Membranes > Calvin Cycle > Flashcards

Calvin Cycle Flashcards

(23 cards)

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1
Q

What are photoautotrophs?

A
  • they can organisms that can generate food from CO2 and H2O using energy from light
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2
Q

What is the Calvins Cycle?

A
  • 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
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3
Q

What are the 3 distinct stages of the Calvin Cycle?

A

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

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4
Q

What happens during Stage 1 of the Calvin Cycle?

A
  1. 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
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5
Q

What is the structure of RuBisCO?

A
  • 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
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6
Q

How does the first intermediate form in Stage 1 of the Calvin Cycle?

A
  • 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

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7
Q

How does the second intermediate form in Stage 1 of the Calvin Cycle?

A
  • the enediol species then reacts with another CO2 molecule to form a carbon-carbon covalent bond
  • forming the β-keto acid intermediate
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8
Q

How does the product form in Stage 1 of the Calvin Cycle?

A
  • addition of water from the β-keto acid intermediate
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9
Q

What happens during Stage 2 of the Calvin Cycle?

A

2 reaction stage:

  1. phosphorylation of 3-phosphoglycerate to 1,3-bisphosphoglycerate
    - uses 2/3 of ATP consumed in calvin cycle
  2. reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate
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10
Q

What enzyme catalyses the phosphorylation of 3-phosphoglycerate in Stage 2 of the Calvin cycle?

A
  • 3-phosphoglyceric acid kinase
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11
Q

What enzyme catalyses the reduction of 1,3-bisphosphoglycerate in Stage 2 of the Calvin cycle?

A
  • glyceraldehyde-3-phosphate dehydrogenase
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12
Q

What happens during Stage 3 of the Calvin Cycle?

A

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
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13
Q

How many Carbon dioxide molecules and ATP molecules are needed to form 1G3P?

A

3 CO2 & 9 ATP

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14
Q

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?

A 
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
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15
Q

What are the different ways to regulate the Calvin Cycle?

A
  • 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
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16
Q

What is photorespiration?

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A
  • 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
17
Q

How have plants living in dry and tropical regions adapted to reduce water loss?

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A
  • 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
18
Q

What is the C4 pathway?

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A
  1. 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
  2. Oxaloacetate is either reduced to malate by malic DH or transaminated to aspartic acid and pyruvate
    - happens in mesophyll cells
  3. 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
  4. to have a balanced amino nitrogen in plants, the pyruvate formed is transaminated to alanine which is transported to the mesophyll cells
19
Q

What is the purpose of the C4 pathway

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A

C4 plants minimize photorespiration by separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types.

20
Q

What is the difference between a C3 and C4 plant?

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A
  • 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
21
Q

What is the Crassulacean acid metabolism (CAM)?

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A
  • metabolism that operates in ecosystems where plants need to keep stomata closed in day to prevent water loss
22
Q

What do CAM plants do?

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A

CAM plants minimize photorespiration and save water by separating steps in time, between night and day.

23
Q

How does CAM work?

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A
  • 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
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