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
What is photorespiration?
- Rubisco-Mg2+ enzyme catalyses an oxygenase reaction in which O2 is consumed and CO2 is released
- produces phosphoglycolate and 3-phosphoglycerate
- one of the carbons is lost as CO2
How have plants living in dry and tropical regions adapted to reduce water loss?
- C4 plants
- vascular components are surrounded by bundle-sheath cells and by layers of mesophyll cells
- stops water being released but also CO2 being absorbed
- C4 compounds are synthesises from CO2 near the leaf surface in mesophyll cells and then transported towards photosynthesis and Calvin Cycle
- reduces wasteful photorespiration as O2 is in lower conc due to lack of O2 diffusion to bundle-sheath cells
What is the C4 pathway?
- Phosphoenol pyruvic carboxylase is used to carboxylate PEP (using CO2) to form oxaloacetate and phosphate
- phosphoenol pyruvic carboxylase has a greater affinity for CO2 than rubisco
- happens in mesophyll cells - Oxaloacetate is either reduced to malate by malic DH or transaminated to aspartic acid and pyruvate
- happens in mesophyll cells - malate is decarboxylated to pyruvate and CO2
- aspartic acid is deaminated to oxaloacetate which is reduced to malate and then decarboxylated to pyruvate and CO2 - to have a balanced amino nitrogen in plants, the pyruvate formed is transaminated to alanine which is transported to the mesophyll cells
What is the purpose of the C4 pathway
C4 plants minimize photorespiration by separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types.
What is the difference between a C3 and C4 plant?
- C4 plants are seen in more dry/tropical environments
- in C3 plant, Calvin cycle is operating in the mesophyll cell at the surface
- Calvin cycle operates in the bundle sheath cells in the C4 plants, beneath the mesophyll cell
- in C4, CO2 is converted metabolically into other products that are then transported into the bundle sheath cell where the Calvin cycle is operating
What is the Crassulacean acid metabolism (CAM)?
- metabolism that operates in ecosystems where plants need to keep stomata closed in day to prevent water loss
What do CAM plants do?
CAM plants minimize photorespiration and save water by separating steps in time, between night and day.
How does CAM work?
- stomata opens at night
- CO2 is absorbed at night
- CO2 is fixed as malate via oxaloacetate applying C4 pathway
- concentrate CO2 around rubisco to maximise Calvin cycle flux
