Calvin cycle

Question 1

What are the products from the light INDEPENDANT reaction that are used in this reaction

Answer 1

Reduced NADP and ATP

Question 2

Why is the light independent reaction not truly independant

Answer 2

It uses products from the light dependant reaction as reactants reaction so without light this cycle will run out

Question 3

Where does the calvin cycle take place

Answer 3

In the stroma of chloroplasts

Question 4

Fixation of carbon dioxide (First step)

Answer 4

Carbon dioxide combines with the 5-carbon chain called ribulose bisphosphate
(RuBP) in a reaction catalysed by RuBisCo
This results in an unstable intermediate which is quickly split into two 3 carbon chains molecules called glycerate 3-phosphate (GP)

Question 5

Second step (Reduction of GP)

Answer 5

Both molecules of GP are reduced using energy from 2 ATP and 2 reduced NADP as the reducing agent. They are reduced into 2x 3 Triose phosphate

Question 6

Third step (Regeneration of RuBP)

Answer 6

One carbon atom out of both of the two Triose Phosphate molecules is used for making useful organic substances (Glucose, Sucrose, Cellulose, Starch, glycerol etc)
The other 5 carbon atoms are converted back into RuBP using energy from ATP

Question 7

Exam tip

Answer 7

while the specification uses abbreviations for GP and RuBP, it does not use an abbreviation for triose phosphate, which is often abbreviated in textbooks and notes to TP; be sure to always write triose phosphate in full the first time you use the term in an exam 🌞

Question 8

How many turns are needed to make a hexose sugar?

Answer 8

1 turn = 2 Triose Phosphate molecules
3 Turns = 6 triose phosphate molecules
However only 1 carbon atom is used for useful organic substances each time and hexose has six carbons therefore:
The calvin cycle must undergo 6 turns to make 6 carbons useful for organic substances
In these 6 turns it requires 18 ATP molecules and 12 reduced NADP