Week 9 Johnson Lecture 8 Flashcards
What is the dark reaction?
Calvin cycle
What is the calvin cycle?
The Calvin cycle uses ATP and NADPH to convert CO2 into carbohydrates, regenerating ADP, Pi and NADP+. The light and dark reactions are therefore mutually dependent upon one another.
What are the 3 phases of the calvin cycle?
The Calvin cycle can be broken down into 3 parts:
- carboxylation
- reduction
- regeneration
What happens during the calvin cycle?
For every complete turn of the cycle 3 molecules of CO2 are reduced using 9 molecules of ATP and 6 molecules of NADPH with a net output of one glyceraldehyde-3-phosphate molecule.
Explain the full calvin cycle
Calvin cycle;
- Phase 1- carboxylation
- -CO2 is combined with a 5- carbon sugar, ribulose 1,5- bisphosphate in a reaction catalyzed by the enzyme ribulose 1,5, bisphosphate carboxylase /oxygenase (Rubisco). The reaction forms an unstable 6 carbon intermediate that immediately splits into 2 molecules of 3-phosphoglycerate This reaction is energetically favourable ΔG ̊ = -51.9 kJ mol-1
- Phase 2- Reduction
- -The reduction phase of the Calvin cycle involves two steps;
- –the first sees 3-phosphoglycerate (3- PGA) phosphorylated by the enzyme phosphoglycerate kinase to form 1,3-bisphosphoglycerate using ATP produced by the light reactions
- –The second reaction is carried out by glyceraldehyde-3-phosphate dehydrogenase and uses NADPH produced by the light reactions to reduce 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate (GAP). GAP is the output product of the Calvin cycle- its major fate is in sucrose synthesis in the cytoplasm
- Phase 3- Regeneration
- -For every 3 x 5C sugar ribulose 1,5-bisphosphate (RuBP) and 3 x CO2 produced, 6 molecules of GAP are formed. But 5 out of 6 of these are required to regenerate ribulose 1,5-bisphosphate (5 x 3 = 3 x 5) Regeneration involves a complex series of reactions that form 3 molecules of the 5C sugar ribulose 5-phosphate
- -In the final step ribulose 5- phosphate is phosphorylated by phosphoribulose kinase using ATP produced by the light reactions to regenerate ribulose 1,5-bisphosphate
What is rubisco?
Rubisco is a slow enzyme (3 s-1) with relatively low affinity for CO2 hence very high concentrations are needed to match potential supply of ATP and NADPH- it represents nearly 50% of total leaf protein.
-Rubisco contains 8 large (catalytic) and 8 small (regulatory) subunits.
How are the light and dark reactions linked?
Many of the enzymes of the Calvin cycle are also involved in glycolysis or the pentose- phosphate pathway and Therefore they must be carefully regulated to avoid futile cycling. This regulation is achieved via the activity of the light reactions, which modify the environment of the dark reactions (i.e. the stroma). Δp formation across the thylakoid membrane increases the pH and the [Mg2+] in the stroma (H+ influx into the lumen is balanced by Mg2+ efflux to the stroma).
What is thioredoxin?
Thioredoxin is a regulatory protein that senses redox changes in the stroma. Thioredoxin regulates the activity of several Calvin cycle enzymes, ensuring the activity of the light and dark reactions is closely coordinated
How is Rubisco regulated?
Rubisco is regulated by the light reactions. Mg2+ is essential for the catalytic function of Rubisco. Both Mg2+ and alkaline conditions are needed for carbamate formation which is provided by the light reactions. The active site of Rubisco contains a lysine, which reacts with another molecule of CO2 to form a carbamate anion that is then able to bind Mg2+.
What is the use of GAP sugars formed during photosynthesis?
starting points in plants for multiple metabolic pathways that lead to amino acid, lipid and nucleotide synthesis
How are plants able to survive without light?
Plants contain mitochondria which can oxidize sugar molecules to produce ATP- this allows energy production to sustain plant metabolism at night and in the roots etc
