Energy flow in the light-dependent reactions of photosynthesis

The
P700 in photosystem I absorbs photons of a 700-nanometer wavelength. The P680
of photosystem II absorbs photons at 680 nanometers. Energy inputs boost P700
and P680 to an excited state in which they lose electrons.
A The noncyclic pathway involves a one-way flow of electrons from water, to
photosystem II, to photosystem I, to NADPH. As long as electrons continue
to flow through the two electron transfer chains, H+ continues to be carried
across the thylakoid membrane, and ATP and NADPH keep forming. Light
provides the energy boosts that keep the pathway going.
B In the cyclic pathway,
electrons ejected
from photosystem I are
returned to it. As long
as electrons continue to
pass through its electron
transfer chain, H+ continues
to be carried across
the thylakoid membrane,
and ATP continues to
form. Light provides the
energy boost that keeps
the cycle going.