The Calvin Cycle

Question 1

Light independent reactions (Overview).

Answer 1

1. In the Chloroplast Stroma (liquid part) molecules of CO2 are joined to make carbohydrates (glucose) using the energy from ATP and the reducing power of NADPH.
2. The chemical bonds that are made contain energy.
3. Light independent reactions are called the Calvin cycle after Melvin Calvin. (Nobel prize for Chemistry in 1961)
   or Calvin-Benson cycle
   or Calvin- Benson-Bassham cycle!

Question 2

What are the first steps of C3 photosynthetic pathway?

Answer 2

• Named C3 because the first product is a 3
carbon compound
• Ribulose 1,5-bisphosphate – RuBP
(5 carbon sugar) + CO2 =
- 2 x 3 phosphoglyceric acid (PGA)
- PGA has 3 carbons ( Mnemonic -Pro Golf Association)
• Enzyme responsible for this is RUBISCO (RuBP carboxylase/oxygenase)
1. Ribulose phosphate (RP) a 5 carbon sugar is phosphorylated (add PO3).
2. ATP is used to phosphorylate RP to form ribulose bis- phosphate (RUBP)
3. The enzyme ribulose bis- phosphate carboxylase oxygenase (RUBISCO) is the most abundant enzyme in nature!
4. It combines CO2 and RUBP to a 6C compound which breaks down to two 3C molecules, phosphoglyceric acid (PGA).
PGA is first detectable compound 14 CO2.
C3 pathway continued
5. PGA is converted to phosphoglyceraldehyde (PGAL) using NADPH and ATP formed by the light reaction.
6. Six turns of the Calvin cycle incorporates 6 CO2 into 12 PGAL and 2 PGAL (3C) leave to make sugars and starch, cellulose etc. 10 PGAL molecules are rearranged to form 6 RP (5C) molecules to start again. Or more precisely imagine thousands of reactions going on at once in the stroma and pooling together. So a lot of the Calvin cycle activity is in regenerating RP to keep the cycle going!

Question 3

PGAL not ATP?

Answer 3

• Why go to the trouble of producing phosphogyceraldehyde (PGAL) and sucrose?
• Why not use ATP as permanent energy stores?
• Not much ATP in cells and is converted back and forth rapidly to provide a quick energy currency within cells
• ADP+P->ATP

Question 4

Currency of photosynthesis

Answer 4

• ATP (adenosine-tri-phosphate)
ADP + Pi + energy = ATP (high energy)
• NADPH (nicotinamide adenine dinucleotide phosphate) NADP + 2e- + H+ = NADPH (high energy)
Making bonds takes and stores energy Breaking bonds releases energy

Question 5

What is Rubisco?

Answer 5

Enzyme name:
4. Recall what RUBISCO stands for and what the C & O mean in the name. The C is cool – fixes carbon!
RUBISCO
Ribulose bisphosphate carboxylase oxygenase
Carboxylase activity – adds carbon from CO2
to RuBP – basis of carbon fixation
In some conditions when plants close stomata
when water stressed will bind oxygen instead
(Oxygenase activity) – wasteful called Photorespiration NB: It is NOT respiration in the light!