Q.2H2O → 4H+ + O2 + 4e-. Based on the above equation, answer the following questions: a. Where does this reaction take place in plants? b. What is the significance of this reaction?
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.
a. This reaction — the splitting of water — takes place at the water splitting complex associated with Photosystem II, which is located on the inner side of the thylakoid membrane, so the products are released into the thylakoid lumen.
b. Its significance is twofold:
It releases oxygen, which is one of the net products of photosynthesis and diffuses out of the chloroplast into the atmosphere. …
This equation represents the splitting of water at Photosystem II, an inner-thylakoid-membrane event that replenishes PS II's electrons, releases oxygen, and helps build the proton gradient for ATP synthesis.
a. Where it occurs: Photosystem II continuously loses electrons to the electron transport chain as part of the Z scheme, and these electrons must be replaced for PS II to keep functioning. This replacement is achieved by splitting water, a reaction carried out by the water splitting complex associated with Photosystem II. Because PS II is physically located on the inner side of the thylakoid membrane, this splitting reaction, and the protons and oxygen it produces, occur there too — so the products of the reaction are released into the thylakoid lumen rather than toward the stroma.
b. Significance: This single reaction serves several purposes at once.
It creates oxygen (O2), one of the two net products of photosynthesis, which then diffuses out of the chloroplast.
The four electrons released replace those that Photosystem II has passed on to the electron transport chain, keeping the flow of electrons through the Z scheme continuous. …
Recognise the equation: 2H2O → 4H+ + O2 + 4e- is the water-splitting (photolysis) reaction.
Locate the site: this occurs at the water-splitting complex associated with Photosystem II, on the inner (lumen-facing) side of the thylakoid membrane (answer a).
List what the reaction supplies: O2 (released as a photosynthesis by-product), electrons (replace those PS II continuously loses to the electron transport chain), and protons (deposited into the lumen). …