5.1 – Photosynthesis

1. Role of Light and Pigments

Photosynthesis is the process by which green plants make their own food using sunlight. But how do they “catch” sunlight? They do this with the help of special molecules called pigments, which are mostly found in the leaves.

• The most important pigment is chlorophyll – it gives leaves their green color.
• These pigments absorb light energy from the sun and convert it into chemical energy.

Simple Example:
Think of chlorophyll like a solar panel. Just as a solar panel captures sunlight and turns it into electricity, chlorophyll captures sunlight and turns it into energy the plant can use to make food.

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2. Summary Equation of Photosynthesis

The whole process of photosynthesis can be summarized by a chemical equation:

6CO₂ + 12H₂O + Light → C₆H₁₂O₆ + 6O₂ + 6H₂O

Let’s break it down:

• CO₂ = Carbon dioxide (taken from the air through tiny pores in the leaves called stomata)
• H₂O = Water (absorbed from the soil through the roots)
• Light = Energy from sunlight

The products are:

• C₆H₁₂O₆ = Glucose (a type of sugar; this is the food for the plant)
• O₂ = Oxygen (released into the air)
• H₂O = Water (some is produced in the process)

Note: Water is used at the beginning and also appears as a product. But because the amount used and released can balance out, there’s no net gain or loss of water.

Compensation Point

1. Relationship Between Photosynthesis and Respiration

• Photosynthesis and respiration are like two sides of the same coin.
• Photosynthesis produces glucose and oxygen.
• Respiration uses glucose and oxygen to make energy.

So, plants produce what they need to survive – and then use it to keep themselves alive.

2. Day and Night Activity

• Photosynthesis only happens during the day, because it needs light.
• But respiration happens all the time, day and night. Plants always need energy to live.

3. Behavior During Darkness

At night, there’s no light, so photosynthesis stops. But respiration continues. This means:

• The plant uses oxygen and releases carbon dioxide, just like animals do when they breathe.

4. What Happens at Dawn and Dusk

During early morning and late evening, when light is very dim, something interesting happens:

• Photosynthesis starts, but very slowly.
• The rate of photosynthesis equals the rate of respiration.
• That means:
  • Oxygen made by photosynthesis = oxygen used in respiration
  • Carbon dioxide used in photosynthesis = carbon dioxide made in respiration

So, there’s no net exchange of gases with the environment. This moment is called the Compensation Point.

In Simple Words:
It’s a balancing point — the plant is neither gaining nor losing oxygen or carbon dioxide to/from the environment.