Phtosynthesis V

Comparison between chloroplast and mitochondria

Chloroplast	Mitochondria
1. Cytb6f	Cyt bc1
2. PQB	QB
3. PC	Cytc
4. Proton transfer ( from outside stroma ---- to inside Lumen	Proton transfer from inside ( matrix ) to outside ( inner membrane )

Sequence of reactions for proton pump

1. PQ_BH₂ participates in Q cycle.

2. Electrons released during oxidation of PQ_BH₂ are equally partitioned in following electron trranfer sequences :

1. Rieske Fe – S protein – Cyt f –PC – PSI
2. Cyt b₆ funnels electron back in Q cycle for reduction of PQ_B ( stroma side )

3. The orientation of Q cycle is such that H⁺ released during oxidation of PQ_BH₂ is pumped towards lumen ( inside thylakoid)

4. Protons required during reduction of PQ_B is derived from stroma.

5. Proton concentration in lumen becomes higher than stroma and proton gradient is established resulting in ATP synthesis.

TOPOLOGY

a) PSII spans across the lamelle with P680 exposed in stroma to capture light and OEC ( oxygen evolving complex ) buried in lumen.

b) PQ on the stroma side and PQH₂ near lumen side

c) Rieske Fe – S protein and Cyt f on lumen side

d) PC on lumen side

e) Cyt b₆ on stroma side.

f) PSI spans across the lamelle with P700 exposed in stroma to capture light

g) Fd on stroma side

Spatial separation of PSII and PSI is managed to prevent larceny ( theft ) of photons

PSII is located in tightly apposed membrane stacks of grana.