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.
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.
Hope things are clear. Watch the video for
better understanding. Thank you and enjoy learning Bio....
