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.
Splitting of water is associated with the inner side of the thylakoid membrane.
The water splitting complex is physically part of Photosystem II, which is itself located on the inner side of the thylakoid membrane.
Because of this location, the protons and oxygen produced when water is split are released into the lumen, the space enclosed on the inner side of the thylakoid, rather than toward the stroma. …
Water splitting is carried out by a complex associated with Photosystem II, which sits on the inner surface of the thylakoid membrane, so its protons and oxygen are released into the lumen.
The splitting of water answers a specific problem: Photosystem II continuously loses electrons to the electron transport chain, and these electrons must be replaced if PS II is to keep functioning. This replacement is achieved by splitting water — two molecules of water are split into four protons, oxygen, and four electrons, and the electrons released refill PS II. …
Method 1 — Locate where the water-splitting complex physically sits
Recall that splitting of water is a function tied to Photosystem II — not to Photosystem I, ruling out (a) and (c).
Recall that the water-splitting (oxygen-evolving) complex is physically embedded in the thylakoid membrane, on the side facing the lumen — i.e. on the inner surface of the thylakoid membrane. …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
AP EAPCET 2022Set ap-2022-07-12-FN1 markMCQ
Q.Identify the first electron acceptor from an excited chlorophyll molecule of photosystem-II
(A) Cytochrome
(B) Ferredoxin
(C) Quinone
(D) Plastocyanin
›Reveal solutionSolution
In PS-II, the excited chlorophyll (P680*) passes its electron to a quinone-type primary acceptor before it moves down the electron transport chain.
Concept and Intuition
In the Z-scheme of photosynthesis, light excites the special chlorophyll P680 of Photosystem II, which then ejects a high-energy electron. This electron is captured first by the primary acceptor, a quinone-type molecule (often represented as Q), before being passed along the electron transport chain (through cytochrome complexes) to plastocyanin and finally to PS-I. Ferredoxin, in contrast, is the terminal acceptor at the end of PS-I's electron chain — not PS-II's first acceptor. Cytochrome and plastocyanin are intermediate carriers between the two photosystems, not the very first acceptor from excited PS-II.
Step-by-Step Solution
Light excites P680 in PS-II, which loses an electron to its primary/first acceptor.