Biology · Photosynthesis in Higher Plants · NEET
Manganese (Mn). This is the most common NEET trap because magnesium (Mg) is the metal in the chlorophyll ring, so students mix them up. Remember: Mg is inside chlorophyll (the pigment), while Mn is inside the oxygen evolving complex (the water-splitting site). The OEC also needs chloride (Cl-) and calcium (Ca2+) ions, but the key catalytic metal is manganese.
It is part of Photosystem II (PS II) only. PS II loses electrons when P680 gets excited by light, so it needs a new supply of electrons. The OEC sits on PS II and splits water to give these replacement electrons. PS I (P700) does not split water; it gets its electrons from PS II through the electron carriers.
When light hits P680 in PS II, it ejects high-energy electrons that travel down the electron transport chain. PS II is now short of electrons. The oxygen evolving complex splits water to replace exactly those lost electrons. So water is the ultimate electron donor in green-plant (oxygenic) photosynthesis, and O2 comes out as a by-product.
On the inner side (lumen side) of the thylakoid membrane. NCERT states the water-splitting complex is located on the inner side of the membrane. This matters because the 2H+ released go into the thylakoid lumen and help build the proton gradient used later by ATP synthase.
Water splits into 2H+ (protons), [O] (which pairs up to form O2) and electrons (e-). NCERT writes it as: 2H2O → 4H+ + O2 + 4e-. The electrons go to refill PS II, the protons build the lumen gradient for ATP, and the oxygen is released — the O2 we breathe comes from water, not from CO2.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a group of manganese atoms attached to Photosystem II that splits water into protons, oxygen and electrons during the light reaction. It is also called the water-splitting complex.
Manganese is the catalytic metal in the oxygen evolving complex. It helps pull electrons out of water so PS II can be refilled. Without manganese, water splitting and oxygen release would stop.
Along with manganese (Mn), the reaction needs chloride (Cl-) and calcium (Ca2+) ions. But the metal NEET most often asks about is manganese.
It comes from the splitting of water at the oxygen evolving complex, not from CO2. This was proven by experiments using water labelled with heavy oxygen.
To PS II. PS II loses electrons first, so the OEC on PS II splits water to replace them. PS I gets its electrons downstream from PS II.
