Photosynthesis

ICSE Class 10 Biology • Chapter 05

1. Definition & Importance

Photosynthesis: The process by which green plants manufacture food (glucose) from Carbon dioxide and Water, in the presence of Sunlight and Chlorophyll, releasing Oxygen as a by-product.

$$ 6CO_2 + 12H_2O \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow $$

Practice Q1: Equation Analysis

BOARD QUESTION Name the gas released during photosynthesis. What is its source?

Ans: Oxygen. Its source is Water ($H_2O$).

2. Site of Photosynthesis: Chloroplast

Located chiefly in the mesophyll cells (Palisade and Spongy parenchyma) of leaves.

Grana (Stacked Thylakoids): Site of Light Reaction. Contains chlorophyll to trap light.
Stroma (Ground Matrix): Site of Dark Reaction. Contains enzymes for carbohydrate synthesis.

3. Introduction to Phases

Photosynthesis occurs in two phases primarily to convert light energy into chemical energy.

A. Light Dependent Phase (Photochemical Phase)

Occurs in Grana. Fastest phase. Requires Light.

Steps:

Activation of Chlorophyll: Photons from sunlight activate chlorophyll.
Photolysis of Water: Splitting of water by activated chlorophyll.
$2H_2O \rightarrow 4H^+ + 4e^- + O_2$ (Oxygen is released)
Reduction of NADP: NADP picks up $H^+$ and electrons to form NADPH.
Photophosphorylation: Formation of ATP from ADP + Pi using light energy.
$ADP + Pi \xrightarrow{\text{Energy}} ATP$

B. Light Independent Phase (Dark/Biosynthetic Phase)

Occurs in Stroma. Does not require specific light (uses ATP & NADPH from light reaction).

Calvin Cycle: A series of reactions where $CO_2$ is fixed/reduced to Glucose ($C_6H_{12}O_6$) using energy from ATP and Hydrogen from NADPH.

Polymerisation: Glucose is converted to Starch for storage (Insoluble).

4. Adaptations for Photosynthesis

Large Surface Area: For maximum light absorption.
Leaf Arrangement: At right angles to light source (Mosaic arrangement).
Numerous Stomata: For rapid exchange of gases ($CO_2/O_2$).
Thinness: Rapid transport of materials.

5. Experiments

De-starching: Essential first step. Keep plant in dark for 24-48 hours to remove all starch from leaves so that starch found after experiment is freshly formed.

A. Iodine Test

Used to test presence of starch. Iodine turns Starch Blue-Black.

B. Chlorophyll is Necessary

Use Variegated Leaf (e.g., Coleus, Croton - has green and white parts).

Observation: Only green parts turn Blue-Black with iodine. White parts remain brown.

C. Sunlight is Necessary

Use Ganong's Light Screen or Black Paper clip.

Observation: Covered part shows no starch (Brown). Exposed part turns Blue-Black.

D. Carbon Dioxide is Necessary (Moll's Half-Leaf)

Half leaf inserted in a bottle with KOH (Potassium Hydroxide) which absorbs $CO_2$.

Observation: Part inside bottle (no $CO_2$) turns brown. Part outside turns Blue-Black.

E. Oxygen Evolution

Use Hydrilla (aquatic plant) in a funnel under a test tube of water.

Observation: Bubbles of gas collect. A glowing splinter bursts into flame (Testing Oxygen).

6. Factors Affecting Photosynthesis

Light Intensity: Increases rate up to a certain point, then stabilizes.
CO2 Concentration: Limiting factor. Increase boosts rate up to 0.05%.
Temperature: Optimum 35°C. Above 40°C, enzymes are destroyed.

Exam Practice Questions (PYQ Trends)

Year: 2021

BOARD QUESTION Give the biological term for: The organelles where photosynthesis takes place.

Ans: Chloroplasts (specifically Thylakoids/Stroma).

Reasoning

THINKING Why is it advised to sleep under a tree during the day but not at night?

Ans: During the day, trees release Oxygen (Photosynthesis). At night, they consume Oxygen and release CO2 (Respiration only, no Photosynthesis), which is harmful.

Differentiation

CONCEPT CHECK Differentiate between Light Reaction and Dark Reaction.

Ans:
- Light Reaction: Occurs in Grana, requires Light, produces ATP/NADPH/O2.
- Dark Reaction: Occurs in Stroma, independent of Light, produces Glucose.