IB Biology - Stages of Photosynthesis II

In IB Biology curriculum, there are three main stages of the Calvin Cycle:

• The cycle begins with a 5C compound called ribulose bisphosphate (or RuBP)
• RuBP carboxylase (or Rubisco), catalyses the attachment of a CO2 molecule to RuBP
• The resulting 6C compound is unstable and breaks down into 2 3C compounds  --> glycerate 3 phosphate or GP
• 1 cycle involves 3 RuBP combining with 3 CO2 to form 6 GP

• GP is converted to triose phosphate using NADPH and ATP
• Reduction by NADPH transfers hydrogen atoms  to the compound and the hydrolysis of ATP provides energy
• Each GP requires 1 NADPH and 1 ATP so 6 molecules of each are necessary for 1 cycle

There are four highlights you need to know under the IB Biology curriculum:

• 1 of the 6 TP molecules is used to make 1/2 a sugar
• 2 cycles are therefore necessary to make a whole sugar
• The 5 other TPs  are recombined to make RuBP
• The regeneration of RuBP requires energy derived from the hydrolysis of ATP

There are seven steps of this experiment in the IB Biology:

1. Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
2. Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
3. After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
4. Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
5. Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
6. By comparing different periods of light exposure, the order by which carbon compounds are generated was determined
7. Calvin used this information to propose a sequence of events known as the Calvin cycle (light-independent reactions)

End of this topic!

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