Dark ReactionDark Reaction“Not for the Faint of Heart”

Here it is!Here it is!

Dark Reaction of Photosynthesis

- Carbon Cycle - Calvin Cycle - Calvin/Benson Cycle - Stephen King Cycle

- Consists of a cycle of chemical reactions that "fix" CO2 into a fragment of glucose (1/6 C6H12O6 = CH2O)

- The cycle must turn 6 times to make a molecule of glucose.

- Each turn of the Dark Reaction is powered by 3 ATP and requires 2 NADPH2, which both come from the Light Reaction.

- The Dark Reaction takes place in the stroma and can happen in the light OR the dark, as long as there is a supply of ATP and NADPH2

Here are the events of one cycle:

1. CO2 joins with a 5-C molecule called Ribulose Diphosphate [RuDP] to produce an unstable 6-C molecule.

2. This 6-C molecule quickly splits into 2 3-C molecules called Phosphoglyceric Acid [PGA]

Dark Reaction cont.Dark Reaction cont.

3. These 2 PGA molecules each pick up two Hydrogen atoms ( from NADPH2) and each pick up a phosphate from the breakdown of 2 of the 3 ATP molecules that come from the Light Reaction.

4. The 2 PGA molecules are now called Phosphoglyceraldehyde [PGAL]

5. The PGAL molecules then temporarily join together to form a a 6-C molecule with 4 phosphates attached.

6. From the 6-C molecule breaks away a fragment of glucose [CH2O] and 3 of the 4 attached phosphates leaving a 5-C molecule with only 1 phosphate attached. This molecule is called Ribulose Phosphate [RuP] or sometimes called [RUMP!]

7. RuP picks up another phosphate when the last ATP is broken down and changes from RuP to RuDP, and now the cycle is ready to take place again with the next CO2.