Capturing Light Energy
The absorption of a photon by a pigment molecule (e.g. chlorophyll a, chlorophyll b), excites a single
electron, moving it from a low energy (ground state), to a high energy (or excited state).
The difference between the energy level of the ground state and the energy level of the excited state is
equivalent to the energy of the photon of light that was absorbed.
Following light absorption there are three possible outcomes for the excited electron within a pigment
molecule:
1. Electron returns to its ground state by emitting a less energetic photon (fluorescence) at a longer
wavelength; or by releasing energy as thermal energy
2. The electron returns to its ground state, and the energy released is transferred to a different
electron in a neighbouring pigment molecule – requires the two molecules to be very closely and
precisely aligned.
3. The excited-state electron may itself be transferred to a nearby electron-accepting molecule
During photosynthesis, chlorophyll a becomes oxidized and donates an electron to a primary electron
acceptor (outcome #3). This electron in turn will be passed on through the electron transport chain.
Chorophyll b and carotenoids are called accessory pigments because after light absorption they transfer
excitation energy to molecules of chlorophyll a (outcome #2). These molecules are part of the antenna
complex.
Light-Dependent Reactions
Photosystem II (P680) 
1) Starts with the absorption of a photon of light. Energy excites an electron in a chlorophyll a molecule
in P680 forming P680*.
The excited electron is then passed to the primary electron acceptor molecule which becomes negatively
charges. This then makes photosystem II have a positive charge (P680+).
P680+ is so electronegative (strongest oxidant in all of biology) that it can exert forces strong enough to
remove electrons from water. This process is facilitated by an enzyme subunit on the lumen side of the
thylakoid called the water-splitting complex.
H20  2 H+ + ½ O2 + 2 e- (oxidation of water by P680+)
Electrons removed from water are used to reduce P680+ back to P680. At the same time, the electron in
the acceptor molecule is passed to a molecule called plastoquinone (PQ), the acceptor molecule
becomes neutral again and the process can start over.
Oxygen gas and protons (H+) are released into the lumen
NOTE: This process occurs twice for each water molecule that is completely oxidized (2 e - are released
per 1 water molecule oxidized).
2) PQ acts as an electron shuttle between photosystem II and the cytochrome complex. When PQ
accepts the electron from P680 it also takes up protons from the stroma. When PQ donates electrons to
cytochrome complex it also releases protons into the lumen, further increasing the proton gradient.
3) From cytochrome complex, electrons pass to the mobile carrier plastocyanin, which shuttles electrons
to photosystem I (P700).
4) P700 also can absorb a photon of light and excite another electron in chlorophyll a. This excited
electron is transferred to an electron acceptor forming P700+. This then gets reduced back to P700 by
the oxidation of plastocyanin which is shuttling the electron originally from P680.
5) The electron acceptor in P700 is then transferred to ferrodoxin. The oxidation of ferrodoxin results in
the transfer of the electron to NADP+, which is reduced to NADP.
6) Another electron is transferred to NADP by ferrodoxin and a proton (H+) from the stroma (done by
the enzyme NADP+ reductase) to form NADPH. Therefore, NADPH is now carrying two high-energy
electrons which will be used in the Calvin cycle (light-independent reactions) downstream.
Proton gradient
As in cellular respiration, a proton gradient is created with a high concentration on the lumen side and a
low concentration on the stroma side. This gradient is then used to drive the chemiosmotic synthesis of
ATP via ATP synthase. The ATP generated is then used in the Calvin cycle downstream.
The concentration gradient is established in three ways:
1. Protons taken into the lumen by the reduction and oxidation of PQ as it moves from
photosystem II to the cytochrome complex and back again.
2. Concentration of protons in the lumen is increased by the addition of two protons for each
molecule of water that is split in the lumen by photosystem II and water splitting complex.
3. The removal of one proton from the stroma for each NADPH molecule formed decreases the
concentration of protons in the stroma outside the thylakoid membrane.