Light and Dark Reactions of Photosynthesis (ICSE Class 10 Biology)
Updated 16 October 2026 · Fact-checked
Photosynthesis runs in two linked phases. The light (photochemical) reaction in the grana uses light to activate chlorophyll, split water, release oxygen and make ATP and NADPH. The dark (biosynthetic) reaction in the stroma uses that ATP and the hydrogen of NADPH to combine with carbon dioxide and build glucose.
Understand Light and Dark Reactions of Photosynthesis
The overall picture. Photosynthesis is the making of glucose from carbon dioxide and water by green cells, using light energy trapped by chlorophyll, with oxygen given off. The balanced equation that ICSE expects is:
6CO₂ + 12H₂O → (sunlight, chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂
Write sunlight above the arrow and chlorophyll below it. Twelve water molecules go in and six come out; showing water on both sides makes the point that all the oxygen released comes from water, not from CO₂.
Why two phases? Light energy cannot be used to build sugar directly. It is first converted into two portable forms, ATP (energy currency) and NADPH (a carrier of hydrogen). These are then spent in a second set of reactions that do not need light themselves. ICSE uses two names for the phases:
- Photochemical phase = light reaction (light drives chemical change).
- Biosynthetic phase = dark reaction (a substance, glucose, is built up).
Light reaction: four events in order (in the grana).
1. Activation of chlorophyll. Chlorophyll in the thylakoids absorbs light; its molecules become energised and give off high-energy electrons.
2. Photolysis of water. Light energy splits water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost by chlorophyll.
3. Release of oxygen. The O₂ from photolysis diffuses out of the leaf through the stomata. It is a by-product.
4. Formation of ATP and NADPH. Part of the energy is stored by adding phosphate to ADP: ADP + Pi → ATP. This use of light to make ATP is photophosphorylation. The hydrogen ions and electrons reduce NADP to NADPH.
Dark reaction (in the stroma). The hydrogen carried by NADPH combines with carbon dioxide to form glucose, using energy from ATP. ATP returns to the grana as ADP, and NADPH as NADP, ready to be charged again. The 2027 syllabus asks only for this idea; detailed equations of the dark reaction are not required. The dark reaction does not need light directly, but it normally runs in daylight because it depends on a steady supply of ATP and NADPH from the light reaction; in a long dark period it stops when these run out.
Expansions to learn by heart.
| Abbreviation | Full form | Role |
|---|---|---|
| ATP | Adenosine triphosphate | stores energy for the dark reaction |
| ADP | Adenosine diphosphate | 'uncharged' form, recharged in grana |
| NADP | Nicotinamide adenine dinucleotide phosphate | hydrogen acceptor |
| NADPH | reduced NADP | carries hydrogen to the stroma |
Light versus dark reaction.
| Point | Light reaction | Dark reaction |
|---|---|---|
| Other name | photochemical phase | biosynthetic phase |
| Site | grana (thylakoids) | stroma |
| Light | needed directly | not needed directly |
| Raw materials | water, light, ADP, NADP | CO₂, ATP, NADPH |
| Products | O₂, ATP, NADPH | glucose, ADP, NADP |
Factors affecting the rate (an idea). The rate depends on light intensity, CO₂ concentration and temperature. Enzymes of the dark reaction work best at about 25-35 °C (the 2025 paper asked the optimum range); at very low temperatures they are slow, and at high temperatures they are damaged and stomata close. The factor in shortest supply limits the rate: on a bright winter morning, cold limits it; in deep shade, light does.
Flow chart. Light → chlorophyll activated → water split (photolysis) → O₂ out; H⁺ + e⁻ → NADPH; energy → ATP (photophosphorylation) → into stroma → H from NADPH + CO₂ (energy from ATP) → glucose → stored as starch; ADP and NADP back to grana.
Draw-and-label checklist: two-phase scheme. Draw a large oval (chloroplast) with a stack of discs on one side (label: granum, light reaction) and the surrounding space (label: stroma, dark reaction). Arrows: light and H₂O into the granum; O₂ out; ATP and NADPH arrows from granum to stroma; CO₂ into stroma; glucose out; ADP and NADP arrows back to the granum.
Light and Dark Reactions of Photosynthesis: key facts
- Overall balanced equation
- 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂
- Above the arrow: sunlight
- Below the arrow: chlorophyll
- Water on both sides; both conditions written; equation balanced.
- Light reaction (grana)
- Activation of chlorophyll
- Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂
- Photophosphorylation: ADP + Pi → ATP
- NADP + hydrogen → NADPH
- Products sent to the stroma: ATP and NADPH. By-product: O₂.
- Dark reaction (stroma)
- Hydrogen from NADPH + CO₂ → glucose
- Energy from ATP
- ADP and NADP return to grana
- Detailed equations are not required.
How to answer Light and Dark Reactions of Photosynthesis questions
ICSE asks for the balanced equation, expansions of NADP or ATP, the name of a step (photolysis, photophosphorylation), the site of each phase, differences between the phases, and the effect of temperature or light on the rate.
- 1For the equation, write 12H₂O on the left, 6H₂O and 6O₂ on the right, sunlight above and chlorophyll below the arrow.
- 2Name steps by meaning: 'lysis' = splitting (of water); 'phosphorylation' = adding phosphate (ADP to ATP).
- 3Always pair each phase with its site: light with grana, dark with stroma.
- 4For 'what links the two phases', answer ATP and NADPH.
- 5For a limiting factor question, name the factor in shortest supply and explain the effect on the rate.
Light and Dark Reactions of Photosynthesis in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Oxygen comes from water
Suppose your younger brother says plants 'breathe in CO₂ and breathe out the same oxygen atoms'. He has read that leaves release oxygen and take in carbon dioxide.
Correct him using the light reaction.
Try it first, then show the solutionSolution
- In the light reaction, water absorbed by the roots is split by light energy in the grana (photolysis).
- The oxygen of the water molecules is released as O₂ gas.
- The carbon and oxygen of CO₂ end up in glucose (and some water) during the dark reaction.
- So the oxygen we breathe from plants comes from water, not from CO₂.
Answer: Released oxygen comes from the photolysis of water; CO₂ is built into glucose in the dark reaction.
Memory hook: Water gives the O₂; CO₂ gives the C.
Solar cell and battery
Suppose a village has a solar panel that charges batteries during the day, and the batteries run a flour mill that grinds grain whenever needed.
Match this system to the two phases of photosynthesis.
Try it first, then show the solutionSolution
- The solar panel traps light, like chlorophyll in the grana.
- Charged batteries are like ATP and NADPH, which store the energy and hydrogen.
- The mill does the building work without needing light itself, like the dark reaction in the stroma using CO₂.
- Flat batteries return to be charged, as ADP and NADP return to the grana.
Answer: Panel = light reaction; charged batteries = ATP and NADPH; mill = dark reaction making glucose.
Memory hook: Light charges the batteries; the dark reaction spends them.
Tricks to remember Light and Dark Reactions of Photosynthesis
Short memory aids. Say each one aloud once, then use it on a question the same day.
G for Grana, G for Go (light)
Grana go first with light; Stroma Synthesises Sugar.
Use it when: Sites of the two phases.
A-P-O-N for the light reaction
Activate chlorophyll, Photolysis, Oxygen out, NADPH and ATP made.
Use it when: Writing the four events in order.
Nicotinamide Adenine Dinucleotide Phosphate
'Nice Ants Do Party': N-A-D-P, and it is adenINE, not adenosine.
Use it when: Expanding NADP without spelling errors.
Light and Dark Reactions of Photosynthesis: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Naming the step · 1 mark · MCQBoard level
In the grana, light energy is used to attach a phosphate group to ADP. This step is called:
(a) photolysis (b) photophosphorylation (c) carbon fixation (d) hydrolysis
Try it first, then show the solutionSolution
- Adding phosphate = phosphorylation; using light = photo-. Photolysis is splitting of water.
Answer: (b) photophosphorylation.
Example 2: The full equation · 2 marks · Short answerBoard level
Write the balanced chemical equation for photosynthesis, showing both conditions, and state why water appears on both sides.
Try it first, then show the solutionSolution
- 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂, with sunlight above and chlorophyll below the arrow (1 mark).
- Water on both sides shows that all the O₂ released comes from the splitting of water (1 mark).
Answer: 6CO₂ + 12H₂O → (sunlight/chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂; the O₂ released comes from water.
Example 3: Phases compared · 3 marks · Short answerHarder board level
Give three differences between the photochemical and the biosynthetic phases of photosynthesis.
Try it first, then show the solutionSolution
- Site: photochemical in the grana; biosynthetic in the stroma (1 mark).
- Light: needed directly in the photochemical phase; not directly in the biosynthetic phase (1 mark).
- Events: photolysis of water, O₂ release, ATP and NADPH formed versus CO₂ combined with hydrogen to form glucose, using ATP (1 mark).
Answer: Grana versus stroma; light needed versus not directly needed; ATP, NADPH and O₂ formed versus glucose formed.
Example 4: Counting molecules · 3 marks · Short answerHarder board level
Using the balanced equation, if a leaf fixes 18 molecules of CO₂, calculate (a) the molecules of glucose formed, (b) the molecules of water split, and (c) the molecules of oxygen released.
Try it first, then show the solutionSolution
- 6 CO₂ give 1 glucose, so 18 ÷ 6 = 3 glucose (1 mark).
- Each equation uses 12 H₂O, so 3 × 12 = 36 water molecules (1 mark).
- Each equation releases 6 O₂, so 3 × 6 = 18 O₂ (1 mark).
Answer: (a) 3 (b) 36 (c) 18.
Example 5: Plant moved into darkness · 2 marks · Short answerHOTS / competency
A plant photosynthesising in bright light is suddenly placed in a dark room. Explain why the dark reaction also stops after a short time, even though it does not need light.
Try it first, then show the solutionSolution
- In darkness the light reaction stops, so no new ATP or NADPH is made (1 mark).
- The dark reaction uses up the ATP and NADPH already present and then stops, so no more CO₂ is converted to glucose (1 mark).
Answer: Without light, ATP and NADPH are not replaced; the dark reaction runs out of them and stops.
Example 6: Greenhouse heaters · 3 marks · Case-basedHOTS / competency
Suppose a tomato grower in Shimla keeps glasshouse plants in bright light on winter mornings when the air is 8 °C. The yield is low. (a) Which factor is limiting? (b) Explain how it limits the dark reaction. (c) Suggest one change and its expected effect.
Try it first, then show the solutionSolution
- (a) Temperature (1 mark).
- (b) The dark reaction is controlled by enzymes, which work slowly at low temperature, so glucose forms slowly even in bright light (1 mark).
- (c) Warm the glasshouse towards the optimum range of about 25-35 °C; the rate rises until another factor becomes limiting (1 mark).
Answer: (a) Temperature. (b) Enzymes of the dark reaction are slow in the cold. (c) Heat to about 25-35 °C; the rate increases.
Where students lose marks in Light and Dark Reactions of Photosynthesis
The slips examiners see most often, and what to write instead.
Mistake: Photolysis written for ADP to ATP
CISCE examiners' comments (2024): for the conversion of ADP to ATP, candidates chose photolysis or polymerisation instead of photophosphorylation.
Do this: Photolysis splits water; photophosphorylation adds phosphate to ADP.
Usually costs: 1 mark.
Mistake: Wrong expansion of NADP
CISCE examiners' comments (2024): candidates wrote adenosine for adenine and diphosphate for dinucleotide phosphate.
Do this: Nicotinamide adenine dinucleotide phosphate.
Usually costs: 1 mark.
Mistake: Incomplete or unbalanced equation
CISCE examiners' comments (2025): candidates missed a product such as water, wrote unbalanced equations, or left out one or both conditions.
Do this: 12H₂O in, 6H₂O and 6O₂ out; sunlight above, chlorophyll below.
Usually costs: 1-2 marks.
Mistake: Saying the dark reaction happens only at night
The name misleads; the dark reaction runs mainly in daylight, using ATP and NADPH made by the light reaction.
Do this: Write 'does not need light directly'.
Usually costs: 1 mark.
Exam tips
- Pair every phase with its site: grana with light, stroma with dark.
- ATP and NADPH are the bridge between the phases.
- Oxygen released comes from water.
- Optimum temperature for photosynthesis: about 25-35 °C.
Practice questions on Light and Dark Reactions of Photosynthesis
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · MCQBoard level
The site of the dark reaction is the:
(a) granum (b) thylakoid (c) stroma (d) outer membraneHint
Fluid matrix.
Answer
(c) stroma.
Q2 · 1 mark · Very short answerBoard level
Expand ADP and state how it differs from ATP.
Hint
Count the phosphates.
Answer
Adenosine diphosphate; it has two phosphate groups, one fewer than ATP, so it stores less energy.
Q3 · 1 mark · Very short answerHarder board level
If the grana of a chloroplast stopped working while the stroma still received CO₂, which two substances would the stroma soon run short of?
Hint
What links the two phases?
Answer
ATP and NADPH.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Oxygen released in photosynthesis is a by-product of the light reaction.
Reason (R): Photolysis splits water molecules into hydrogen ions, electrons and oxygen.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).Hint
Where does O₂ come from?
Answer
(c).
Q5 · 2 marks · Short answerHarder board level
Why is the light reaction called the photochemical phase and the dark reaction the biosynthetic phase?
Hint
Photo + chemical; bio + synthesis.
Answer
Light energy drives chemical changes (activation of chlorophyll, photolysis, ATP and NADPH formation) in the first phase; in the second, a biological compound, glucose, is built up (synthesised) from CO₂ and hydrogen.
Q6 · 2 marks · Short answerHarder board level
A student lists the light reaction as: 'ATP formed, water split, chlorophyll activated, oxygen released'. Rewrite the events in the correct order and name the step in which ADP becomes ATP.
Hint
A-P-O-N.
Answer
Chlorophyll activated by light; water split (photolysis); oxygen released; ATP (and NADPH) formed. ADP becomes ATP by photophosphorylation.
Q7 · 2 marks · Short answerHarder board level
A leaf releases 30 molecules of oxygen. Using the balanced equation, how many molecules of CO₂ were fixed and how many glucose molecules were formed?
Hint
6 O₂ for every 6 CO₂ and 1 glucose.
Answer
30 CO₂ fixed (one CO₂ per O₂); 30 ÷ 6 = 5 glucose.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a Hydrilla shoot gives 15 bubbles per minute at 20 °C and 33 bubbles per minute at 30 °C, in the same light.
(a) Which gas forms the bubbles? (1)
(b) By how many times did the rate increase? (1)
(c) Predict and explain the rate at 55 °C. (1)Hint
Enzymes and optimum temperature.
Answer
(a) Oxygen. (b) 33 ÷ 15 = 2.2 times. (c) Much lower, because the enzymes of the dark reaction are damaged above the optimum range of about 25-35 °C.
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.What is photolysis?Show answer
The splitting of water by light energy in the light reaction.
2.Where does the light reaction occur?Show answer
In the grana (thylakoids) of the chloroplast.
3.What does NADPH carry to the stroma?Show answer
Hydrogen.
4.Which gas is used in the dark reaction?Show answer
Carbon dioxide.
5.What is the source of the oxygen given off?Show answer
Water.
Light and Dark Reactions of Photosynthesis: frequently asked questions
Do I need the Calvin cycle for ICSE Class 10?
No. The syllabus asks only that hydrogen from NADPH combines with CO₂ to form glucose; detailed equations of the dark reaction are not required.
Why are there 12 water molecules on the left?
Six O₂ need twelve oxygen atoms, all from water, so twelve H₂O are split; six new water molecules are formed in the dark reaction.
Is 'photochemical' the same as 'light reaction'?
Yes. ICSE uses photochemical for the light phase and biosynthetic for the dark phase.
Why does temperature matter if light provides the energy?
The dark reaction is run by enzymes, whose speed depends on temperature.