Biology · Photosynthesis in Higher Plants · NEET
P700 is in Photosystem I (PS I) and P680 is in Photosystem II (PS II). Remember the bigger number goes with the smaller Roman numeral: P700 = PS I, P680 = PS II. The systems are named by the order they were discovered, not the order they work in the light reaction (PS II actually acts first).
The 'P' means pigment (the reaction-centre chlorophyll a), and the number is the wavelength of red light it absorbs most strongly. In PS I the reaction-centre chlorophyll a has an absorption peak at 700 nm, so it is named P700. In PS II it peaks at 680 nm, so it is named P680. NCERT states this directly.
Both are chlorophyll a molecules, so chemically they are the same pigment type. But they sit in different protein environments in PS I and PS II, which shifts their absorption peaks slightly — one to 700 nm, the other to 680 nm. So they are the same pigment (chlorophyll a) but function as two different reaction centres.
Shorter wavelength means higher energy. 680 nm is shorter than 700 nm, so P680 (PS II) uses slightly higher-energy light than P700 (PS I). This matches the Z-scheme: PS II must lift electrons high enough to eventually reduce NADP+ after passing through PS I.
No. Each photosystem has hundreds of pigments in the antenna (light-harvesting complex) that only trap and funnel light. Just one single chlorophyll a molecule at the centre — P700 or P680 — actually loses the excited electron and starts the electron transport. That single molecule is the reaction centre.
The reaction centre in PS II has an absorption maxima at
Which pigment has an absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700)?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the single chlorophyll a molecule in a photosystem that receives energy from the antenna pigments and releases an excited electron to start the electron transport chain. P700 (PS I) and P680 (PS II) are the two reaction centres.
P700 is the reaction-centre chlorophyll a of PS I that absorbs light at 700 nm; P680 is the reaction-centre chlorophyll a of PS II that absorbs light at 680 nm.
P680 (PS II). The splitting of water (photolysis) that releases O2 is associated with Photosystem II, which contains P680.
No. They are reaction-centre chlorophyll a molecules. Accessory pigments (chlorophyll b, carotenoids, xanthophylls) sit in the antenna and only pass energy to these reaction centres.
It is a direct one-line NCERT fact that is very easy to twist. Questions swap the wavelengths (700/680) or swap the photosystems (PS I/PS II), so knowing the exact pairing gives a guaranteed mark.
