Dark Reaction: Calvin Cycle - Practice Questions & MCQ

• The products of the light reactions are ATP, NADPH, and O2. 
• Of these O2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars.
• This is the biosynthetic phase of photosynthesis.
• This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO2 and H2O.
• The use of radioactive 14C by Melvin Calvin in algal photosynthesis studies led to the discovery that the first CO2 fixation product was a 3-carbon organic acid. 
• He also contributed to working out the complete biosynthetic pathway; hence it was called the Calvin cycle after him. 
• The first product identified was 3-phosphoglyceric acid or in short PGA.
• It is a 3 carbon compound.

The Primary Acceptor of CO2:

• The acceptor molecule of CO2 is a 5-carbon ketose sugar – ribulose bisphosphate (RuBP). 
• CO2 combines with RuBP to form an unstable 6C compound called enediol.
• Enediol is quickly broken into two 3C compounds called 3-phosphoglyceric acid or in short PGA.
• This is why the Calvin cycle is also called the C3 cycle.
• Experiments conducted over a wide range of plants led to the discovery of another group of plants, where the first stable product of CO2 fixation was again an organic acid, but one which had 4 carbon atoms in it. 
• This acid was identified to be oxaloacetic acid or OAA. 
• These plants are said to perform the C4 cycle.
• However, the Calvin pathway occurs in all photosynthetic plants.
   

• It occurs in the stroma of the chloroplast.
• It involves the action of many enzymes, hence it is an enzymatic reaction.
• It is slower than the photochemical or light reaction.
• In order to make one molecule of glucose, 6 turns of the Calvin cycle are needed:

• Calvin cycle is completed in three steps as follows:

1. Carboxylation
2. Reduction
3. Regeneration

Carboxylation:

• Carboxylation is the fixation of CO2 into a stable organic intermediate. 
• Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilized for the carboxylation of RuBP. 
• This reaction is catalyzed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. 
• Since this enzyme also has an oxygenation activity, it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

Reduction:

• These are a series of reactions that lead to the formation of glucose.
• The steps involve the utilization of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO2 molecule fixed. 
• The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway.
• During this process, reactions opposite to glycolysis occur. Thus, this process is also called glycolytic reversal.

Regeneration:

• Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted.
• The regeneration steps require one ATP for phosphorylation to form RuBP.
• Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required. 
• It is probably to meet this difference in the number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.