Plant Physiology — Photosynthesis
Introduction
Photosynthesis is the process by which green plants manufacture carbohydrates using carbon dioxide and water in the presence of sunlight, with the release of oxygen. In ICSE Class 10 Biology, you study the mechanism, factors, and experimental demonstration of photosynthesis.
The Photosynthesis Equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll)
Raw Materials and Products
| Raw materials | Source | Products |
|---|---|---|
| Carbon dioxide (CO₂) | Air (enters through stomata) | Carbohydrates (glucose/starch) |
| Water (H₂O) | Soil (absorbed by roots) | Oxygen (released through stomata) |
| Sunlight | Sun | — |
| Chlorophyll | Chloroplasts of leaf cells | — |
Light and Dark Reactions
Light Reaction (Photochemical Phase)
- Occurs in the grana of chloroplasts.
- Requires sunlight.
- Chlorophyll absorbs light energy.
- Water splits (photolysis of water): 2H₂O → 4H⁺ + 4e⁻ + O₂
- Products: ATP (from photophosphorylation), NADPH (reduced NADP), and O₂.
Dark Reaction (Calvin Cycle / Biosynthetic Phase)
- Occurs in the stroma of chloroplasts.
- Does NOT require light directly (but needs ATP and NADPH from light reaction).
- CO₂ is fixed (reduced) to form carbohydrates.
- Calvin cycle: CO₂ → 3-phosphoglycerate → G3P → glucose.
Factors Affecting Photosynthesis
| Factor | How it affects | Limiting factor (at high intensity) |
|---|---|---|
| Light intensity | Rate increases with light up to a point | CO₂ concentration, temperature |
| CO₂ concentration | Rate increases with CO₂ up to a point | Light intensity, temperature |
| Temperature | Rate increases up to ~35°C, then declines | CO₂, light (at optimum temp) |
| Water | Deficiency closes stomata, reducing CO₂ intake | — |
| Chlorophyll | Necessary for light absorption | — |
Blackman's Law of Limiting Factors: When a process is affected by multiple factors, the rate is limited by the factor that is in shortest supply.
Experiments on Photosynthesis
Experiment 1: Starch Test (Product of Photosynthesis)
Principle: Starch is produced during photosynthesis and turns blue-black with iodine.
Procedure:
- A potted plant is kept in darkness for 2−3 days (destarching).
- A leaf is partially covered with black paper and exposed to light.
- After a few hours, the leaf is tested for starch (boiled in water → alcohol → iodine).
- The covered part remains colourless; the exposed part turns blue-black.
Conclusion: Light is necessary for photosynthesis.
Experiment 2: CO₂ is Necessary for Photosynthesis
Procedure:
- Two destarched plants are taken.
- One plant is kept in a bell jar with KOH (absorbs CO₂).
- The other is kept in a bell jar without KOH (control).
- After a few hours in sunlight, leaves are tested for starch.
Result: The plant without CO₂ (with KOH) shows no starch; the control shows starch.
Conclusion: CO₂ is necessary for photosynthesis.
Experiment 3: Oxygen is Released
Procedure:
- Hydrilla (aquatic plant) is placed in a beaker of water with a funnel and test tube.
- The setup is kept in sunlight.
- Bubbles of gas are collected in the test tube.
- The gas is tested — it relights a glowing splint (oxygen).
Chloroplast Structure
| Part | Function |
|---|---|
| Outer membrane | Protects the chloroplast |
| Inner membrane | Controls entry/exit of substances |
| Stroma | Site of dark reaction (Calvin cycle) |
| Grana (thylakoids) | Site of light reaction |
| Chlorophyll | Pigment that absorbs light energy |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Thinking the dark reaction only occurs at night | It occurs during the day too (uses products of light reaction) |
| Confusing grana and stroma | Grana: light reaction; Stroma: dark reaction |
| Writing the wrong gas released | Oxygen comes from water (photolysis), not from CO₂ |
| Forgetting to destarch before an experiment | Destarching ensures any starch present is used up before testing |
| Thinking chlorophyll is the only pigment | Carotenoids (yellow/orange) are accessory pigments |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: equation, light/dark reactions, limiting factors, starch test, CO₂ and O₂ experiments, chloroplast structure.
Marks Blueprint: Equation and raw materials — 2 marks, Light/dark reactions — 2 marks, Experiments — 2 marks, Factors — 2 marks.
Self-Test Questions
-
Write the balanced chemical equation for photosynthesis.
-
Distinguish between light and dark reactions of photosynthesis.
-
Describe an experiment to show that CO₂ is necessary for photosynthesis.
-
Explain Blackman's law of limiting factors with respect to photosynthesis.
-
Describe the starch test to show that light is necessary for photosynthesis.
-
Draw and label the structure of a chloroplast.
