Light Dependent Process
Photosynthesis is frequently described in two stages the
light dependent process, which requires light, and the light independent
process, which does not require light.
The light dependent process occurs in the thylakoid membranes of
the chloroplast. The process, illustrated below, begins when light
strikes a molecule of chlorophyll and excites electrons to a higher
energy state. Water is split to replace the electrons, releasing
oxygen as a by-product. In a series of reactions within a redox
chain, or electron transport chain, the energy from the electron
is converted into ATP and NADPH. The ATP and NADPH are used to create
carbohydrate during the light independent process.
eukaryotes and cyanobacteria have two photosystems Photosystem
II and Photosystem I. Light energy causes the excitation and loss
of an electron from a PSII reaction center chlorophyll (P680). Water
is oxidized to replace the lost electron, generating H+
ions and oxygen (O-2) ions. These O-2 ions
combine to form the diatomic O2. The excited electron
is boosted to a higher energy state. Electrons are passed from a
photosystem into a redox or electron transport chain, eventually
attaching to a chlorophyll molecule in Photosystem I (P700).
acts on chlorophyll in Photosystem I, causing an electron to be
boosted to a still higher potential. The electron is attached to
a primary electron acceptor (different from the one associated with
Photosystem II). The electron is passed again through a series of
redox reactions, eventually joining NADP+ and H+
to form NADPH. Electrons flow continuously from water to NADPH.
In Photosystem II, the pumping of H+ ions into the interior
space of the thylakoid membranes creates a proton gradient, which
is coupled to ATP production.