DBQ p. 112- Identifying the First Products of Carbon Fixation
- Glycerate-3-phosphate is the first product of CO2 fixation because:
- Glycerate-3-phosphate is the most radioactive molecule in the first 1 minute. It starts off being responsible for more than 70% of the total radioactivity.
- The radioactivity of glycerate-3-phosphate, along with that of triose phosphate and other sugar phosphates, is observable from less than 2 seconds after the introduction of 14C. This means these molecules are being synthesized very rapidly.
- The % radioactivity of glycerate-3-phosphate only decreases over time (especially after 1 minute). It is also the first of the molecules where the % radioactivity decreases.
- Other molecules like malate, aspartate, and alanine only start to synthesize shortly before 1 minute after introducing 14C. This could mean that these molecules are being formed by the glycerate-3-phosphate that was already synthesized in the first minute or less.
- It can be seen that glycerate-3-phosphates is being converted into triose phosphates and other sugar phosphates:
- From around 2-16 seconds, the % radioactivity of glycerate-3-phosphate declines, whereas the % radioactivity of triose phosphates and other sugar phosphates increase.
- A similar trend can be seen shortly before and after 1 minute: As the % radioactivity decreases in glycerate-3-phosphate, it increases in triose phosphates and other sugar phosphates.
- The rate of % radioactivity decrease in glycerate-3-phosphate and the rate of % radioactivity increase in triose phosphates and other sugar phosphates is similar.
- This implies that the radioactive 14C is being transferred from glycerate-3-phosphate to triose phosphates and other sugar phosphates, as the molecules are converted.
- The graph shows how the % radioactivity of glycerate-3-phosphates can be observed from less than 4 seconds after the introduction of 14C. It can be estimated that CO2 diffuses into cells and is converted with RuBP to glycerate-3-phosphates in about 2 seconds.
14liedan 
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DBQ Identifying the 1st Products of Photosynthesis
1.
• 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
2.
• As concentration of G3P falls with time
• concentration of TP rises.
• G3P is a raw material for TP production
3.
• 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