The light reactions drive the movement of electrons from water to NADP+. Molecular oxygen is generated as a by-product of this reaction. Light energy is used to remove the electrons from water. Once removed, the electrons spontaneously flow through electron carriers from photosystem II to photosystem I. The energy released from the spontaneous flow of electrons is used to pump protons into the thylakoid lumen. Photosystem I absorbs additional light energy, which is required to complete the transfer of electrons to NADP.

The flow of electrons from water to NADP+ is noncyclic. Molecular oxygen and NADPH are generated only from the noncyclic flow of electrons. ATP is produced by both the noncyclic and cyclic flow of electrons.

In the cyclic flow of electrons, photosystem I absorbs a photon. An excited electron from its reaction center moves through electron carriers and then returns. The energy released as the electron is transferred drives the movement of protons into the lumen.

The increased concentration of protons in the lumen creates a proton-motive force. When the protons flow out of the lumen through ATP synthase, ATP is made.

Optimal plant growth, and hence optimal rates of photosynthesis, require balanced lighting. This means light rich in wavelengths of blue and red.

In addition, the rate of photosynthesis depends on the number of photons available to the photosystems. More intense light, with greater numbers of photons, has more energy and supports increased rates of photosynthesis compared to dim or low-intensity light.

Photosynthesis transforms the energy of sunlight into chemical energy. Photosynthesis is the ultimate source of chemical energy on our planet.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education