BiologyICSE Class 10 · Chapter 6

Photosynthesis ICSE Class 10: Process, Chloroplast, Experiments and Carbon Cycle

Updated 14 October 2026 · Fact-checked

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen. The light (photochemical) phase in the grana splits water and forms ATP and NADPH; the dark (biosynthetic) phase in the stroma uses them to turn carbon dioxide into glucose.

What Photosynthesis is about

Almost every food chain on Earth starts with photosynthesis. Green plants trap light energy and store it as chemical energy in glucose, releasing oxygen as a by-product.

The CISCE 2027 syllabus covers: the process and its importance; the internal structure of the chloroplast as the site of the light and dark reactions; opening and closing of stomata by the potassium ion exchange theory; the overall balanced equation; the terms 'photochemical' (light phase) and 'biosynthetic' (dark phase); the light reaction (activation of chlorophyll, photolysis of water, release of O₂, formation of ATP by photophosphorylation and NADPH); the dark reaction (only the combination of hydrogen from NADPH with CO₂ to form glucose; detailed equations not required); adaptations for photosynthesis; experiments on the necessity of light, carbon dioxide and chlorophyll, formation of starch and release of oxygen, with emphasis on destarching and the starch test; and the carbon cycle.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Photosynthesis is in the 2. Plant Physiology (iii) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

The equation and importance of photosynthesis

Photosynthesis is the process by which green plants, in the presence of sunlight and chlorophyll, make glucose from carbon dioxide and water, releasing oxygen.

$6 C O_{2} + 12 H_{2} O sunlight chlorophyll C_{6} H_{12} O_{6} + 6 H_{2} O + 6 O_{2}$

(Writing 12 H₂O on the left and 6 H₂O on the right shows that the oxygen released comes from water.) Glucose is converted to starch for storage.

Importance:

Source of food (directly or indirectly) for all organisms.
Releases oxygen for respiration and keeps O₂ and CO₂ in balance in the air.
Converts light energy into chemical energy, the energy that flows through food chains.
The source of fossil fuels (coal, petroleum formed from ancient plants).

Worked example

Write the balanced equation for photosynthesis, naming the two conditions.

Try it first, then show the solutionSolution

Reactants: CO₂ and water; products: glucose, water and oxygen.
$6 C O_{2} + 12 H_{2} O sunlight chlorophyll C_{6} H_{12} O_{6} + 6 H_{2} O + 6 O_{2}$
Conditions: sunlight and chlorophyll.

Answer: As above.

Structure of the chloroplast

The chloroplast is the site of photosynthesis. It is a disc-shaped plastid with:

A double membrane (outer and inner).
Grana (singular: granum): stacks of flattened, coin-like sacs called thylakoids, which contain chlorophyll. Grana are joined by stroma lamellae (intergranal membranes). The light reaction takes place in the grana.
Stroma: the colourless, protein-rich fluid matrix around the grana, containing enzymes (and some DNA, ribosomes and starch grains). The dark reaction takes place in the stroma.

In a leaf, chloroplasts are most numerous in the palisade cells just below the upper epidermis.

Worked example

Name the parts of a chloroplast where (a) light is absorbed, (b) glucose is made.

Try it first, then show the solutionSolution

Chlorophyll is in the thylakoids of the grana: light reaction.
Enzymes in the stroma fix CO₂ into glucose: dark reaction.

Answer: (a) Grana (b) Stroma

Light reaction (photochemical phase)

Takes place in the grana and needs light.

1. Activation of chlorophyll: chlorophyll absorbs light energy and becomes activated (it gives off energised electrons).
2. Photolysis of water: the energy splits water molecules into hydrogen ions, electrons and oxygen: $2 H_{2} O \to 4 H^{+} + 4 e^{-} + O_{2}$ . The oxygen is released into the air.
3. Photophosphorylation: light energy is used to make ATP (adenosine triphosphate) from ADP and phosphate.
4. Formation of NADPH: the hydrogen (H⁺ and electrons) reduces NADP (nicotinamide adenine dinucleotide phosphate) to NADPH.

Products carried to the dark reaction: ATP (energy) and NADPH (hydrogen). By-product: O₂.

Worked example

What is photophosphorylation? Where does it occur?

Try it first, then show the solutionSolution

It is the formation of ATP from ADP and phosphate using light energy.
It occurs in the grana (thylakoids) during the light reaction.

Answer: Light-driven formation of ATP, in the grana.

Dark reaction (biosynthetic phase)

Takes place in the stroma and does not need light directly (it can occur in light or dark as long as ATP and NADPH are available). It is called biosynthetic because glucose is built up.

Hydrogen from NADPH combines with carbon dioxide to form glucose, using energy from ATP.
ATP becomes ADP and NADPH becomes NADP, which return to the grana to be used again.

The detailed equations of the dark reaction are not required for 2027.

Light reaction	Dark reaction
grana	stroma
needs light	light not directly needed
photolysis of water; O₂ released	CO₂ fixed into glucose
ATP and NADPH formed	ATP and NADPH used

Worked example

Give two differences between the light and dark reactions.

Try it first, then show the solutionSolution

Site: grana versus stroma.
Light reaction releases O₂ and forms ATP and NADPH; dark reaction uses them to convert CO₂ to glucose.

Answer: As above.

Destarching and the starch test

Experiments on photosynthesis detect starch (the stored form of glucose) in a leaf.

Destarching: keep the potted plant in darkness for 24-48 hours before the experiment. The leaves use up or move out their stored starch, so any starch found later must have been made during the experiment.

Starch test (on a leaf):
1. Boil the leaf in water for a few minutes: kills the cells and softens the leaf, and makes cells permeable to iodine.
2. Boil in alcohol (methylated spirit) over a water bath: removes chlorophyll, so colour changes can be seen; a water bath is used because alcohol is inflammable.
3. Dip in warm water: softens the leaf, which becomes brittle in alcohol.
4. Add dilute iodine solution: parts with starch turn blue-black; parts without starch stay brown (colour of iodine).

Worked example

Why is the leaf boiled in alcohol, and why over a water bath?

Try it first, then show the solutionSolution

Alcohol dissolves chlorophyll, decolourising the leaf so the iodine colour can be seen.
Alcohol is inflammable, so it is heated indirectly in a water bath, not over a flame.

Answer: To remove chlorophyll; for safety.

Experiments: light, CO₂, chlorophyll and oxygen

All use a destarched plant and the starch test, except the oxygen experiment.

Light is necessary: fix a strip of black paper (or a Ganong's light screen) over part of a destarched leaf on both sides; keep the plant in sunlight for a few hours; test for starch. The covered part stays brown; the exposed part turns blue-black.
CO₂ is necessary (Moll's half-leaf experiment): put half of a destarched leaf inside a bottle containing potassium hydroxide (KOH) solution, which absorbs CO₂, with the other half outside; seal with a split cork. After sunlight, the part inside the bottle gives no starch.
Chlorophyll is necessary: use a variegated leaf (croton or coleus) with green and white patches; draw its pattern; after sunlight, test for starch. Only the green parts turn blue-black.
Oxygen is released: place Hydrilla (a water plant) under an inverted funnel in a beaker of water, with a water-filled test tube inverted over the funnel's stem. In sunlight, bubbles rise and gas collects in the test tube; it relights a glowing splint: oxygen. Adding a little sodium bicarbonate supplies CO₂ and speeds it up.

Worked example

In the black paper experiment, which part of the leaf turns blue-black with iodine, and what does it prove?

Try it first, then show the solutionSolution

The exposed part received light and made starch: blue-black.
The covered part received no light and made no starch: brown.

Answer: The uncovered part; light is necessary for photosynthesis.

Adaptations of leaves and the carbon cycle

Adaptations for photosynthesis:

Broad, flat lamina: large surface to absorb light; thin: short path for gases.
Many stomata for gas exchange; air spaces in the spongy mesophyll.
Palisade cells packed with chloroplasts just below the upper surface.
A network of veins (xylem brings water; phloem carries food away).
Leaves arranged to avoid shading one another (leaf mosaic).

Opening and closing of stomata (potassium ion exchange theory): in light, K⁺ ions move into the guard cells from neighbouring cells; the guard cell sap becomes more concentrated, water enters by osmosis, the guard cells become turgid and curve apart, and the stoma opens. In darkness, K⁺ ions move out, water leaves, the guard cells become flaccid and the stoma closes.

Carbon cycle (diagrammatic):

CO₂ in air (and dissolved in water) → photosynthesis → carbon in plant food.
Plants → eaten by animals.
CO₂ returned to the air by respiration of plants and animals, decomposition of dead bodies and wastes by decomposers, and combustion of wood and fossil fuels.
Some carbon is locked in fossil fuels and in carbonates (shells, limestone).

Worked example

Name three processes that return carbon dioxide to the atmosphere.

Try it first, then show the solutionSolution

Respiration by plants and animals.
Decomposition of dead organisms.
Combustion of fuels.

Answer: Respiration, decomposition, combustion.

Photosynthesis 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.

Necessity of light

Suppose a child sticks a star-shaped black sticker on a leaf of a potted plant that was kept in a dark cupboard for two days, then leaves the plant on a sunny window sill for a day.

If the leaf is tested with iodine, what pattern will appear and why? Why was the plant kept in the cupboard first?

Try it first, then show the solutionSolution

Keeping it in the dark destarched the leaves.
In sunlight, the uncovered parts made starch; the star-shaped covered part did not.
After the starch test, the leaf turns blue-black except for a brown star.

Answer: A brown star on a blue-black leaf; destarching ensures any starch was made during the test.

Memory hook: No light, no starch: the sticker leaves its shape.

Oxygen release

Suppose an aquarium owner notices more bubbles rising from water plants in the afternoon sun than in the evening.

Name the gas, explain the difference, and suggest how to confirm the gas.

Try it first, then show the solutionSolution

Water plants photosynthesise and release oxygen.
More light in the afternoon gives a faster rate, so more bubbles.
Collect the gas in an inverted test tube over a funnel; it relights a glowing splint.

Answer: Oxygen; more light, more photosynthesis; glowing splint test.

Memory hook: Bright light, more bubbles of oxygen.

Carbon cycle

Suppose a town burns large amounts of coal and also cuts down a forest for building.

Explain how both actions affect the carbon dioxide in the air, using the carbon cycle.

Try it first, then show the solutionSolution

Burning coal returns carbon locked in fossil fuels to the air as CO₂.
Cutting forests reduces photosynthesis, which removes CO₂ from the air.
Both actions increase atmospheric CO₂.

Answer: Both raise CO₂: more is added and less is removed.

Memory hook: Burn more, plant less: CO₂ goes up.

Topics in this chapter

Photosynthesis: key facts

Overall equation: $6 C O_{2} + 12 H_{2} O sunlight chlorophyll C_{6} H_{12} O_{6} + 6 H_{2} O + 6 O_{2}$; Oxygen released comes from water.
Photolysis of water: $2 H_{2} O \to 4 H^{+} + 4 e^{-} + O_{2}$; Light reaction, in grana.
Photophosphorylation: $A D P + P_{i} light A T P$; Energy store for the dark reaction.
Reduction of NADP: $N A D P + H \to N A D P H$; Hydrogen (from photolysis) carried to the stroma.

Photosynthesis: diagrams to practise

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

Chloroplast

Draw a neat, labelled diagram of a chloroplast.

Draw

An oval (lens-shaped) body with two outer lines (double membrane).
Several stacks of coin-like discs (grana) inside.
Thin lines (lamellae) joining the grana.
Fill the background as stroma.

Label

outer membrane
inner membrane
granum
thylakoid
stroma lamella
stroma

Watch out: Write 'light reaction' beside grana and 'dark reaction' beside stroma.

Experiment to show oxygen is released

Draw a labelled diagram to show that oxygen is evolved during photosynthesis.

Draw

A beaker of water containing Hydrilla.
An inverted glass funnel over the plant, completely under water.
A test tube full of water inverted over the funnel's stem.
Bubbles rising and gas collecting at the top of the test tube; sun rays falling on the beaker.

Label

beaker
water (with a little sodium bicarbonate)
Hydrilla
funnel
test tube
oxygen
sunlight

Watch out: Keep the water level above the funnel's stem.

Moll's half-leaf experiment

Experiment to show CO₂ is necessary.

Draw

A wide-mouthed bottle containing KOH solution.
A split cork through which half of a destarched leaf (still on the plant) is inserted into the bottle.
The other half of the leaf outside, in air.

Label

KOH solution
split cork
leaf part inside (no CO₂)
leaf part outside
potted plant

Watch out: Mention: grease the cork to make it airtight.

Carbon cycle

Diagrammatic representation of the carbon cycle.

Draw

'CO₂ in air' at the top centre.
Arrow to plants labelled 'photosynthesis'.
Arrow from plants to animals labelled 'feeding'.
Arrows back to CO₂ from plants and animals ('respiration'), from dead matter ('decomposition'), and from fuels ('combustion').
A box 'fossil fuels' fed from dead organisms.

Label

photosynthesis
respiration
decomposition
combustion
fossil fuels

Watch out: Every arrow needs a process name.

How the board asks Photosynthesis

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

Question type	How it is asked	Method
Experiment diagrams	3 marks: draw the set-up to show that oxygen is released (Hydrilla); label Moll's experiment.	Show funnel under water, test tube over the stem, bubbles, sunlight rays, water level above the funnel stem.
Starch test colours	Fill in blanks for a leaf diagram: which parts turn blue-black and which stay brown.	Starch (light + CO₂ + chlorophyll) = blue-black; no starch = brown.
Chloroplast diagram	Draw a neat, labelled chloroplast.	Double membrane; grana stacks; stroma; lamellae.
Light and dark reactions	Site, products, terms (photophosphorylation, photolysis, photochemical, biosynthetic), expand NADP and ATP.	Learn the table and the spellings.
Equation and conditions	Write the overall balanced equation with conditions.	Include 6H₂O on the right; write sunlight and chlorophyll over the arrow.

Tricks to remember Photosynthesis

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

Grana Grab light, Stroma Stores sugar
Light reaction in grana; sugar made in stroma.
Use it when: Sites of reactions.
Light makes ATP, NADPH and O₂
'A-N-O': ATP, NADPH, Oxygen come from the light reaction.
Use it when: Products of the light phase.
Water, Alcohol, Water, Iodine
W-A-W-I: boil in water, alcohol, warm water, then iodine.
Use it when: Starch test steps.
Blue-black = Busy (made starch)
Blue-black means the part was busy photosynthesising; brown means it was idle.
Use it when: Not swapping iodine colours.

Photosynthesis: board-level solved questions

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

Example 1: Events of the light reaction · 1 mark · MCQBoard level

Which pair correctly matches an event of photosynthesis with its name?
(a) splitting of water by light: photophosphorylation
(b) ADP + phosphate → ATP using light: photolysis
(c) ADP + phosphate → ATP using light: photophosphorylation
(d) CO₂ combined with hydrogen to form glucose: photolysis

Try it first, then show the solutionSolution

Photolysis ('light splitting') is the splitting of water; photophosphorylation ('light adding phosphate') makes ATP from ADP.
Combining CO₂ with hydrogen is the dark (biosynthetic) reaction, so only (c) is correct.

Answer: (c)

Example 2: Sites · 1 mark · Assertion-reasonHarder board level

Assertion (A): The dark reaction can take place in light.
Reason (R): The dark reaction does not directly need light but depends on ATP and NADPH from the light reaction.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

'Dark' means light-independent, not 'only in darkness'.

Answer: (a)

Example 3: Expand abbreviations · 2 marks · Short answerBoard level

Expand NADP and ATP, and state the role of each in photosynthesis.

Try it first, then show the solutionSolution

NADP: nicotinamide adenine dinucleotide phosphate; it carries hydrogen (as NADPH) to the dark reaction.
ATP: adenosine triphosphate; it supplies energy for the dark reaction.

Answer: As above.

Example 4: Moll's experiment · 3 marks · Short answerHarder board level

In Moll's half-leaf experiment: (a) what is the role of KOH? (b) which part of the leaf gives a negative starch test? (c) what is the conclusion?

Try it first, then show the solutionSolution

(a) KOH absorbs CO₂ from the air in the bottle.
(b) The part inside the bottle.
(c) CO₂ is necessary for photosynthesis.

Answer: As above.

Example 5: Variegated leaf · 3 marks · Short answerBoard level

A destarched variegated croton leaf is kept in sunlight and tested with iodine. (a) What colours do the green and white parts show? (b) Why? (c) What does this prove?

Try it first, then show the solutionSolution

(a) Green parts blue-black; white parts brown.
(b) Only green parts have chlorophyll and made starch.
(c) Chlorophyll is necessary for photosynthesis.

Answer: As above.

Example 6: Light reaction events · 4 marks · Long answerHarder board level

Describe the light reaction under four headings.

Try it first, then show the solutionSolution

Activation of chlorophyll by light.
Photolysis of water into hydrogen ions, electrons and oxygen.
Release of oxygen into the air.
Formation of ATP (photophosphorylation) and NADPH.

Answer: As above.

Example 7: Oxygen experiment diagram · 3 marks · Short answerHarder board level

List the essential parts and labels of a diagram to show that oxygen is released during photosynthesis, and the test for the gas.

Try it first, then show the solutionSolution

Beaker of water with Hydrilla under an inverted funnel; water level above the funnel's stem.
Test tube filled with water inverted over the stem; gas collecting at the top; bubbles; sunlight.
Test: the gas relights a glowing splint (oxygen).

Answer: As above.

Example 8: HOTS: design a control · 3 marks · Short answerHOTS / competency

A student shows that light is needed by covering part of a leaf with black paper. Another student says the paper might simply block CO₂, not light. Suggest an improvement and explain.

Try it first, then show the solutionSolution

The objection: paper pressed on the leaf could reduce gas exchange through stomata.
Use a light screen that keeps light out but allows air to circulate (Ganong's light screen), or cover with transparent and black strips side by side.
If only the black-covered part lacks starch while the transparent-covered part has starch, light (not CO₂) is the factor.

Answer: Use a control that blocks only light (transparent strip versus black strip, or a light screen allowing air).

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 2 + 1 + 1 marks.

The leaf with a stencil

Suppose a destarched leaf is covered on both sides with a black paper stencil with the letters 'QB' cut out, placed in sunlight for six hours, then tested for starch.

(i) · 1 mark
What will the leaf look like after the iodine test?
Answer
Answer: Blue-black letters 'QB' on a brown background.
(ii) · 2 marks
Why must the leaf be boiled in alcohol before adding iodine?
Answer
Answer: To remove chlorophyll so that the blue-black colour can be seen.
(iii) · 1 mark
Why is the leaf dipped in warm water after alcohol?
Answer
Answer: To soften it, as alcohol makes it brittle.
(iv) · 1 mark
Name the product formed first in photosynthesis that is later stored as starch.
Answer
Answer: Glucose.

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.

A student writes CO₂ + H₂O → C₆H₁₂O₆ + O₂ for photosynthesis. Write the correct balanced equation with its conditions. (2 marks) (2 marks)

Step	Marks
Correct reactants and products: $6 C O_{2} + 12 H_{2} O \to C_{6} H_{12} O_{6} + 6 H_{2} O + 6 O_{2}$	1
Conditions: sunlight and chlorophyll written over the arrow	1

Draw a labelled chloroplast and mark on it the sites of the light and dark reactions. (3 marks) (3 marks)

Step	Marks
Oval outline with a double membrane	1
Grana (stacks of thylakoids) joined by lamellae, in a stroma	1
Labels: outer and inner membrane, granum (light reaction), thylakoid, stroma (dark reaction), lamella	1

Where students lose marks in Photosynthesis

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

Mistake: Photolysis for photophosphorylation
CISCE examiners' comments, 2024: for the conversion of ADP to ATP, candidates chose photolysis or polymerisation.
Wrong working
ADP → ATP is photolysis.
Right working
Photophosphorylation; photolysis is the splitting of water.
Do this: Phospho = phosphate added; lysis = splitting.
Usually costs: 1 mark.
Mistake: Swapping iodine colours
CISCE examiners' comments, 2024: in a leaf diagram, candidates wrote blue-black for the part without starch and brown for the part with starch.
Wrong working
Covered part: blue-black.
Right working
Covered part: brown; exposed part: blue-black.
Do this: Blue-black = starch present.
Usually costs: 1 mark per blank.
Mistake: Misspelling NADP
CISCE examiners' comments, 2024: expanding NADP, candidates wrote adenosine for adenine and diphosphate for dinucleotide phosphate.
Wrong working
Nicotinamide adenosine diphosphate
Right working
Nicotinamide adenine dinucleotide phosphate
Do this: Write it out five times.
Usually costs: 1 mark.
Mistake: Single membrane in chloroplast
CISCE examiners' comments, 2024: candidates drew a single membrane and mislabelled parts.
Wrong working
One outline around the grana.
Right working
Two concentric membranes.
Do this: Chloroplast = double membrane.
Usually costs: 1 mark.
Mistake: Incomplete equation
CISCE examiners' comments, 2025: candidates missed a product such as water, wrote unbalanced equations, or omitted the conditions.
Wrong working
CO₂ + H₂O → C₆H₁₂O₆ + O₂
Right working
$6 C O_{2} + 12 H_{2} O sunlight chlorophyll C_{6} H_{12} O_{6} + 6 H_{2} O + 6 O_{2}$
Do this: Balance and add both conditions.
Usually costs: 1-2 marks.
Mistake: Poor oxygen-release diagram
CISCE examiners' comments, 2025: candidates merged the funnel and test tube, showed the water level below the funnel stem, omitted sunlight and the oxygen in the tube, or drew unrelated diagrams.
Wrong working
Funnel stem above the water surface.
Right working
Funnel fully under water; test tube full of water over the stem; gas at the top of the tube; sunlight shown.
Do this: Follow the checklist below.
Usually costs: Up to 3 marks.

Practice questions on 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 light reaction of photosynthesis takes place in the:
(a) stroma
(b) grana
(c) cytoplasm
(d) mitochondria
Hint
Grana Grab light.
Answer
(b)
Q2 · 1 mark · MCQBoard level
The gas used up in the dark reaction is:
(a) oxygen
(b) nitrogen
(c) carbon dioxide
(d) hydrogen
Hint
Fixed into glucose.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Oxygen released in photosynthesis comes from carbon dioxide.
Reason (R): Water is split by light energy during photolysis.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.
Hint
Where does O₂ come from?
Answer
(d)
Q4 · 3 marks · Very short answerBoard level
Give the term: (a) splitting of water by light; (b) keeping a plant in darkness to remove starch; (c) the fluid matrix of a chloroplast.
Hint
One term each.
Answer
(a) Photolysis (b) Destarching (c) Stroma
Q5 · 3 marks · Short answerHarder board level
Give three adaptations of a leaf for photosynthesis.
Hint
Shape, stomata, cells.
Answer
Broad, thin lamina for light absorption and short gas paths; many stomata and air spaces for gas exchange; palisade cells rich in chloroplasts (also network of veins).
Q6 · 2 marks · Short answerHarder board level
Why is the leaf boiled in water before alcohol in the starch test, and what would happen if the alcohol step were skipped?
Hint
Kill cells; remove green.
Answer
Boiling in water kills the cells and softens the leaf so iodine can enter. Without alcohol, chlorophyll would remain and the green colour would hide the blue-black colour.
Q7 · 2 marks · Short answerHarder board level
List the products of the light reaction and state which are used in the dark reaction.
Hint
A-N-O.
Answer
ATP, NADPH and O₂. ATP and NADPH are used in the dark reaction; O₂ is released.
Q8 · 2 marks · Short answerHarder board level
In the Hydrilla experiment, predict and explain the effect on the number of bubbles per minute when (a) the beaker is moved further from the lamp, (b) sodium bicarbonate is added.
Hint
Light and CO₂.
Answer
(a) Fewer bubbles: less light, slower photosynthesis. (b) More bubbles: more CO₂ available.
Q9 · 3 marks · Case-basedHOTS / competency
A destarched variegated leaf has its green part partly covered with black paper. After sunlight and the starch test, three regions are seen. Predict the colour of each region (green-uncovered, green-covered, white) and what each shows.
Hint
Two factors.
Answer
Green-uncovered: blue-black (light + chlorophyll). Green-covered: brown (no light). White: brown (no chlorophyll). Both light and chlorophyll are necessary.
Q10 · 2 marks · Short answerHOTS / competency
Explain why photosynthesis is called the source of almost all energy in food chains and in fossil fuels.
Hint
Light to chemical energy.
Answer
Photosynthesis converts light energy into chemical energy in glucose; animals get this energy by eating plants or plant-eaters; fossil fuels formed from ancient plants (and organisms) store this energy too.

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.Site of the light reaction?Show answer
Grana (thylakoids) of the chloroplast.
2.Site of the dark reaction?Show answer
Stroma.
3.Source of oxygen released in photosynthesis?Show answer
Water (by photolysis).
4.Why is a plant destarched?Show answer
So that any starch found was made during the experiment.
5.Chemical that absorbs CO₂ in Moll's experiment?Show answer
Potassium hydroxide (KOH).
6.Colour of iodine with starch?Show answer
Blue-black.
7.Another name for the light phase?Show answer
Photochemical phase.
8.Another name for the dark phase?Show answer
Biosynthetic phase.

Best order to study it

Learn the equation, importance and the chloroplast diagram.
Learn the light reaction (four events) and the dark reaction (one step).
Learn destarching and the starch test steps with reasons.
Learn the four experiments with diagrams.
Finish with leaf adaptations, stomatal opening and the carbon cycle.

Quick revision checklist

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

Can you write the balanced equation with conditions?
Can you draw and label a chloroplast with a double membrane?
Can you list the products of the light reaction and what the dark reaction does?
Can you explain each step of the starch test?
Can you describe the black paper, Moll's, variegated leaf and Hydrilla experiments?
Can you draw the carbon cycle with process names?

Photosynthesis: frequently asked questions

Do I need the Calvin cycle for ICSE Class 10 Biology 2027?

No. The syllabus asks only that hydrogen from NADPH combines with CO₂ to form glucose; detailed equations of the dark reaction are not required.

What is the optimum temperature for photosynthesis?

For most plants it lies roughly between 25 °C and 35 °C; at much higher temperatures the enzymes of the dark reaction are damaged and the rate falls.

Why is sodium bicarbonate added in the Hydrilla experiment?

It supplies extra carbon dioxide in the water, so the rate of photosynthesis and oxygen release increases.

Based on: CISCE ICSE 2027 Biology (Science Paper 3) syllabus; CISCE ICSE 2027 Biology (Science Paper 3) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Biology (Science Paper 3), 2025; CISCE Analysis of Pupil Performance, ICSE Biology (Science Paper 3), 2024