question 70 of DAT Destroyer Biology section

[Meas]

In question 70 of DAT Destroyer Biology section, it says in the answer key that 6 turns of Calvin cycle are needed to produce a glucose molecule. I’ve been reviewing the Calvin Cycle in the Barron’s AP Biology Book, it shows that one Calvin Cycle involves 3 CO2 & 3 RuBP to start with, and ending with 1 PGAL or G3P that becomes glucose & 5 G3P that continues in the cycle to produce 3 RuBP. Based on this information, doesn’t it mean that two cycles of Calvin Cycle is sufficient to produce glucose since G3P is a 3 carbon sugar but glucose is a 5 carbon sugar?

Any help will be highly appreciated! Thanks!

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[nze82]

In question 70 of DAT Destroyer Biology section, it says in the answer key that 6 turns of Calvin cycle are needed to produce a glucose molecule. I've been reviewing the Calvin Cycle in the Barron's AP Biology Book, it shows that one Calvin Cycle involves 3 CO2 & 3 RuBP to start with, and ending with 1 PGAL or G3P that becomes glucose & 5 G3P that continues in the cycle to produce 3 RuBP. Based on this information, doesn't it mean that two cycles of Calvin Cycle is sufficient to produce glucose since G3P is a 3 carbon sugar but glucose is a 5 carbon sugar?

Any help will be highly appreciated! Thanks!

Each turn of the Calvin cycle siphons off 1 out of the 6 carbon atoms of PGAL toward production of glucose. Since glucose is a 6C sugar, then 6 turns of the cycle are needed to produce one molecule of glucose.
Also, remember that PGAL is a 6C compound. So, 6 turn of the cycle produces 6x6 = 36C. As I mentioned above 6 of these carbon atoms will be devoted to production of glucose. So, we're left with 36 - 6 = 30C atoms, which are used to regenerate RuBP at the end of the cycle. Since RuBP is a 5C compound, 30C/5 = 6 RuBP molecules will be generated for every 6 turns of the cycle.

 

[Shinpe]

You're either getting the information wrong from Barron's AP bio or the book is wrong. Each cycle, one CO2 enters, so you can remember from this that it takes 6 cycles (since you need 6C for glucose) to make a glucose molecule

Also, remember that PGAL is a 6C compound.

PGAL is a 3C compound. After CO2 joins RuBP, a transient 6C compound is formed which splits into essentially two PGAL which continue the cycle.

 

[Meas]

Thanks for your replies! Let me see if I am understanding this correctly:

-RuBP + CO2 --> one 3-PGA
-With 2 ATP from light rxn, becomes two 1,3-biphosphoglycerate
-With 2 NADPH from light rxn, becomes two G3P
-One G3P goes on and helps to form glucose.
(question: G3P is a 3C compound & glucose is a 6C compound, is it not correct to think that all 3C in G3P will form 3 out 6 C of the glucose compound? If I think this way, I predict 2 cycles of Calvin Cycle is needed only to form glucose)
-The other G3P continues on with the Calvin cycle, using ATP to reform RuBP
(question: Is 1ATP sufficient to reform this RuBP? The Barron's book showed that 3 ATP are needed!)

Thanks!! Looking forward to all your replies

 

[Shinpe]

Comments below ;P

Thanks for your replies! Let me see if I am understanding this correctly:

Remember the number of carbons in each compound.
-RuBP (5C) + CO2 (1C)--> one (TWO) 3-PGA (3C) (So you will get TWO and not one 3-C compound)
-With 2 ATP from light rxn, becomes two 1,3-biphosphoglycerate
-With 2 NADPH from light rxn, becomes two G3P
Each PGAL will need one ATP and one NADPH to become one G3P. so if you have two PGAL, you will need two each of ATP and NADPH, yes.

-One G3P goes on and helps to form glucose.
(question: G3P is a 3C compound & glucose is a 6C compound, is it not correct to think that all 3C in G3P will form 3 out 6 C of the glucose compound? If I think this way, I predict 2 cycles of Calvin Cycle is needed only to form glucose)
-The other G3P continues on with the Calvin cycle, using ATP to reform RuBP
(question: Is 1ATP sufficient to reform this RuBP? The Barron's book showed that 3 ATP are needed!)

You can't just get rid of one 3-C compound (G3P) and leave one in the cycle because you need to go to RuBP (5C), where will you get that other 2C from? You have to continue the cycle, so every 6 cycles, you will have had 12 G3Ps at this point. Out of this 12, 2 can go to make glucose, while the other 10 G3Ps (10*3=30 carbons) will be able to give you back your 6 RuBPs (6*5=30 carbons). Each reaction that makes RuBP needs one ATP, so you need a total of 6 ATPs here.

In general you can say 6 CO2 + 12 ATP (6 for the PGAL step and 6 for the RuBP step) + 6 NADPH -> glucose


Thanks!! Looking forward to all your replies