Photosynthesis

ICSE Class 10 · Biology · Chapter 6

What this chapter covers

Photosynthesis: the process and its importance to life
The chloroplast: site of the light and dark reactions
Opening and closing of stomata
The overall balanced chemical equation
The light (photochemical) phase
The dark (biosynthetic) phase
Adaptations in plants for photosynthesis
Destarching and the starch test
Experiments on the factors essential for photosynthesis
The carbon cycle

Key points

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water using light energy trapped by chlorophyll, releasing oxygen.
Photosynthesis is the source of food for all organisms, of atmospheric oxygen and of the energy stored in fossil fuels.
The balanced equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O; all the oxygen released comes from water.
A chloroplast has a double-membrane envelope, a fluid stroma and stacks of thylakoids called grana; the grana are the site of the light reaction and the stroma of the dark reaction.
Stomata open when potassium ions enter the guard cells, water follows by osmosis and the turgid guard cells bend apart; they close when potassium ions leave.
The light (photochemical) phase involves activation of chlorophyll, photolysis of water, release of oxygen and formation of ATP (photophosphorylation) and NADPH.
The dark (biosynthetic) phase combines hydrogen from NADPH with carbon dioxide, using ATP, to form glucose; it does not need light directly.
Leaves are adapted by a broad thin lamina, leaf mosaic, chloroplast-rich palisade cells, air spaces, stomata and a dense network of veins.
Destarching in darkness for 24 to 48 hours is essential before experiments; the starch test uses boiling water, hot alcohol, warm water and iodine.
Experiments with black paper, potassium hydroxide, variegated leaves and Hydrilla show that light, carbon dioxide and chlorophyll are needed and that oxygen is released.
The carbon cycle moves carbon between air, plants, animals, decomposers and fossil fuels; photosynthesis is its main route for removing carbon dioxide from the air.

Past board exam questions

ICSE 2027 sample paper · 2 marks

Write the balanced equation for photosynthesis. Name the mineral element essential for the formation of chlorophyll.

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6CO₂ + 12H₂O xrightarrow[Chlorophyll]Sunlight C₆H₁₂O₆ + 6H₂O + 6O₂ Magnesium

ICSE 2027 sample paper · 2 marks

A potted plant under a bell jar with a watch glass containing potassium hydroxide is placed under sunlight for three hours. After exposure to sunlight none of the leaves of the plant “X” turned blue black when tested with iodine, while the leaves of the control plant “Y” did. The figure shows two potted plants, each covered by a bell jar: under bell jar X a watch-glass containing potassium hydroxide is placed beside the pot; bell jar Y contains only the potted plant. Explain the conclusion drawn from this experiment after the iodine test.

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Plant X – the CO₂ inside the jar was absorbed by KOH, so photosynthesis did not take place and no starch was formed; the leaves did not turn blue black. Plant Y – CO₂ was available, photosynthesis occurred and starch was formed, so the leaves turned blue black. Hence CO₂ is necessary for photosynthesis.

ICSE 2026 sample paper · 1 mark

Assertion (A): Oxygen is produced during light dependent reactions of photosynthesis. Reason (R): Oxygen is released as a byproduct during photolysis.

(A) is true and (R) is false.
(A) is false and (R) is true.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true but (R) is not the correct explanation of (A).

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Both (A) and (R) are true and (R) is the correct explanation of (A).

ICSE 2026 sample paper · 3 marks

Surya used a plant with variegated leaves (leaves with green regions and white, non-green stripes) for an experiment on photosynthesis. He tested a leaf with iodine after keeping the plant in sunlight for a few hours. (a) Which parts of the leaf will turn blue black after the iodine test? (b) What does this tell us about the role of chlorophyll in photosynthesis? (c) Write the overall chemical equation for photosynthesis.

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(a) Only the green parts of the leaf turn blue-black; the white (non-green) parts do not. (b) Starch is formed only where chlorophyll is present, so chlorophyll is necessary for photosynthesis – it traps the light energy. (c) 6CO₂ + 12H₂O xrightarrow[chlorophyll]sunlight C₆H₁₂O₆ + 6O₂ + 6H₂O (carbon dioxide + water → glucose + oxygen + water)

ICSE 2026 board paper · 2 marks

Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis. She plucked one of the leaves and tested it for starch. The leaf did not turn blue-black on adding Iodine solution. (a) Why was the plant placed in the dark for 48 hours? (b) What is the significance of boiling the leaf in alcohol during the starch test?

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(a) To destarch the leaves — in the dark, the starch already present in the leaves is used up (or moved out), so that any starch found later is formed during the experiment. (b) Boiling in alcohol dissolves and removes the chlorophyll (decolourises the leaf), so that the colour change with iodine can be seen clearly.

ICSE 2026 board paper · 2 marks

Expand the abbreviations: (a) NADP (b) ADP

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(a) NADP – Nicotinamide Adenine Dinucleotide Phosphate (b) ADP – Adenosine Diphosphate

ICSE 2025 board paper · 1 mark

The optimum temperature for photosynthesis is:

10 – 15 °C
60 – 65 °C
40 – 45 °C
25 – 35 °C

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25 – 35 °C

ICSE 2025 sample paper · 1 mark

Assertion (A): All food chains begin with herbivores. Reason (R): Green plants are heterotrophs.

A is True and R is False
A is False and R is True
Both A and R are True
Both A and R are False

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Both A and R are False