Q.a. Briefly describe the photosynthetic units or light harvesting complexes. OR b. Give three significance of the Krebs cycle.

The Problem: Capturing Sunlight

Imagine a solar panel. It doesn't just sit there — it has to absorb photons and convert that energy into something useful. In photosynthesis, the "solar panels" are photosystems. But there's a catch: a single chlorophyll molecule can only absorb a photon and get excited, but it can't do much with that energy on its own. It needs help.

So nature builds a complex — a cluster of hundreds of pigment molecules (chlorophylls and carotenoids) surrounding a special pair of chlorophyll molecules at the reaction centre. The surrounding pigments act like an antenna: they absorb light from different wavelengths and funnel the energy inward, like a crowd passing a message to the stage. This is called resonance energy transfer.

When the energy reaches the special pair, that pair gets excited and ejects an electron. That electron is the start of the whole electron transport chain.

Two Photosystems, Two Jobs

Why two? Because a single photon doesn't carry enough energy to push an electron all the way from water to NADP⁺. The electron needs a boost — twice. So evolution split the job into two photosystems that work in series, like two pumps lifting water up a hill.

Photosystem II (PS II) comes first in the sequence. Its reaction centre chlorophyll absorbs best at 680 nm, so it's called P680. PS II is the water-splitting photosystem. It uses light energy to pull electrons from water, releasing oxygen as a byproduct. The excited electron from P680 is passed to a primary acceptor, and P680⁺ becomes the strongest biological oxidising agent known — strong enough to rip electrons from water.

Photosystem I (PS I) comes later. Its reaction centre absorbs best at 700 nm, so it's called P700. PS I receives electrons that have already lost some energy after passing through the electron transport chain from PS II. Light excites P700, and this second boost gives the electron enough energy to reduce NADP⁺ to NADPH.

The Precise Statement

A photosystem is a pigment-protein complex embedded in the thylakoid membrane, consisting of:

• Antenna complex (light-harvesting complex): hundreds of pigment molecules that absorb photons and transfer energy to the reaction centre.
• Reaction centre: a special pair of chlorophyll molecules (P680 in PS II, P700 in PS I) that undergoes charge separation upon excitation — losing an electron to a primary acceptor.

Photosystem II (P680) catalyses the light-driven oxidation of water:

The excited electron from P680* reduces plastoquinone (PQ), which then carries the electron toward the cytochrome b₆f complex.

Photosystem I (P700) catalyses the light-driven reduction of NADP⁺:

The excited electron from P700* reduces ferredoxin (Fd), which then reduces NADP⁺ via the enzyme ferredoxin-NADP⁺ reductase.

Why the Different Wavelengths? …