Lesson 5 of 12 · 6 min
The light reaction and the photosystems
NCERT §11.5
Two antenna arrays sit side by side in every thylakoid membrane in Kavya's rice leaf, gathering light all morning, and only one tiny molecule in each is ever allowed to actually give up an electron.
The lesson in notes
In short
The light reactions (the photochemical phase) cover light absorption, oxygen release, water splitting and the production of ATP and NADPH, the high-energy chemical intermediates.
Pigments are organised into two discrete light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).
Each LHC, which acts as an antenna, consists of hundreds of pigment molecules held by proteins; by absorbing different wavelengths it makes photosynthesis more efficient.
Every photosystem has all pigments except one chlorophyll a molecule that forms its reaction centre.
The reaction centre chlorophyll a of PS I has its absorption peak at 700 nm, hence the name P700; that of PS II peaks at 680 nm and is called P680.
The photosystems are numbered in the order they were discovered, not in the order they act during the light reaction.