📘 PHOTOSYNTHESIS

Photosynthesis is the process by which green plants, algae, and some bacteria manufacture their own food (glucose) using sunlight, carbon dioxide, water, and chlorophyll.

It is the foundation of life on Earth because it produces oxygen and is the main source of energy for almost all organisms.

1️⃣ Definition

Photosynthesis is the process by which green plants use chlorophyll and sunlight to convert carbon dioxide and water into glucose and oxygen.

Word Equation:

Carbon dioxide + Water → Glucose + Oxygen

Balanced Chemical Equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(in the presence of sunlight and chlorophyll)

2️⃣ Conditions Necessary for Photosynthesis

Plants must have:

1. Sunlight

Provides the energy needed for the reaction.

2. Chlorophyll

The green pigment in leaves that traps sunlight.

3. Carbon Dioxide (CO₂)

Taken in through the stomata.

4. Water (H₂O)

Absorbed from the soil by roots.

5. Suitable Temperature

Enzymes work best between 25°C–35°C.

3️⃣ Structure of the Leaf (Adaptations for Photosynthesis)

Leaves are perfectly built for photosynthesis:

Broad surface → absorbs more sunlight
Thin → gases diffuse easily
Chloroplasts present → contain chlorophyll
Veins (xylem & phloem) → transport water and food
Stomata → allow gas exchange

4️⃣ Stages of Photosynthesis

Photosynthesis occurs in two main stages:

A. Light-Dependent Stage (Light Reaction)

Happens in the thylakoid membranes of the chloroplast.

What happens:

Sunlight splits water into hydrogen and oxygen
(This process is called photolysis)
Oxygen is released into the atmosphere
ATP (energy molecule) is formed
NADPH (hydrogen carrier) is produced

Main Products:

Oxygen (O₂)
ATP
NADPH

B. Light-Independent Stage (Dark Reaction / Calvin Cycle)

Happens in the stroma of the chloroplast.
Does not require light directly, but uses the products of the light reaction.

What happens:

Hydrogen (from NADPH) + CO₂ → Glucose
ATP provides the energy for sugar formation

Main Product:

Glucose (C₆H₁₂O₆)

5️⃣ Importance of Photosynthesis

1. Produces Oxygen

Essential for respiration of living organisms.

2. Produces Food

Forms the base of the food chain.

3. Removes Carbon Dioxide

Helps control global CO₂ levels.

4. Forms Plant Biomass

Glucose is stored as starch, proteins, oils.

5. Provides Energy for All Life

Directly or indirectly.

6️⃣ Factors Affecting Photosynthesis

1. Light Intensity

More light = faster rate (until saturation point).

2. Carbon Dioxide Concentration

Higher CO₂ = faster rate.

3. Temperature

Enzymes work best within specific range.

4. Water Supply

Water shortage reduces rate drastically.

5. Chlorophyll Supply

Yellow leaves = low photosynthetic activity.

7️⃣ How to Test a Leaf for Starch (WAEC Practical)

Boil leaf in water (kills cells).
Boil in alcohol (removes chlorophyll).
Rinse in warm water (softens leaf).
Add iodine solution.

Positive test:

Leaf turns blue-black → starch present → photosynthesis occurred.

8️⃣ WAEC/JAMB Likely Questions

Define photosynthesis.
State the raw materials and products.
Write the balanced photosynthesis equation.
Describe the light and dark reactions.
List four factors affecting photosynthesis.
Explain how to test a leaf for starch.
Give three leaf adaptations for photosynthesis.