D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 18
PHOTOSYNTHESIS III
4.
Calvin Cycle (fig, 10 - 18 & ppt. 1):
• Calvin cycle occurs in stroma, independent of light:
- carbon dioxide fixation & sugar production
- carbon dioxide is fixed to a pentose bisphosphate, catalyzed by rubisco
enzyme
- very unstable hexose bisphosphate that is formed immediately splits into two
3-carbon phosphoglyceric acids (PGAs)
- two PGAs are phosphorylated (cost of 2 ATPs) & reduced (oxidation of 2
reduced NADPs) to yield two PGAL (glyceradehyde – 3 – phosphate) molecules
- two more turns of cycle generate a total of six PGALs (five to regenerate
the 3 pentose acceptors {cost of 3 ATPs} & one stored as 3-C sugar)
- three carbon dioxides generate one 3-C sugar (cost of 9 ATPs + 6 reduced
NADPs)
- six carbon dioxides must be fixed to generate one glucose (cost of 18
ATPs & 12 reduced NADPs)
Biology 102, lec 18 - Spring ‘13
page 2
5.
Review coupling of light reactions and Calvin cycle (fig. 10 - 21 & ppt. 5):
• linear electron flow involved two photosystems, each absorbing photons
- P680 donated electrons to photophosphorylation electron transport
(chemiosmotic mechanism, confirmed by experimentally immersing acidified chloroplasts in
neutral pH: ATP synthesis occurred) (p. 205 #9 & ppt. 2), producing ATP
- P680 regained electrons from splitting of water, which produced oxygen
- P700 donated electrons to short electron transport chain that reduced
NADP (stored high energy electrons)
- P700 regained electrons from P680 driven electron transport
• cyclic electron flow involved only P700 photosystem that donated electrons
to ATP-synthesizing electron transport chain and regained electrons that had passed
through the photophosphorylation chain (only ATP produced)
- Calvin cycle used ATP and reduced NADP from light reactions to fix carbon
dioxides; CO2 combined with 5-C carrier to eventually yield two
phosphoglyceraldehydes (PGals), following activation (ATP used) & reduction steps
(reduced NADP used); 3 cycles of CO2 fixation yield six PGals, one stored as 3-C
sugar & 5 used to regenerate three 5-C carriers; six cycles produce glucose
• overall process: approx. 2000 kcal of photon energy; 686 captured in
glucose; approx. 1300 lost to surroundings; coupled reactions -- highly exergonic