The blue-green algae made another great invention that released them completely from the need for H₂S as a reducing agent in photosynthesis. They evolved a method to turn a very poor reducing agent, H₂0, into a usable one by activating it with light. Although H₂0 is a bad reducing agent, it is available everywhere. Any organism that found a way to take electrons away from water obviously would have a great advantage over its more pedestrian cousins. The key was the development of two photocenters, one to excite electrons for reduction of NAD⁺ (actually, NADP⁺) in the usual way, the other to provide the energy required to strip electrons away from water molecules to leave 0₂ gas and hydrogen ions:

These are Photocenters I and II, diagrammed on next page. Photocenter I, which is analogous to the photocenter in bacteria, absorbs light in the far-red region at wavelengths of 700 nanometers (7000 A) and longer. Its chlorophyll is designated as P700 for "700-nm pigment." Photocenter II absorbs slightly shorter wavelengths, with a maximum absorption around 680 nm.

This energy is used to excite electrons on that chlorophyll, send them cascading down an electron-transport chain to Photocenter I, and remove electrons from water to make up the deficit.

This is the two-photocenter, water-using, oxygen-liberating form of photosynthesis that has been adopted by all green plants. It is more versatile because it enables the organism to use two photons of light to make a good reducing agent out of a bad one, rather than forcing the organism to seek out a better reducing agent such as H₂S.

We know more about the electron-transport chain that bridges the photocenters than we do about the corresponding chain in bacteria, and its resemblance to the respiratory chain is striking. The molecule that accepts electrons from excited Photocenter II may be a flavoprotein analogous to the flavoprotein that accepts electrons from NADH in respiration.