🔶 Types of Photosystems

In plants, there are two types of photosystems:

🌞 Photosystem-I (PS-I)

• Discovered after PS-II, but works after PS-II during the process.
• Has a chlorophyll-a molecule called P700 in its reaction centre.
• It is named P700 because it absorbs light best at 700 nanometers, which is red light.

🌞 Photosystem-II (PS-II)

• Discovered first, but works before PS-I in the photosynthesis pathway.
• Has a chlorophyll-a molecule called P680 in its reaction centre.
• It absorbs light best at 680 nanometers, which is also in the red part of the light spectrum.

Note: The “P” in P680 and P700 stands for “pigment”, and the number tells us the wavelength of light it absorbs best.

🔶 Mechanism of Photosynthesis

Now that we know about photosystems, let’s understand what they do during photosynthesis.

🌱 Photosynthesis Is a Redox Process

This means it involves two main chemical actions:

1. Oxidation – When a substance loses electrons.
2. Reduction – When a substance gains electrons.

In photosynthesis:

• Water (H₂O) is oxidized — It loses electrons and hydrogen ions, and produces oxygen gas (O₂).
• Carbon dioxide (CO₂) is reduced — It gains electrons and hydrogen ions to form glucose (sugar).

Example:

Let’s take the overall equation of photosynthesis:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
(Carbon dioxide + water → glucose + oxygen)

In this process:

• Water is broken down (oxidized) to give off oxygen.
• Carbon dioxide is built up (reduced) into sugar.

🔶 Complexity of Photosynthesis

Photosynthesis is not a simple one-step process. It actually happens in two main stages:

☀️ 1. Light-dependent reactions (require sunlight)

• Happen on the thylakoid membranes.
• Involve photosystems I and II.
• Light energy is captured and used to:
  • Split water into oxygen, hydrogen ions, and electrons.
  • Generate ATP (energy molecule) and NADPH (electron carrier).

🌑 2. Light-independent reactions (do not require sunlight directly)

• Also called the Calvin cycle.
• Happen in the stroma (the fluid part inside chloroplasts).
• Use ATP and NADPH (from light reactions) to convert CO₂ into glucose.

🎓 In Simple Words

Imagine a plant cell as a small solar factory. Inside this factory:

• Light is captured by solar panels (antenna complexes).
• The energy is passed into a machine (reaction centre).
• The machine starts an electric flow (electron transport chain).
• Water is broken, oxygen is released.
• Carbon dioxide is taken in and sugar is made.