niel; confirmed with isotopes). That is why 12 H₂O appear in the equation.

  Usually costs: 1 mark.

• ✗Mistake: Writing that dark reactions occur only at night

  The name is taken literally.

  ✓Do this: Biosynthetic reactions do not directly need light but use ATP and NADPH from light reactions; they occur in daytime and stop soon after dark.

  Usually costs: 1 mark.

• ✗Mistake: Stating that cyclic photophosphorylation produces NADPH

  Products of the two types are mixed.

  ✓Do this: Cyclic: ATP only. Non-cyclic: ATP, NADPH and O₂.

  Usually costs: 1 mark.

• ✗Mistake: Saying C4 plants do not have the Calvin cycle

  C3 and C4 are thought to be alternative pathways.

  ✓Do this: C4 plants run the Calvin cycle in bundle sheath cells; the C4 cycle only concentrates CO₂.

  Usually costs: 1 mark.

• ✗Mistake: Placing RuBisCO in the mesophyll of C4 plants

  In C3 plants mesophyll does have RuBisCO.

  ✓Do this: C4 mesophyll has PEPcase and lacks RuBisCO; bundle sheath has RuBisCO and lacks PEPcase.

  Usually costs: 1 mark.

• ✗Mistake: Getting the proton gradient backwards

  Mitochondrial and chloroplast gradients are confused.

  ✓Do this: In chloroplasts H⁺ accumulates in the thylakoid lumen and flows out to the stroma through CF₀; CF₁ faces the stroma.

  Usually costs: 1-2 marks.

NCERT exercise guide: Photosynthesis in Higher Plants

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Photosynthesis in Higher Plants

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which of these is the chief photosynthetic pigment?
   (a) Chlorophyll b (b) Xanthophyll (c) Chlorophyll a (d) Carotene

   Hint

   Reaction centre.

   Answer

   (c) Chlorophyll a

2. Q2 · 1 mark · MCQExam level

   The enzyme that fixes CO₂ in the mesophyll cells of C4 plants is:
   (a) RuBisCO (b) PEP carboxylase (c) ATP synthase (d) NADP reductase

   Hint

   Mesophyll lacks RuBisCO.

   Answer

   (b) PEP carboxylase

3. Q3 · 2 marks · Very short answerExam level

   List the four events of the light reaction. (2 marks)

   Hint

   Absorb, split, release, make.

   Answer

   Light absorption; splitting of water; release of oxygen; formation of ATP and NADPH (½ each).

4. Q4 · 2 marks · Very short answerHarder exam level

   What did Julius von Sachs show about photosynthesis? (2 marks)

   Hint

   Product and location.

   Answer

   Plants produce glucose, usually stored as starch (1); the green substance is located in special bodies (chloroplasts) within plant cells (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   Explain why chloroplasts turn their edges towards very strong light. (2 marks)

   Hint

   Too much light damages chlorophyll.

   Answer

   Very high light can break down chlorophyll and reduce photosynthesis (1); turning edge-on reduces the light received, protecting the pigments, while in moderate light they turn flat surfaces to catch more (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   Explain why water stress reduces photosynthesis even though only a little water is used as a reactant. (2 marks)

   Hint

   Indirect effects.

   Answer

   Water stress closes stomata, reducing CO₂ entry (1); leaves wilt, reducing surface area and metabolic activity (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Describe the Hatch and Slack pathway. (3 marks)

   Hint

   PEP → OAA → malate → CO₂ released.

   Answer

   In mesophyll, PEP (3C) accepts CO₂ by PEPcase to form OAA (4C), converted to malic or aspartic acid (1). These move to bundle sheath cells, where they release CO₂ and a 3C acid; CO₂ enters the Calvin cycle with RuBisCO (1). The 3C acid returns to the mesophyll and is converted back to PEP, completing the cycle (1).

8. Q8 · 3 marks · Short answerHarder exam level

   Leaves of two plants are given ¹⁴CO₂ for a few seconds. In plant P the label first appears in a 3-carbon acid; in plant Q it first appears in a 4-carbon acid. (a) Name the first product and the primary CO₂ acceptor in each plant. (b) In which cells of Q will the label show up in 3-PGA a little later, and why? (c) Which plant would lose more fixed carbon to photorespiration on a hot, bright day? (3 marks)

   Hint

   Calvin traced the first product with ¹⁴C; C4 plants still run the Calvin cycle.

   Answer

   (a) P: 3-PGA, acceptor RuBP (C3 plant); Q: oxaloacetic acid (OAA), acceptor PEP (C4 plant) (1). (b) In the bundle sheath cells: the 4C acid moves there and releases CO₂, which RuBisCO fixes with RuBP into 3-PGA in the Calvin cycle (1). (c) P: in C3 plants RuBisCO also binds O₂, whereas Q concentrates CO₂ around RuBisCO in the bundle sheath (1).

9. Q9 · 5 marks · Long answerHarder exam level

   (a) Explain the chemiosmotic hypothesis of ATP formation in chloroplasts. (b) Isolated thylakoids are lit only with light of wavelengths longer than 680 nm. Which type of photophosphorylation runs, what does it produce, and why is no oxygen released? (5 marks)

   Hint

   Three causes of the gradient; CF₀/CF₁. Which reaction centre can still be excited beyond 680 nm?

   Answer

   (a) Proton gradient across the thylakoid membrane: water splitting releases H⁺ in the lumen; an H carrier moves H⁺ from stroma to lumen; NADP reductase removes H⁺ from the stroma (1½). Protons flow back through CF₀ of ATP synthase; the energy changes CF₁'s shape and ATP forms on the stroma side (1). (b) Only PS I (P700) is excited, so cyclic photophosphorylation runs: excited electrons pass along the transport chain and return to PS I (1). It makes ATP only, with no NADPH (1). PS II (P680) is not excited, so water is not split and no O₂ is released (½).

10. Q10 · 3 marks · Short answerHOTS / competency

    Engelmann's action spectrum peaks in blue and red light. Predict what would happen to the bacteria in his experiment if Cladophora were replaced by a mutant alga lacking chlorophyll b and carotenoids but with normal chlorophyll a. Explain. (3 marks)

    Hint

    Accessory pigments widen the spectrum.

    Answer

    Bacteria would still gather in blue and red because chlorophyll a absorbs these and drives photosynthesis (1). The narrower spectrum would give fewer bacteria in regions where accessory pigments normally absorb (for example, parts of the blue-green region) (1). The alga might also suffer photo-oxidation of chlorophyll a in bright light without carotenoid protection (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    Suppose a C3 plant is suddenly moved from bright light to darkness. Predict how the levels of 3-PGA and RuBP in its chloroplasts change over the next minute, and explain. (3 marks)

    Hint

    Which steps need ATP/NADPH?

    Answer

    In darkness, ATP and NADPH run out quickly (1). Carboxylation (RuBP + CO₂ → 3-PGA) does not need ATP, so it continues briefly, using up RuBP and forming 3-PGA (1). Reduction of 3-PGA and regeneration of RuBP need ATP/NADPH and stop, so 3-PGA accumulates and RuBP falls (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): C4 plants are more productive than C3 plants in hot, sunny conditions.
    Reason (R): RuBisCO has a greater affinity for O₂ than for CO₂.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Check R carefully.

    Answer

    (c) RuBisCO has a much greater affinity for CO₂; C4 plants are more productive because they concentrate CO₂ and avoid photorespiration.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which plant did Priestley use in his bell-jar experiment?Show answer

   A mint plant.

2. 2.Which alga did Engelmann use?Show answer

   Cladophora.

3. 3.What is the reaction centre of PS II?Show answer

   P680 (chlorophyll a absorbing maximally at 680 nm).

4. 4.What is the first stable product of the Calvin cycle?Show answer

   3-phosphoglyceric acid (3-PGA).

5. 5.What is the primary CO₂ acceptor in C3 plants?Show answer

   RuBP, a 5-carbon ketose sugar.

6. 6.What is the primary CO₂ acceptor in C4 mesophyll cells?Show answer

   Phosphoenolpyruvate (PEP), a 3-carbon compound.

7. 7.Name the two parts of the chloroplast ATP synthase.Show answer

   CF₀ (membrane channel) and CF₁ (projecting into stroma).

8. 8.Where does cyclic photophosphorylation probably occur?Show answer

   Stroma lamellae.

9. 9.What are the products of the oxygenation of RuBP?Show answer

   One 3-PGA and one phosphoglycolate.

10. 10.At what fraction of full sunlight does light saturation occur?Show answer

    About 10%.

11. 11.Name two greenhouse crops grown in CO₂-enriched air.Show answer

    Tomatoes and bell pepper.

12. 12.Who stated the law of limiting factors, and when?Show answer

    Blackman, 1905.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you state what each early experiment proved?
• ☐Can you list the four pigments with their colours and two roles of accessory pigments?
• ☐Can you draw the Z scheme with P680, P700, cytochromes and NADP⁺?
• ☐Can you compare cyclic and non-cyclic photophosphorylation in four points?
• ☐Can you explain three causes of the proton gradient and the role of CF₀ and CF₁?
• ☐Can you work out ATP and NADPH needed for any number of glucose molecules?
• ☐Can you describe the Hatch and Slack pathway and Kranz anatomy?
• ☐Can you explain why photorespiration is wasteful and why C4 plants avoid it?

Photosynthesis in Higher Plants: frequently asked questions