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.