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MSF2 - Photosynthesis > Carbon Reactions > Flashcards

Carbon Reactions Flashcards

1
Q

Under what circumstances can the carbon reactions occur without light?

A

If there is an electron source available to produce NADPH.

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2
Q

What is the Calvin cycle?

A

The Calvin cycle is the metabolic pathway that incorporates carbon dioxide into carbohydrates - carbon fixation.

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3
Q

Where do the reactions of the Calvin cycle take place and why is this possible?

A

All the enzymes in the Calvin cycle are soluble and therefore the reactions take place in the stromal matrix of the chloroplasts.

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4
Q

How much carbon dioxide in the atmosphere is converted to carbohydrates by photosynthetic organisms per year?

A

> 10%

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5
Q

How does fixed carbon return to the atmosphere?

A

As carbon dioxide produced by microbial, plant and animal metabolism and by the combustion of biomass and fossil fuels.

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6
Q

Describe the experiments involved in the discovery of the Calvin cycle.

A

Isotopic labelling of sodium bicarbonate with 14C. Followed the conversion of bicarbonate to carbon dioxide in cells and the following use of carbon dioxide. Placed chlorella algae onto thin glass plates and shone pulses of light onto the plate. Samples killed in hot methanol after each pulse. Application of samples to paper chromatography. This separates the products in the sample and the radiolabel could be followed using X-Ray film. Identification of eluted spots by mass spectrometry.

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7
Q

What were the results of the pulse-chase experiments?

A

After 5s, the radiolabel was mostly in phosphoglycerate - this implied there was a precursor (later discovered to be RuBP) that acted as a carbon acceptor.

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8
Q

What did the breakdown of phosphoglycerate following radiolabelled pulse chase experiments show?

A

After 5s, most of the radiolabel was on the COOH group in phosphoglycerate. After 30s, here was radiolabelling of the other two carbons in phosphoglycerate. This suggested that the precursor (carbon acceptor) was being regenerated as part of a cycle of reactions.

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9
Q

What is the carbon acceptor in the Calvin cycle?

A

Ribulose-1,5-bisphosphate (RuBP) which is a pentode sugar.

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10
Q

What are the 3 main stages of the Calvin cycle?

A
  1. Fixation
  2. Reduction
  3. Regeneration
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11
Q

Describe the fixation stage of the Calvin cycle.

A

RuBP carbon acceptor is carboxylated to give two molecules of phosphoglycerate.

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12
Q

Under what circumstances can the carbon reactions occur without light?

A

If there is an electron source available to produce NADPH.

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13
Q

What is the Calvin cycle?

A

The Calvin cycle is the metabolic pathway that incorporates carbon dioxide into carbohydrates - carbon fixation.

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14
Q

Where do the reactions of the Calvin cycle take place and why is this possible?

A

All the enzymes in the Calvin cycle are soluble and therefore the reactions take place in the stromal matrix of the chloroplasts.

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15
Q

How much carbon dioxide in the atmosphere is converted to carbohydrates by photosynthetic organisms per year?

A

> 10%

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16
Q

How does fixed carbon return to the atmosphere?

A

As carbon dioxide produced by microbial, plant and animal metabolism and by the combustion of biomass and fossil fuels.

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17
Q

Describe the experiments involved in the discovery of the Calvin cycle.

A

Isotopic labelling of sodium bicarbonate with 14C. Followed the conversion of bicarbonate to carbon dioxide in cells and the following use of carbon dioxide. Placed chlorella algae onto thin glass plates and shone pulses of light onto the plate. Samples killed in hot methanol after each pulse. Application of samples to paper chromatography. This separates the products in the sample and the radiolabel could be followed using X-Ray film. Identification of eluted spots by mass spectrometry.

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18
Q

What were the results of the pulse-chase experiments?

A

After 5s, the radiolabel was mostly in phosphoglycerate - this implied there was a precursor (later discovered to be RuBP) that acted as a carbon acceptor.

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1
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19
Q

What did the breakdown of phosphoglycerate following radiolabelled pulse chase experiments show?

A

After 5s, most of the radiolabel was on the COOH group in phosphoglycerate. After 30s, here was radiolabelling of the other two carbons in phosphoglycerate. This suggested that the precursor (carbon acceptor) was being regenerated as part of a cycle of reactions.

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20
Q

What is the carbon acceptor in the Calvin cycle?

A

Ribulose-1,5-bisphosphate (RuBP) which is a pentode sugar.

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21
Q

What are the 3 main stages of the Calvin cycle?

A
  1. Fixation
  2. Reduction
  3. Regeneration
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22
Q

Describe the fixation stage of the Calvin cycle.

A

RuBP carbon acceptor is carboxylated to give two molecules of phosphoglycerate.

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23
Q

Describe the reduction stage in the Calvin cycle.

A

Phosphorylation of phosphoglycerate followed by reduction to give two molecules of glyceraldehyde-3-phosphate. This is a sugar that the cells can use in biosynthesis - can exit cycle.

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24
Q

Describe the regeneration stage of the Calvin cycle.

A

The conversion of ribulose-5-phosphate to RuBP - can then bind another molecule of carbon dioxide.

25
Q

Where does the Calvin cycle occur in Cyanobacteria?

A

In the cytoplasm

26
Q

What does the Calvin cycle require?

A

ATP and NADPH (products of the light reactions), Carbon dioxide, RuBP and RuBisCO.

27
Q

How many turns of the Calvin cycle are required to produce one molecule of glucose?

A

6 turns of the Calvin cycle.

28
Q

What enzyme catalyses the carbon fixation stage?

A

RuBisCO

29
Q

Describe the reaction that takes place in the carbon fixation stage.

A

RuBP is carboxylated to give two molecules of phosphoglycerate. This occurs via a 6C intermediate which breaks down immediately.

30
Q

Describe the phosphorylation reaction in the reduction stage.

A

3-phosphoglycerate + ATP -> 1,3-bisphosphoglycerate
Catalysed by phosphoglycerate kinase
First ATP consuming step of the Calvin cycle

31
Q

Describe the reduction reaction in the reduction stage of the Calvin cycle.

A

1,3-bisphosphoglycerate -> Glyceraldehyde-3-P
Reduction uses NADPH
Catalysed by GAPDH

32
Q

Why are there two copies of the GAPDH gene in plants?

A

The NAD version of the enzyme is used during glycolysis. Therefore, one gene copy is needed for photosynthesis and one for glycolysis.

33
Q

What happens to the 6 molecules of GAP generated at the end of the reduction reaction?

A

1 molecule is exported to the cytosol for biosynthesis and the remaining 5 molecules are required for the regeneration of RuBP.

34
Q

What happens during GAP isomerisation immediately after the reduction reaction?

A

GAP is isomerised to give DHAP.

35
Q

What enzyme catalyses the isomerisation of GAP to give DHAP?

A

Triose phosphate isomerase

36
Q

Other than photosynthesis, where else are phosphoglycerate kinase, GAPDH and triose phosphate isomerase used?

A

During glycolysis

37
Q

What are the 5 main steps involved in regeneration of RuBP?

A
  1. Aldolase reactions
  2. Phosphatase reactions
  3. Transketolase reactions
  4. Rearrangment reactions
  5. Final rearrangment of R5P to give RuBP
38
Q

Describe the final regeneration reaction in the Calvin cycle.

A

Phosphorylation of ribulose-5-phosphate to give RuBP. Second ATP consuming step in the cycle. Catalysed by phosphoribulose kinase.

39
Q

Why do most species use the Calvin cycle instead of alternative pathways of carbon fixation?

A

It would hard to evolve another mechanism to regenerate RuBP without losing any carbon during the reactions.

40
Q

Give the overall reaction for the Calvin cycle.

A

6CO2 + 16ATP + 12NADPH + 12H20 -> 6Glc + 18ADP + 18Pi + 6H+ + 12NADP+

41
Q

How is the Calvin cycle autocatalytic?

A

Generates more RuBP than it started with, if no intermediates are removed.

42
Q

What is the benefit of the autocatalytic nature of the Calvin cycle?

A

Intermediates can be removed for biosynthesis without stopping the cycle.

43
Q

What is fructose-6-phosphate used for if it leaves the Calvin cycle?

A

Starch store in chloroplasts.

44
Q

What is GAP used for if it leaves the Calvin cycle?

A

Glucose synthesis in the cytosol.

45
Q

How is GAP transported from the chloroplast to the cytosol?

A

By a phosphate antiporter.

46
Q

How big is Rubisco?

A

540kDa

47
Q

How many subunits are there in Rubisco?

A

8 large dimeric subunits - L subunits

8 smaller subunits - S subunits

48
Q

What is the purpose of the L subunits?

A

Contain the active site and catalyse the reaction.

49
Q

What is the purpose of the S subunits

A

Enhance the catalytic activity of the L subunits

50
Q

Where does the magnesium required by Rubisco come from?

A

Mg2+ ions move out of the thylakoid membranes to counterbalance proton translocation.

51
Q

What reaction can be catalysed by Rubisco instead of carboxylation and what does this involve?

A

Oxygenation - oxygen added to RuBP instead of carbon dioxide. Produces one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate.

52
Q

Why does Rubisco have a poor turnover rate?

A

Some of the reactions will be oxygenation, meaning that the overall efficiency of Rubisco is poor.

53
Q

Why must phosphoglycolate be recovered after an oxygenation reaction?

A

It is toxic

54
Q

How is magnesium coordinated in the active site of Rubisco?

A

via lysine-201 which is carbamated

55
Q

What is the role of the magnesium ion in the active site of Rubisco?

A

Activates bound RuBP so that it can react with the substrate carbon dioxide.

56
Q

Why is it wasteful for there to be an oxygenation reaction?

A

Carbon dioxide is released during the recovery of phosphoglycolate.

57
Q

Why did photorespiration evolve?

A

Possibly to dissipate electrons and to protect from photoxidative damage when there is insufficient carbon dioxide available

58
Q

How is the glycolate produced in oxygenation recovered?

A

By conversion to glycerate, via glycine and serine. Involves reactions taking place in the peroxisome and mitochondrion. Glycerate can then be phosphorylated using ATP in the chloroplast - producing phosphoglycerate which enters the Calvin cycle.

MSF2 - Photosynthesis (6 decks)

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