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Chemiosmosis and ATP synthesis in Photosynthesis Simplified steps

How is ATP synthesized in photosynthesis by chemiosmosis?
  • Chemiosmotic Hypothesis by Peter Mitchell 1961
  • Awarded the Nobel Prize for Chemistry in 1978.
  • This process is similar to the ATP synthesis during cellular respiration by Chemiosmosis.

Chemiosmosis and ATP synthesis in Photosynthesis

EXACT SITE OF REACTION
  • Organelle: Chloroplast during light dependent reaction
  • Site of Electron transport chain: Thylakoid membrane of chloroplast
  • Proton (H+) pumped into the thylakoid lumen or thylakoid space
  • ATP synthesis occurs towards the stromal side (see the above figure)
  • ATP produced during light reaction is used to fix carbon dioxide to carbohydrates in Calvin cycle
Chemiosmotic Theory states that Electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane.
Let’s simplify this statement
The 3 major events in chemiosmosis are
1. Electron flow & energy release
2. Proton movement and gradient formation
3. proton motive force (PMF) driven ATP synthesis

Background info: Light absorbing pigments are arranged on the thylakoid membrane as photosystems (PS-I and PSII). Light energy is absorbed by the pigment molecules of the photosystem. The absorbed energy is transferred to nearby pigment molecules finally reaching the reaction centre chlorophyll-a molecule by resonanace transfer (vibratory transfer).

Now let as move into the detail
1. Electron flow and Energy release:
High energy electrons are released from the reaction centre chl-a molecule that is transferred through series of electron carriers in cyclic and non cyclic photophosphorylation. During the movement of electrons from high energy level to low energy level, some amount of energy is released

2. Proton movement and gradient formation
The energy released during electron flow is used to pump proton (H+ ions) from stromal side to the thylakoid lumen or thylakoid space of chloroplast (see figure). This creates a proton gradient or (Electrochemical gradient or proton motive force) across the thylakoid membrane (that is higher concentration of H+ ions in the thylakoid space compared to the stroma).

3. Proton motive force (PMF) driven ATP synthesis
The H+ ions should move to stroma to maintain equilibrium (to balance H+ ion concentration). As phopholipid bilayer of thylakoid membrane is impermeable, the only way out is through the protein complex called ATP synthase which spans the thylakoid membrane and has a proton channel.
The flow of H+ ions through ATP synthase provides energy for the addition of phosphate to ADP thus forming ATP towards the stromal side which is utilized in Calvin cycle for fixing CO2 to glucose (just like turbine in hydroelectric power plant where water forces turbine movement, here flow of H+ ions drives ATP synthesis).
The proton gradient (Proton motive force) driven ATP synthesis is called Chemiosmosis.
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