The Calvin cycle (continued)
Phase 1: Carbon fixation
The cycle begins when carbon dioxide is fixed to the 5-carbon sugar RuBP. This step is catalyzed by RuBP carboxylase (rubisco), possibly the most abundant protein molecule on the planet. The resulting 6-carbon intermediate is so unstable that it immediately breaks into two molecules of the 3-carbon acid, 3-phosphoglycerate.
Phase II: Reduction
ATP transfers its phosphate group to the PGA molecule generating the
1, 3 diphosphoglycerate molecule. The DPGA molecule is unstable and ready to receive high energy electrons from NADPH. Glyceraldehyde 3-phosphate is the result of this reduction. The NADP+ and the ADP return to the thylakoid to continue the light reactions.
Phase III: Regeneration of RuBP
Each turn of the cycle does not represent the net gain of a carbohydrate for the chloroplast. Five of every six molecules of G3P must remain in the cycle to regenerate the starting material, RuBP. A complex series of reactions convert the five 3-carbon sugars to three molecules of RuBP. An ATP must be consumed for each RuBP regenerated.
Once RuBP is available, the cycle continues.
Three fates of G3P
1. Regeneration of RuBP
2. Incorporation into glucose and/or other complex sugars for export from the chloroplast (sucrose is the most common sugar destined for transport).
2 G3P go to 1 glucose
3. Incorporation into glucose and subsequent storage in starch molecules to be held in the chloroplast.
CANEHOP (in class)
Cyclic phosphorylation
overview
overview
Although ATP and NADPH are generated in approximately equal amounts, ATP is consumed at a higher rate than is NADPH (3ATP:2NADPH). To correct this imbalance, the chloroplast is able to redirect the electrons flowing from P700. Rather than leaving the primary acceptor and then going to the NADP+ reductase, the electrons travel to the plastiquinone, through the ETC, and return to the reaction centre of P700. A proton-motive force is generated and ATP is synthesized.
What's important? No NADPH is synthesized in cyclic electron flow. ATP is generated to keep up with the demand by the Calvin cycle. No water is split and no oxygen is released as a by-product. Electrons are recycled. Only P700 is involved in this pathway.
Summary (in class)
Photorespiration
Photorespiration refers to a light-dependent pathway that consumes oxygen, releases carbon dioxide, yet produces no ATP. This pathway drains the Calvin cycle of carbon material and can seriously affect plant growth potential and crop yields.