How
is ATP synthesized in Cellular respiration by chemiosmosis?
Chemiosmotic
Hypothesis by Peter Mitchell 1961
Awarded
the Nobel Prize for Chemistry in 1978.
EXACT SITE OF REACTION
- Organelle: Mitochondrion
- Site of Electron transport chain: Mitochondrial
Inner membrane
- Proton (H+) pumped into the
intermembrane space creating proton gradient
- ATP synthesis occurs towards the
matrix region (see the above figure)
Chemiosmotic Theory states that Electron transport and ATP synthesis are
coupled by a proton gradient across the inner mitochondrial membrane.
Lets 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: At the beginning of
electron transport chain we have NADH and FADH2 synthesized during kerb’s cycle
and glycolysis. Approximately only 4 ATP are synthesized directly (2 from
glycolysis and 2 from Krebs cycle) from a glucose molecule. The rest ~32-34 ATP
are synthesized during Electron transport chain (ETC) by chemiosmosis.
Now
let as move into the detail
1.
Electron flow and Energy release:
NADH and FADH2 donates high energy electrons that
pass through different protein complexes and electron carriers in the ETC.
As the electrons moves from high energy to low
energy level, some amount of energy is released. The final electron acceptor is
O2 which splits and takes up H+ to form water (H2O)
2.
Proton movement
and gradient formation
The energy
released during electron flow is used to pump proton (H+ ions) from matrix side
to the intermembrane space of mitochondrion. (see figure). This creates a
proton gradient or (Electrochemical
gradient or proton motive force) across the inner mitochondrial membrane (that
is higher concentration of H+ ions in the intermembrane space compared to the
matrix).
3. proton motive force (PMF) driven
ATP synthesis
The H+ ions should move to matrix to maintain
equilibrium (to balance H+ ion concentration). As phopholipid bilayer of inner mitochondrial
membrane is impermeable, the only way out is through the protein complex called
ATP synthase which spans the inner mitochondrial 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. (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....
