8.2b Photosynthesis Light-Independent Reactions

12/01/2013 § 1 Comment

Hi, still doing HL stuff. Today’s blog has to do with the reactions involved in the Calvin cycle!

Carbon fixation and carbohydrate synthesis

All organisms carry out their photosynthesis with carbon dioxide. For plants, and organisms that do photosynthesis, the conversion of carbon dioxide into another carbon compound occur in the stroma of the chloroplast, the fluid surrounding the thylakoids (think of a cytoplasm in a cell, that’s what I do). The product of this reaction is glycerate 3-phosphate, a 3-carbon compound. What happens is that carbon dioxide reacts with the five-carbon compound ribulose biphosphate (abbreviated to RuBP) to produce two molecules of glycerate 3-phosphate. As with all reactions, this carbon fixation reaction is done by an enzyme called ribose biphosphate carboxylase, which is clearly a mouthful, so we abbreviate it to rubisco. There is a large amount of rubisco in the stroma of the chloroplast in order to maximize this reaction.

Next, to produce carbohydrates, hydrogen is added to glycerate 3-phosphate via a reduction reaction (reduction is gain of electrons or hydrogen!). In this reaction, ATP and NADPH provide energy and hydrogen atoms (respectively) to the glycerate 3-carboxylase to create two molecules of triose phosphate, another 3-carbon compound.

Regeneration of RuBP

The products of the carbon fixation reactions and the reduction reactions in the first part of the Calvin cycle can be used to create carbohydrates such as starch and hexose phosphates. Eventually, the supply of RuBP (the original molecule) would be used up, therefore it would have to be regenerated through its own product. Five triose phosphates (the products of the first two reactions) react with three ATP molecules (which then changes back to ADP and an independent phosphate molecule) to produce three ribulose biphosphate molecules, which are five-carbon molecules. (What I’m trying to say is that the original fifteen carbon molecules with the five triose phosphate molecules – 5 x 3 carbons – at the beginning of regeneration are conserved at the end of generation, when there are three ribose biphosphate molecules as a product – 3 x 5 carbons.)

This means that three RuBP molecules are used at the beginning of the Calvin cycle to make six triose phosphates. But five of these triose phosphates are needed to regenerate the original three RuBP molecules, so that leaves one triose phosphate to convert into a sugar molecule. Furthermore, this means that the Calvin cycle needs to turn six times to produce one yield of sugar, like glucose.

Essay Questions

  1. Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. 6 marks
  2. Explain the effect of light intensity and temperature on the rate of photosynthesis. 8 marks
  3. Explain how the rate of photosynthesis can be measured. 7 marks
  4. Explain the role of water in photosynthesis. 4 marks
  5. Outline the light-dependent reactions of photosynthesis. 6 marks
  6. Explain photophosphorylation in terms of chemiosmosis. 8 marks
  7. Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast. 8 marks
  8. Outline the light-independent reactions of photosynthesis. 8 marks
  9. Explain why the light-independent reactions of photosynthesis can only continue for a short time in darkness. 6 marks
  10. Explain how the light-independent reactions of photosynthesis rely on light-dependent reactions. 8 marks
  11. Outline the formation of carbohydrate molecules in photosynthesis starting from the absorption of light energy. 6 marks
  12. Compare the structure of a chloroplast and a mitochondrion in relation to function. 8 marks
  13. Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope. 4 marks
  14. Photosynthesis and transpiration occur in leaves. Explain how temperature affects these processes. 8 marks


Page 112, identifying the first products of carbon fixation

1. Explain the evidence from the graph that convinced Calvin that glycerinate-3-phosphate is the first product of carbon dioxide fixation.

At the very beginning of the evidence – therefore the first product of carbon dioxide fixation – the product with the highest radioactivity is glycerate-3-phosphate. Calvin probably deduced this as the first product because the radioactive carbon is the evidence that shows which products reacted with the algae that was added. Glycerate-3-phosphate had the highest radioactivity percentage at the beginning of the experiment.

2. Explain the evidence from the graph for the conversion of glycerinate-3-phosphate to triose phosphate and other sugar phosphates.

The concentration of glycerate-3-phosphate is much higher than triose phosphate (or any other sugar phosphate) at the beginning of the experiment. This concentration diminishes as time passes and the concentration of triose phosphate increases. As more time passes (minutes instead of seconds), other sugars also start to enter the concentration while glycerate-3-phosphate continues to diminish. This is concrete evidence that glycerate-3-phosphate converts to sugars as part of photosynthesis. 

3. Using the data in the graph, estimate how rapidly carbon dioxide can diffuse into cells and be converted with RuBP to glycerinate-3-phosphate.

Carbon dioxide can diffuse into cells and be converted with RuBP to glycerate-3-phosphate in roughly one minute (which is where the concentrations of triose phosphates and glycerate-3-phosphate become equal).

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§ One Response to 8.2b Photosynthesis Light-Independent Reactions

  • Dave Ferguson says:

    Grade 7 A consistent and thorough understanding of the required knowledge and skills, and the ability to apply them almost faultlessly in a wide variety of situations.

    DBQ Identifying the 1st Products of Photosynthesis

    • The highest amount of radioactivity was in the G3P initially, and then fell with time
    • Other molecules being synthesized depend on the radioactive G3P as a raw material

    • As concentration of G3P falls with time
    • concentration of TP rises.
    • G3P is a raw material for TP production

    • Because radioactive carbon appears in G3P in 1-2 seconds after its introduction
    • CO2 must be able to diffuse into cells within 1-2 seconds
    • and Rubisco must be able to fix carbon within 1-2 seconds

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