The Light Dependent Reactions: Linear Electron Flow
What follows is a summary of the steps to explain how linear electron flow during the light reactions
generates ATP and NADPH that are required for the light independent (dark) reactions.
1. A photon of light strikes a pigment molecule in Photosystem II. The energy captured is relayed to
other pigment molecules until it reaches the P680 pair of chlorophyll a molecules. An electron in the
chlorophyll a is excited to a higher energy state.
2. This electron is transferred from the excited P680 to the primary electron acceptor.
3. This leaves the chlorophyll with a +ve charge. An enzyme splits a water molecule into 2e-, 2 H+ and
an oxygen atom. The electrons are used to fill the electron vacancy when the excited electrons are
transferred from P680 to the primary electron acceptor. The oxygen will combine with another (the
result of another water molecule splitting) to form O2.
4. Each photo-excited electron passes from the primary electron acceptor of PS II to PS I via an electron
transport chain. It is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a
protein called plastocyanin (Pc).
5. The “fall” of electrons to a lower energy level provides the energy required for the synthesis of ATP.
As electrons pass through the cytochrome complex, the pumping of protons (H+) builds a proton
gradient that is used in chemiosmosis. (Explained later)
6. At the same time, light energy is absorbed by a pigment molecule in PS I exciting a pair of P700
chlorophyll a molecules. The photo-excited electrons are transferred to the primary electron acceptor
for PS I. As with Photosystem II, this leaves an electron vacancy in the chlorophyll a. The P700+ will
now accept the electrons that have reached the bottom of the electron transport chain from PS II.
7. Photo-excited electrons are passed in a series of redox reactions from the primary electron acceptor of
PS I down a second electron transport chain through the protein ferredoxin (Fd).
8. The enzyme NADP+ reductase catalyses the transfer of electrons from Fd to NADPH+. Two
electrons are required for its reduction to NADPH. This molecule is at a higher energy level that water
and its electrons are more readily available for the reactions of the Calvin Cycle than were those of
water.
In summary: Light reactions use: water and energy from the sun
Light reactions produce: atmospheric oxygen, ATP and NADPH which are both required
to build the carbohydrates that result from the Calvin Cycle.
Chemiosmosis:
In the thylakoid membrane there are many copies of a protein complex called ATP synthase, the enzyme
that makes ATP from ADP and Pi. Here’s how it works….
At certain steps along the electron transport chain, electron transfer causes protein complexes to move
H+ ions from the Stroma of the chloroplast to the Thylakoid space….inside the thylakoid… storing
energy as a proton-motive force (H+ gradient). As H+ diffuses back into the stroma through ATP
synthase, its passage drives the phosphorylation (addition of P) to ADP, resulting in the production of
ATP.