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.
Important
The numbering (I and II) is historical — PS I was discovered first, but PS II actually works before PS I in the electron flow. Think of it as: PS II → electron transport chain → PS I → 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:
2H2O4hνO2+4H++4e−
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⁺:
NADP++H++2e−2hνNADPH
The excited electron from P700* reduces ferredoxin (Fd), which then reduces NADP⁺ via the enzyme ferredoxin-NADP⁺ reductase.
Photosystem I's reaction-centre chlorophyll a is named P700 for its 700 nm absorption peak.
Photosystems are named for the wavelength (in nanometres) at which their special reaction-centre chlorophyll a molecule shows peak light absorption. Photosystem I (PS I) has its reaction centre chlorophyll a absorbing maximally at 700 nm, hence it is called P700. In contrast, Photosystem II (PS II) has its reaction centre chlorophyll a absorbing maximally at 680 nm, and is called P680.
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.What is the absorption peak of the reaction centre chlorophyll a, in PS I?
(a) 700 nm
(b) 600 nm
(c) 680 nm
(d) 780 nm
›Reveal solutionSolution
Photosystem I's reaction-centre chlorophyll a is named P700 for its 700 nm absorption peak.
Photosystems are named for the wavelength (in nanometres) at which their special reaction-centre chlorophyll a molecule shows peak light absorption. Photosystem I (PS I) has its reaction centre chlorophyll a absorbing maximally at 700 nm, hence it is called P700. In contrast, Photosystem II (PS II) has its reaction centre chlorophyll a absorbing maximally at 680 nm, and is called P680.
(b) Chl-a, 680 — the reaction centre of Photosystem II is designated P680 because it absorbs light maximally at 680 nm.
PS II's reaction centre is P680 (Chl-a absorbing at 680 nm); PS I's is P700.
Each photosystem has a special pair of chlorophyll-a molecules at its reaction centre that actually loses an electron on excitation, while the surrounding antenna pigments merely funnel light energy to it. Photosystem II's reaction-centre chlorophyll-a has its absorption peak at 680 nm, so it is named P …