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Linear Electron Flow

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Linear electron flow is the process that produces ATP and NADPH during the light reactions of photosynthesis. These products are then used for the next process, the Calvin cycle. Linear electron flow occurs in the photosystems that are embedded in the thylakoid membrane within a chloroplast. There are two photosystems (PSI and PSII), each made of a reaction-center complex surrounded by a light-harvesting complex, both of which will be discussed later. Photosystem II actually comes before photosystem I (they were named in order of discovery). The following steps describe linear electron flow in detail.
  1. A photon of light strikes the light-harvesting complex of photosystem II (PSII). The light-harvesting complex consists of various pigment molecules (e.g. chlorophyll a, chlorophyll b, and carotenoids) bound to proteins. The photon of light boosts an electron to a higher energy state. As it falls back to the ground state, it releases energy. This energy is then passed along to the neighboring pigment molecules, exciting their electrons. Energy (not electrons) is transferred from pigment to pigment.
  2. The energy is transferred from pigment to pigment until it reaches the P680 pair of chlorophyll a molecules in the reaction center complex of PSII. The pigment is known as P680 because it is best at absorbing light with a wavelength of 680 nanometers. An electron is excited and moves from P680 to the primary electron acceptor. Note that this time an actual electron is being transferred, not just the energy. 
  3. P680 is now missing an electron and is therefore known as P680+ (the strongest biological oxidizing agent).
  4. An enzyme catalyzes the splitting of water into two hydrogen ions (H+), two electrons, and one oxygen atom.
  5. The two H+ are released into the thylakoid space.
  6. The oxygen atom combines with an oxygen atom from another split water molecule, forming O2 (a gas). The oxygen gas is released out of the plant (or any photosynthesizing organism) and into the atmosphere.
  7. The two electrons travel to the P680+ pair of chlorophyll molecules, replacing the previous electrons that had already moved to the primary electron acceptor.
  8. The electrons at the primary electron acceptor travel down a transport chain (a series of redox reactions) that connects PSII to PSI. The transport chain is made of the electron carrier plastoquinone (Pq), a cytochrome complex, and the protein plastocyanin (Pc).
  9. As the electrons travel down the transport chain, they create an electrochemical gradient. This gradient is used to form ATP by chemiosmosis.
  10. In PSI, another photon of light strikes the light-harvesting complex and excites an electron, transferring energy to its neighboring pigment molecule as it falls back to the ground state. The energy is transferred from pigment to pigment until it reaches the P700 pair of chlorophyll a molecules. These pigments are best at absorbing light with wavelengths of 700 nm.
  11. The energy excites an electron of the P700 pair of chlorophyll molecules. The excited electron is then transferred to the primary electron acceptor.
  12. P700 becomes P700+ since it's missing an electron.
  13. P700+ accepts electrons at the bottom of the transport chain coming from PSII.
  14. The electrons at the primary electron acceptor of PSI travel down another transport chain made of the protein ferredoxin (Fd). There is no protein gradient; therefore, no ATP is produced.
  15. The enzyme NADP+ reductase catalyzes the transfer of electrons from ferredoxin to NADP+. Two electrons are required for this process. NADP+ is reduced to form NADPH.
The electron transport chain has now produced ATP and NADPH, which are readily available for the next process, the Calvin cycle. There is also an alternative to linear electron flow, known as cyclic electron flow. You can read about it here.
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