Biology · Photosynthesis in Higher Plants · NEET
Because of its shape when drawn on a redox potential (energy) scale, not because of the order of parts. Light energy at PS II pushes electrons up to a high energy point. They then move downhill through carriers to PS I. At PS I, light again pushes them up, and they move downhill once more to NADP+. When you plot these ups and downs, the line looks like a sideways 'Z'. NEET often gives a graph and asks you to identify this shape.
It begins at PS II. In NCERT the sequence of the light reaction is P680 (PS II) first, then the electron transport chain, then P700 (PS I). This is confusing because PS I is numbered '1'. Remember: the numbers show the order they were discovered, not the order electrons flow. Electrons flow PS II to PS I.
Uphill means the electron gains energy. This happens only at the two reaction centres (P680 and P700) when they absorb light. Downhill means the electron loses energy as it passes through carriers (like plastoquinone, the cytochrome b6f complex, and plastocyanin). NCERT clearly says the movement of electrons through the transport chain is 'downhill' on the redox potential scale.
Water is split (photolysis) on the inner side of the thylakoid membrane and it supplies the electrons that fill the gap left in PS II. So water sits at the very start, at the bottom-left of the Z. It donates electrons to P680 and releases O2 and protons. Water is the electron source; NADP+ is the final electron acceptor.
The classic Z-scheme shown in NCERT is the non-cyclic path, because it uses both PS II and PS I, splits water, and forms NADPH. Cyclic photophosphorylation uses only PS I, does not form a 'Z' shape, and makes only ATP. So if a question shows both photosystems and NADPH, it is the non-cyclic Z-scheme.
In light reaction, plastoquinone facilitates the transfer of electrons from
Emerson's enhancement effect and Red drop have been instrumental in the discovery of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The two uphill points are the reaction centres P680 (in PS II) and P700 (in PS I). Each absorbs light and boosts electrons to a higher energy level. Everything between and after them is downhill.
NADP+ is the final acceptor. It gains the electrons (and a proton) at the end of the path and becomes NADPH, which is later used in the Calvin cycle to fix CO2.
The main carriers are plastoquinone, the cytochrome b6f complex, and plastocyanin between PS II and PS I. After PS I, ferredoxin and NADP reductase pass electrons to NADP+.
Yes. As electrons move downhill through the carriers, protons build up inside the thylakoid. This proton gradient drives ATP synthase to make ATP, along with the NADPH formed at the end.
NEET frequently tests the exact order of the electron path, the meaning of 'downhill', the role of specific carriers like plastoquinone, and whether the scheme is cyclic or non-cyclic. Knowing the shape and sequence lets you answer graph-based and match-the-column questions quickly.
