ICSE 10 Biology Photosynthesis Basic

1.0 Photosynthesis: Food for All

Photosynthesis is the fundamental process by which green plants manufacture glucose from carbon dioxide and water using sunlight energy trapped by chlorophyll. It is the ultimate source of energy for almost all living organisms.

The Balanced Chemical Equation

6CO₂ + 12H₂O → (Light / Chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂↑

(Carbon Dioxide + Water → Glucose + Water + Oxygen)

The Site: Chloroplast

Photosynthesis occurs in the Mesophyll cells of leaves, which contain numerous chloroplasts. Understanding the structure of a chloroplast is key to understanding the two phases of photosynthesis.

Part	Description	Reaction Site
Grana (Thylakoids)	Stacks of flattened sacs containing chlorophyll.	Light Reaction
Stroma	Colorless ground substance (matrix).	Dark Reaction

Fig: Detailed Structure of a Chloroplast

⚠️ Exam Tip (The Equation):

In ICSE, always write 12H₂O on the reactant side and 6H₂O on the product side. Using 6H₂O only on the reactant side is often marked incorrect in board evaluations!

💡 Did You Know?

Chlorophyll contains the element Magnesium. It is structurally similar to the "heme" in our hemoglobin, but with Magnesium instead of Iron!

2.0 The Mechanism: Two Main Phases

Photosynthesis is not a single-step reaction. It occurs in two distinct series of steps. The first requires light directly, while the second uses the products of the first phase to build glucose.

Phase I: Light-Dependent Reaction (Photochemical Phase)

This phase occurs in the Thylakoids (Grana) of the chloroplast. It involves the following key steps:

1. Activation of Chlorophyll: Chlorophyll absorbs photons (light energy) and becomes excited.
2. Photolysis of Water: Light energy splits water molecules into Hydrogen ions (H⁺), Electrons (e^-), and Oxygen.
   $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
3. Photophosphorylation: ADP is converted into ATP (Adenosine Triphosphate) using light energy.
4. Formation of NADPH: NADP picks up the hydrogen ions to form NADPH.

Phase II: Light-Independent Reaction (Biosynthetic Phase)

Commonly known as the Calvin Cycle, this occurs in the Stroma. It does not require light directly but depends on the ATP and NADPH produced in Phase I.

• 🟠 Fixation: Carbon dioxide from the air is "fixed" or combined with a 5-carbon sugar (RuBP).
• 🟠 Reduction: Using ATP and NADPH, CO₂ is reduced to form Glucose (C₆H₁₂O₆).
• 🟠 Polymerization: Excess glucose is converted into Starch for storage.

⚠️ Common Misconception:

The "Dark Reaction" does not necessarily happen at night. It is called "light-independent" because it doesn't use light directly. In reality, it usually happens simultaneously with the light reaction during the day.

Key End-Products of Light Reaction

ATP (Energy) NADPH (Reducing Power) O₂ (By-product)

💡 Did You Know?

The oxygen we breathe is actually a "waste product" of photolysis. The plant splits water purely to get the electrons and protons; it releases the oxygen as it isn't needed for making sugar!

3.0 Factors Affecting Photosynthesis

The rate of photosynthesis is influenced by several factors. In the ICSE syllabus, we focus on the Principle of Limiting Factors, which states that the rate is determined by the factor present in the least amount.

Factor	Effect on Rate	Condition
Light Intensity	Increases	Up to a certain limit; very high light can destroy chlorophyll.
CO2 Concentration	Increases	Standard air has 0.03%. Increase up to 0.1% speeds it up.
Temperature	Optimum: 35°C	Above 40°C, enzymes are denatured and the rate drops.

4.0 Testing a Leaf for Starch

The presence of starch is proof that photosynthesis has occurred. The Iodine Test is the standard procedure used in all ICSE experiments.

Step-by-Step Procedure:

1. Boiling in Water: Kills the cells and stops metabolic activity.
2. Boiling in Methylated Spirit: Removes chlorophyll (leaf becomes pale/white).
3. Washing in Warm Water: Softens the leaf (alcohol makes it brittle).
4. Iodine Solution: Add a few drops. Starch turns Blue-Black.

What is Destarching?

Before starting any experiment, the plant is kept in the dark for 24-48 hours. This ensures all existing starch is moved to the storage organs (like roots/stems), so any starch found after the experiment is newly made.

⚠️ Safety Precaution:

Never boil methylated spirit directly over a flame! It is highly inflammable. Always use a water bath.

💡 Did You Know?

Potassium Hydroxide (KOH) is used in experiments to prove CO₂ is necessary because it has the unique ability to absorb all Carbon Dioxide from the air!

5.0 Classic Experimental Setups

In the ICSE Biology exam, Section B often features diagrams of specific experiments. You must be able to identify the setup, the aim, and the chemicals used.

1. Moll's Half-Leaf Experiment

Aim: To prove that Carbon Dioxide (CO₂) is necessary for photosynthesis.

• Part of a leaf is inserted into a bottle containing Potassium Hydroxide (KOH).
• The KOH absorbs all CO₂ inside the bottle.
• Result: Only the part of the leaf outside the bottle turns blue-black with iodine.

2. Oxygen Release in Hydrilla

Aim: To prove that Oxygen is evolved during photosynthesis.

• An aquatic plant (Hydrilla) is placed under a funnel in a beaker of water.
• A test tube filled with water is inverted over the funnel.
• Result: Bubbles of gas collect at the top of the tube, which rekindles a glowing splinter (confirming Oxygen).

6.0 Importance of Photosynthesis

1. Provision of Food: It is the primary way energy enters the biosphere. All animals (including humans) depend on plants directly or indirectly.

2. Oxygen for Respiration: It maintains the balance of Oxygen in the atmosphere, which is consumed by all living beings and for combustion.

⚠️ Identification Hack:

If you see a black paper strip on a leaf in a diagram, the experiment is to prove Light is necessary. If you see KOH, it's for CO₂. If you see a Variegated leaf (like Coleus), it's for Chlorophyll.

💡 Did You Know?

The rate of photosynthesis can be increased in greenhouses by burning paraffin lamps or adding dry ice to increase the CO₂ concentration, leading to faster crop growth!