Calvin Cycle Flashcards
What are photoautotrophs?
- they can organisms that can generate food from CO2 and H2O using energy from light
What is the Calvins Cycle?
- set of chemical reactions that take place in chloroplasts during photosynthesis
- light independent reactions as it uses NADPH and ATP create in light reactions
- reduction of CO2 to carbohydrates
What are the 3 distinct stages of the Calvin Cycle?
Stage 1: CO2 fixation
- carboxylation of ribulose-1,5-biphosphate to produce 3-phosphoglycerate
Stage 2: Reduction
- reduction of fixed carbon to commence synthesis of hexoses
Stage 3: Regeneration of starting molecule ribulose-1,5-biphosphate
What happens during Stage 1 of the Calvin Cycle?
- carboxylation of ribulose-1,5-bisphosphate by Rubisco
- starts off with ribulose-1,5-bisphosphate and creates 2 intermediates (enediol and β-keto acid) and ends up with 2 moles of 3-phosphoglyceric Acid
What is the structure of RuBisCO?
- 8 large and 8 small subunits
- equilibrium is to the right of the reaction
- requires Mg and carbamate (needs CO2 for carbamate) to form a complex to bind to ribulose-1,5-bisphosphate
How does the first intermediate form in Stage 1 of the Calvin Cycle?
- The carbamate formed in RuBisCo binds to Mg2+
- the ribulose-1,5-bisphosphate the binds to Mg2+ and deprotonates, forming the first intermediate, the enediol
How does the second intermediate form in Stage 1 of the Calvin Cycle?
- the enediol species then reacts with another CO2 molecule to form a carbon-carbon covalent bond
- forming the β-keto acid intermediate
How does the product form in Stage 1 of the Calvin Cycle?
- addition of water from the β-keto acid intermediate
What happens during Stage 2 of the Calvin Cycle?
2 reaction stage:
- phosphorylation of 3-phosphoglycerate to 1,3-bisphosphoglycerate
- uses 2/3 of ATP consumed in calvin cycle - reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate
What enzyme catalyses the phosphorylation of 3-phosphoglycerate in Stage 2 of the Calvin cycle?
- 3-phosphoglyceric acid kinase
What enzyme catalyses the reduction of 1,3-bisphosphoglycerate in Stage 2 of the Calvin cycle?
- glyceraldehyde-3-phosphate dehydrogenase
What happens during Stage 3 of the Calvin Cycle?
regeneration of ribulose-1,5-bisphosphate to keep the Calvin Cycle operating
- complex collection of reactions including transketolase and aldolase
- using products of the PPP such as xyulose-5-phosphate and sedoheptulose-7-phosphate
How many Carbon dioxide molecules and ATP molecules are needed to form 1G3P?
3 CO2 & 9 ATP
Of every 6 G3P produced, how many is used in non-Calvin cycle metabolism and how many are converted in regeneration steps back to ribulose-1,5-bisphosphate?
One G3P (3 moles of CO2) is used in non-Calvin cycle metabolism - carbohydrate synthesis 5 G3P (15 moles of CO2) are converted in regeneration steps back into the cycle
What are the different ways to regulate the Calvin Cycle?
- rate-limiting step is stage 1 carboxylation of ribulose-1,5-bisphosphate involving rubisco
- regulation by light, whereby stromal Mg2+ conc could increase and also pH
- ATP and NADPH both produced in light reactions are essential for Calvin cycle to operate