☀️ Photosynthesis – Full Breakdown
ICSE Class 10
Biology
June 2026
Photosynthesis is the most important plant physiology chapter in ICSE Class 10 Biology — it always carries 10-15 marks across MCQs, short notes, diagram questions, and experiment-based reasoning. The key is understanding what goes in, what comes out, where each reaction happens, and how to prove it with experiments. Written by someone who scored 93.4% in ICSE 2026.
📌 FIGURE 1 — Chloroplast (Site of Photosynthesis)
Fig: Structure of a chloroplast. Light reaction occurs on thylakoid membranes (grana). Dark reaction (Calvin cycle) occurs in the stroma. ICSE 2019, 2022.
1) What is Photosynthesis?
Photosynthesis: The process by which green plants (and some bacteria) convert light energy into chemical energy (glucose), using carbon dioxide and water, releasing oxygen as a by-product.
Word equation: Carbon Dioxide + Water → Glucose + Oxygen
Chemical equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Photosynthesis is an anabolic, endothermic, redox reaction — it builds glucose using light energy. Water is oxidised (loses electrons → O₂ is released), and CO₂ is reduced (gains electrons → glucose is formed).
⚠️ Note — the source of oxygen: In the photosynthesis equation, the oxygen released comes from water (H₂O), NOT from carbon dioxide (CO₂). This was proven by the isotopic tracer experiment using O¹⁸. Board exam loves this distinction — many students wrongly think O₂ comes from CO₂.
2) Conditions Necessary for Photosynthesis
The following four things are absolutely necessary for photosynthesis to occur:
| Condition | Why Needed | How to Prove (Experiment) |
|---|---|---|
| Sunlight (Light Energy) | Provides energy for photolysis of water (splitting H₂O) and for generating ATP & NADPH in the light reaction | Variegated leaf experiment — only green parts (with chlorophyll) exposed to light show a positive starch test after iodine test |
| Chlorophyll | Pigment that traps light energy and converts it to chemical energy. Without chlorophyll, light cannot be absorbed | Variegated leaf experiment — white/yellow areas (no chlorophyll) → no starch → no blue-black colour with iodine |
| Carbon Dioxide | CO₂ is fixed (reduced) to form glucose in the Calvin cycle (dark reaction) | Mohl's half-leaf experiment (or Bell jar experiment with KOH). Leaf in a flask with KOH (absorbs CO₂) → no starch. Leaf in flask with water → starch formed |
| Water | Provides electrons for photosystems. Split by photolysis → releases O₂ and H⁺ ions. H⁺ is used to reduce NADP to NADPH | If a plant is kept without water, it wilts and photosynthesis stops. Also shown by the destarched plant experiment |
📋 Syllabus Note: ICSE specifically tests the four necessary conditions and their related experiments. Know each experiment setup, control, and result.
3) Chlorophyll — The Photosynthetic Pigment
- Chlorophyll is a green pigment found in chloroplasts
- It absorbs light mainly in the blue (430-450 nm) and red (640-680 nm) regions — reflects green light → plants look green
- Types of chlorophyll:
- Chlorophyll a: Primary pigment — directly involved in the light reaction (reaction centre P680 and P700)
- Chlorophyll b: Accessory pigment — helps absorb light and transfers energy to chlorophyll a
- Carotenoids (yellow-orange): Protect chlorophyll from photo-oxidation and absorb light in other wavelengths
- Where found: In the thylakoid membranes of chloroplasts, arranged in photosystems (PS II and PS I)
- Only chlorophyll a can directly convert light energy → chemical energy (it is the reaction centre)
📌 FIGURE 2 — Cross-Section of a Leaf
Fig: T.S. of a leaf. Palisade cells (packed with chloroplasts) are the main photosynthetic site. Spongy mesophyll has air spaces for gas exchange. ICSE 2018, 2021, 2023.
Leaf Adaptations for Photosynthesis
| Feature | How It Helps |
|---|---|
| Large surface area | Flat, broad lamina maximises light absorption |
| Thin / transparent epidermis | Light easily reaches the photosynthetic cells below. No chloroplasts in upper epidermis (except guard cells) to allow light to pass through |
| Palisade mesophyll | Tightly packed, columnar cells with maximum chloroplasts — main site of photosynthesis |
| Extensive air spaces (spongy mesophyll) | Allows CO₂ to diffuse efficiently through the leaf. Also allows O₂ to diffuse out |
| Many stomata (mostly on lower surface) | Entry of CO₂ and exit of O₂. Guard cells regulate opening/closing to balance gas exchange vs water loss |
| Reticulate venation | Xylem brings water & minerals. Phloem carries away the glucose produced |
| Thin cuticle (or none on lower surface) | Allows gas exchange. (Upper cuticle is thicker to prevent water loss) |
4) The Two Stages of Photosynthesis
Stage 1 — Light Reaction (Photochemical Phase)
- Where: On the thylakoid membranes (grana) of chloroplasts
- Requires: Sunlight (directly)
- What happens (4 steps):
- Photolysis of water: Light energy splits water molecules → H⁺ + OH⁻. OH⁻ gives up electrons → forms O₂ + H⁺. The electrons are fed into the photosystems. 2H₂O → 4H⁺ + 4e⁻ + O₂
- Photosystem II (P680): Absorbs light → excites electrons → electrons pass through an electron transport chain
- ATP formation (photophosphorylation): As electrons move through the chain, energy is released — this energy is used to add a phosphate group to ADP → ATP
- Photosystem I (P700) & NADPH formation: The electrons reach PS I → get re-energised by light → combine with NADP⁺ and H⁺ → NADPH
- Products: ATP, NADPH, O₂ (by-product)
Stage 2 — Dark Reaction (Calvin Cycle, Biosynthetic Phase)
- Where: In the stroma of chloroplasts
- Requires: CO₂, ATP & NADPH (from light reaction). Does NOT require light directly — can happen in the dark as long as ATP & NADPH are available
- What happens (3 steps of Calvin cycle):
- Carbon fixation: CO₂ combines with RuBP (a 5-carbon compound) → forms 2 molecules of 3-PGA (3-carbon compound). Enzyme: RuBisCO
- Reduction: 3-PGA is reduced to G3P (glyceraldehyde-3-phosphate) using ATP (energy) and NADPH (reducing agent)
- Regeneration & Glucose synthesis: Most G3P molecules are used to regenerate RuBP (using ATP). Every 6 turns of the cycle, 2 G3P molecules combine to form 1 glucose (C₆H₁₂O₆)
- Products: Glucose (stored as starch), ADP, NADP⁺ (recycled back to light reaction)
💡 Quick memory aid — Light vs Dark:
Light reaction: L → Light (needs sunlight directly). Location = Lamellae / Grana. Products = ATP, NADPH, O₂ (remember: L-A-N-D — Light, ATP, NADPH, Discharge O₂)
Dark reaction: D → Day (doesn't need light directly, but happens during the day when ATP/NADPH are available). Location = Stroma. Products = Glucose, ADP, NADP⁺.
Light reaction: L → Light (needs sunlight directly). Location = Lamellae / Grana. Products = ATP, NADPH, O₂ (remember: L-A-N-D — Light, ATP, NADPH, Discharge O₂)
Dark reaction: D → Day (doesn't need light directly, but happens during the day when ATP/NADPH are available). Location = Stroma. Products = Glucose, ADP, NADP⁺.
| Feature | Light Reaction (Photochemical) | Dark Reaction (Biosynthetic / Calvin Cycle) |
|---|---|---|
| Location | Thylakoid membranes (grana) | Stroma of chloroplasts |
| Requires light directly? | Yes — can only happen in the presence of light | No — can occur in darkness (but only if ATP & NADPH available from light reaction) |
| Reactants | H₂O, ADP, NADP⁺, Light energy | CO₂, ATP, NADPH |
| Products | ATP, NADPH, O₂ (by-product) | Glucose (as starch), ADP, NADP⁺ |
| Main process | Photolysis of water, Electron transport, Photophosphorylation | Carbon fixation (RuBisCO), Reduction, Regeneration of RuBP |
| Enzyme involved | Various (ATP synthase, etc.) | RuBisCO (Ribulose bisphosphate carboxylase-oxygenase) |
| End result | Chemical energy (ATP + NADPH) + O₂ | Synthesis of glucose |
5) Experiments on Photosynthesis — ICSE Favourites
ICSE has a strong focus on experimental biology. You must know the setup, the control variables, the result, and the conclusion. These are the 4 most important experiments:
📌 FIGURE 3 — Variegated Leaf Experiment
Fig: Variegated leaf experiment proving that chlorophyll is necessary for photosynthesis. ICSE 2018, 2020, 2022, 2024.
Experiment 1 — Chlorophyll is Necessary (Variegated Leaf)
- Take a destarched plant (kept in dark for 48 hours to use up starch)
- Expose the variegated leaf to sunlight for a few hours
- Pluck the leaf, boil in water (to kill cells and break cell membranes)
- Boil in alcohol (to remove chlorophyll — leaf becomes pale/white)
- Perform the iodine test — add a few drops of iodine solution
- Result: Green areas → blue-black (starch present). White/yellow areas → brown (no starch, no photosynthesis)
- Conclusion: Chlorophyll is necessary for photosynthesis — without it, light energy cannot be trapped
Experiment 2 — Light is Necessary (Covered Leaf)
- Destarched plant + a leaf partially covered with black paper (or a strip of foil) so that only part of the leaf receives light
- Expose to sunlight for a few hours
- Remove cover, test for starch (same method — boil in water, alcohol, iodine)
- Result: Exposed part → blue-black. Covered part → brown
- Conclusion: Light is necessary for photosynthesis; no light = no starch produced
⚠️ Common exam mistake — "why boil in alcohol?"
Water is boiled first to kill cells and make the leaf permeable to iodine. Alcohol is boiled separately (in a water bath — NEVER directly) to dissolve chlorophyll so that the blue-black colour of the iodine test is visible. If you don't remove chlorophyll, the green colour masks the iodine result.
Water is boiled first to kill cells and make the leaf permeable to iodine. Alcohol is boiled separately (in a water bath — NEVER directly) to dissolve chlorophyll so that the blue-black colour of the iodine test is visible. If you don't remove chlorophyll, the green colour masks the iodine result.
📌 FIGURE 4 — Mohl's Half-Leaf Experiment
Fig: Mohl's half-leaf experiment proving CO₂ is necessary for photosynthesis. Half leaf in KOH flask → no CO₂ → no starch. Other half exposed to air → starch present. ICSE 2019, 2021, 2023.
Experiment 3 — CO₂ is Necessary (Mohl's Half-Leaf)
- Take a destarched plant (kept in dark for 48 hours)
- Insert part of a leaf into a conical flask containing KOH solution. KOH absorbs CO₂ from the air inside the flask
- A split cork seals the flask — the leaf stalk passes through the cork: one half inside the flask (no CO₂), the other half outside (normal air with CO₂)
- Keep the setup in sunlight for a few hours
- Pluck the leaf, perform starch test
- Result: Half outside flask → blue-black (photosynthesis happened). Half inside flask → brown (no starch → no photosynthesis because no CO₂)
- Conclusion: Carbon dioxide is necessary for photosynthesis
Experiment 4 — Oxygen is Released (Hydrilla / Water Plant Experiment)
- Take a beaker of water with some Hydrilla (an aquatic plant) kept under a glass funnel
- Fill the beaker with water. Invert a test tube over the funnel's stem to collect gas
- Place the setup in bright sunlight
- After a few hours, a gas collects in the test tube
- Test the gas: bring a glowing splinter near the mouth of the test tube. If it relights → the gas is oxygen
- Result: Bubbles of O₂ are released from Hydrilla in sunlight. The glowing splinter relights
- Conclusion: Oxygen is released as a by-product of photosynthesis. Rate of bubbles indicates the rate of photosynthesis
🧠 Master Experiment — from my exam prep:
For each experiment, memorise this 4-point formula: Setup → Procedure → Result → Conclusion.
Board exam questions will always ask ONE of these experiments as a 5-mark question, often with a diagram. Practise drawing: (a) the Mohl's half-leaf setup, (b) the Hydrilla experiment setup, and (c) the starch test steps (leaf in water → leaf in alcohol → iodine).
For each experiment, memorise this 4-point formula: Setup → Procedure → Result → Conclusion.
Board exam questions will always ask ONE of these experiments as a 5-mark question, often with a diagram. Practise drawing: (a) the Mohl's half-leaf setup, (b) the Hydrilla experiment setup, and (c) the starch test steps (leaf in water → leaf in alcohol → iodine).
📌 FIGURE 5 — Hydrilla Experiment (O₂ Release)
Fig: Hydrilla experiment — oxygen bubbles are seen rising from the cut ends of the plant in sunlight. The gas collected relights a glowing splinter, confirming it is oxygen. ICSE 2018, 2020, 2022.
6) Factors Affecting Photosynthesis
External Factors
| Factor | Effect on Photosynthesis | Limiting Factor |
|---|---|---|
| Light Intensity | Rate increases with light intensity (up to a point). Beyond saturation point, rate plateaus (another factor becomes limiting) | In low light conditions, light is the limiting factor. Increase light → increase rate |
| CO₂ Concentration | Rate increases with CO₂ concentration up to a saturation point (~0.1%). Normal air has ~0.03% CO₂ | CO₂ is the most common limiting factor — increasing CO₂ in greenhouses boosts crop yield |
| Temperature | Optimum: 25-35°C. Below 10°C → rate is very slow (enzymatic reactions slow down). Above 40°C → enzymes (RuBisCO) denature → rate drops sharply | On a hot summer day (>35°C), temperature becomes limiting because stomata close (to save water), reducing CO₂ entry |
| Water | Water shortage → stomata close (to reduce transpiration) → CO₂ can't enter → photosynthesis stops. Also leaf wilts → less surface area for light absorption | During drought, water becomes the limiting factor |
| O₂ Concentration | High O₂ → Photorespiration — RuBisCO fixes O₂ instead of CO₂, wasting energy and reducing photosynthesis. This is a problem in C₃ plants in hot conditions | High O₂ reduces efficiency but rarely acts as the primary limiting factor |
⚠️ Limiting Factor — Blackman's Law of Limiting Factors:
"When a process depends on multiple factors, the rate of the process is limited by the factor that is present in the minimum quantity (relative to requirement)."
Example: If light is abundant but CO₂ is scarce, increasing light further won't increase photosynthesis — CO₂ is the limiting factor. The factor closest to its minimum determines the rate.
"When a process depends on multiple factors, the rate of the process is limited by the factor that is present in the minimum quantity (relative to requirement)."
Example: If light is abundant but CO₂ is scarce, increasing light further won't increase photosynthesis — CO₂ is the limiting factor. The factor closest to its minimum determines the rate.
7) The Fate of Glucose
Glucose produced during photosynthesis is used by the plant in several ways:
| Use | Details |
|---|---|
| Immediate energy (respiration) | Glucose is broken down in mitochondria to release ATP (energy for cellular activities) |
| Stored as starch | Excess glucose is converted to starch (insoluble) and stored in leaves, stems, roots, tubers (potato), or seeds. Starch is a storage polysaccharide |
| Cell wall (cellulose) | Glucose is polymerised into cellulose — a structural component of the plant cell wall |
| Fats & oils | Glucose can be converted into lipids for storage (e.g., in seeds like groundnut, mustard) |
| Proteins | Glucose (carbohydrate) combines with nitrogen (from soil nitrates) to form amino acids → proteins (enzymes, structural proteins) |
| Transported as sucrose | Glucose is converted to sucrose (disaccharide) for transport through phloem to other plant parts (sink tissues like roots, fruits, developing leaves) |
8) Why is Photosynthesis Important?
- Primary source of food — all life on Earth depends on glucose produced by plants (directly or indirectly through food chains)
- Releases oxygen — the O₂ we breathe comes from photosynthesis. Most atmospheric O₂ (~90%) comes from marine algae (phytoplankton)
- Removes CO₂ — reduces the greenhouse effect. Deforestation increases CO₂ → global warming
- Source of fossil fuels — coal, oil, natural gas are formed from ancient photosynthetic organisms
- Builds organic matter — forms biomass that sustains all heterotrophs (animals, fungi, bacteria)
- Ozone layer — O₂ released by early photosynthetic organisms formed the ozone layer, enabling life on land (by blocking UV radiation)
9) Common ICSE Board Questions
1-Mark / MCQ Style:
- What is the site of the light reaction in a chloroplast?
- Name the gas released during photosynthesis.
- What is the source of oxygen released during photosynthesis?
- Name the enzyme involved in carbon fixation.
- What is the primary pigment that traps light energy in plants?
- Name the process by which water is split during the light reaction.
- What is the function of KOH in Mohl's half-leaf experiment?
- Define: Limiting factor, Photolysis, Photophosphorylation.
- Which part of the spectrum is most effective for photosynthesis?
- Why is the leaf boiled in alcohol before the iodine test?
3-Mark / Short Answer:
- Draw a neat labelled diagram of a chloroplast. Label: granum, stroma, thylakoid, lamella. (ICSE 2019, 2022)
- Write the overall chemical equation for photosynthesis. Identify the source of released oxygen.
- Differentiate between light reaction and dark reaction (4 points in a table).
- Describe the Mohl's half-leaf experiment. What does it prove? (ICSE 2021, 2023)
- Explain the variegated leaf experiment step by step. (ICSE 2018, 2020, 2022)
- Describe the Hydrilla experiment to show that oxygen is released during photosynthesis. (ICSE 2018, 2022)
- What are the four necessary conditions for photosynthesis? Explain any two with experiments.
- List three leaf adaptations that make it efficient for photosynthesis.
- Explain Blackman's Law of Limiting Factors. Give an example. (ICSE 2020)
- How does temperature affect the rate of photosynthesis? Draw a graph showing the relationship.
5-Mark / Long Answer:
- Explain the two stages of photosynthesis — light reaction and dark reaction. Where does each occur, and what are the products? Draw a labelled diagram of a chloroplast to support your answer.
- Describe with a labelled diagram the set-up of Mohl's half-leaf experiment. What is the role of KOH? How do you confirm that starch was not formed in half of the leaf? (ICSE 2019, 2021, 2023)
- Explain the starch test (iodine test) procedure step by step. Why is the leaf boiled in water first, then in alcohol? What does a blue-black colour indicate? (ICSE 2018, 2020, 2022)
- Draw a labelled cross-section of a leaf (T.S.) showing all tissues. How is the internal structure of a leaf adapted for photosynthesis? (ICSE 2020, 2023)
- Discuss four external factors that affect the rate of photosynthesis. Explain the concept of a limiting factor with the help of a graph. (ICSE 2021, 2024)
- What happens to the glucose produced during photosynthesis? Describe any 4 fates of glucose in a plant.
🧠 From my exam — experiment diagrams to practise (from memory):
1. Mohl's half-leaf experiment — the single most diagram-repeated experiment (2019, 2021, 2023).
2. Hydrilla / O₂ release setup — asked in 2018, 2022, likely 2025/2026.
3. Variegated leaf + starch test steps — know the sequence (leaf in water → leaf in alcohol → iodine) — asked as a diagram in 2020.
4. Chloroplast labelled diagram — easy marks — know grana, stroma, thylakoid, lamella.
5. T.S. of leaf — be ready to label all layers (upper epidermis, palisade, spongy mesophyll, xylem, phloem, stoma, guard cells).
Draw each in under 2 minutes. A clean, well-labelled diagram can score you full marks even if the accompanying explanation is incomplete.
1. Mohl's half-leaf experiment — the single most diagram-repeated experiment (2019, 2021, 2023).
2. Hydrilla / O₂ release setup — asked in 2018, 2022, likely 2025/2026.
3. Variegated leaf + starch test steps — know the sequence (leaf in water → leaf in alcohol → iodine) — asked as a diagram in 2020.
4. Chloroplast labelled diagram — easy marks — know grana, stroma, thylakoid, lamella.
5. T.S. of leaf — be ready to label all layers (upper epidermis, palisade, spongy mesophyll, xylem, phloem, stoma, guard cells).
Draw each in under 2 minutes. A clean, well-labelled diagram can score you full marks even if the accompanying explanation is incomplete.
10) Quick Reference Tables (Print This)
Necessary Conditions vs Experiments
| Condition | Experiment | Observation |
|---|---|---|
| Light | Partially covered leaf | Covered part → no starch (brown with iodine) |
| Chlorophyll | Variegated leaf | White areas → no starch; Green areas → starch |
| CO₂ | Mohl's half-leaf (KOH) | Half in KOH flask → no starch; Other half → starch |
| O₂ released | Hydrilla experiment | Gas collected relights glowing splinter (O₂) |
Light vs Dark Reaction — One Look
| Feature | Light Reaction | Dark Reaction |
|---|---|---|
| Location | Grana (thylakoid) | Stroma |
| Needs Light? | Yes | No (needs ATP/NADPH) |
| Inputs | H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | ATP, NADPH, O₂ | Glucose, ADP, NADP⁺ |
| Key Process | Photolysis, ETC, Photophosphorylation | Carbon fixation (RuBisCO), Reduction, Regeneration |
⚠️ Final exam strategy: Photosynthesis is a high-weightage chapter with 10-15 marks in the board exam. Focus heavily on experiments — ICSE almost always asks one full experiment question with a diagram (5 marks) plus 2-3 short questions on limiting factors or light/dark reactions (3+3 marks). My advice — create a single A4 revision sheet with: (1) the chemical equation, (2) the 4 conditions + experiments table, (3) the light vs dark comparison, and (4) diagrams of Mohl's half-leaf and Hydrilla experiments. Review this sheet 3 times in the week before the exam.