Lesson 3 Part 2: Light-Dependent Reactions

Photosynthesis happens in two major steps:

1. Light-dependent reactions (which need sunlight)
2. Light-independent reactions (which do not need direct sunlight, like the Calvin cycle)

Today, we will focus only on the light-dependent reactions, which are the first phase of photosynthesis.

🔆 What are Light-Dependent Reactions?

Light-dependent reactions are the reactions in photosynthesis that need sunlight to happen. These reactions take place in the thylakoid membranes inside chloroplasts.

Let’s understand the key events step by step:

🔑 Key Events in Light-Dependent Reactions

1. Absorption of light energy
   Special molecules called photosynthetic pigments (like chlorophyll) absorb sunlight. These pigments are like solar panels that catch light energy.
2. Excitation of electrons
   When the pigment molecules absorb light, their electrons gain energy and become “excited.” Think of it like giving a ball a push up a hill — the ball now has more energy.
3. Formation of ATP and NADPH
   The excited electrons are used to make two important energy-rich molecules:
   • ATP (Adenosine Triphosphate): The energy currency of the cell
   • NADPH: A high-energy electron carrier

These two molecules store energy and are later used in the second phase of photosynthesis (the light-independent reactions or Calvin cycle) to make glucose (sugar).

🔋 Why is ATP Formation Important?

The formation of ATP in light-dependent reactions is a vital step. This process is called photophosphorylation. The term comes from:

• Photo = light
• Phosphorylation = adding a phosphate group to make ATP

There are two types of photophosphorylation:

• Non-cyclic photophosphorylation
• Cyclic photophosphorylation

Let’s explain both.