1. Dark Reaction
The Calvin Cycle

2. 6CO2 +6H2O C6H12O6 +6O2
The dark reactions use the energy stored in ATP and NADPH the light reactions.
Carbon atoms from CO2 are bonded, or fixed, into organic compounds = carbon fixation.
THIS OCCURS IN THE STROMA

4. STEP 1 An enzyme (rubisco), combines CO2 with a 5-carbon sugar RuBP
The product, 6-C sugar, immediately splits into
2, -3C molecules (PGA)
PGA– Phosphoglyceric Acid

5. Step 2
PGA is converted to another 3- Carbon molecule PGAL in a 2 part process:
Each PGA receives a P group from ATP
The resulting compound receives a proton from NADPH and releases the P, producing PGAL
( ADP & NADP+ return to light rxn., to make ATP and NADPH)

6. Step 3 Most of the PGAL is converted back to RuBP
Requires a P from another ATP
Some PGAL leave and used by plants create organic compounds

7. Balance Sheet for Photosynthesis
How much ATP & NADH are required to make 1 molecule of PGA from carbon dioxide?
Each turn fixes one CO2
PGAL is a 3-C molecule (takes 3 turns to make each molecule)
Each turn of the cycle:
3 ATP ( 2 in step 2 & 1 in step 3)
2 NADPH (step 3)

8. About 50% made to fuel cell Respiration
Some of the PGAL is used to make amino acids, lipids, carbohydrates like glucose and fructose, glycogen, starch, and cellulose.

10. Alternative Pathways
Plants that fix carbon exclusively through the Calvin Cycle- C3 Plants
Because of the 3-C compound PGA, that is initially formed.
Example: Rice, wheat, oats, and soybeans

11. Alternative Pathways
Plants in hot, dry climates use alternative pathways
Plant lose H2O to the air-through small pores called stomata (underside of leaf)
Can be partially closed to prevent water loss
Stomata are the major passageways through which CO2 enters and O2 leaves
(when stomata are closed CO2 levels decrease and O2 levels increase)

12. Transpiration-Evaporation of water from leaves
Rate of transpiration related to the degree of stomata opening and evaporation demand of environment

13. C4 Plants Fix CO4 into 4-C compounds
Partially close stomata during hottest part of the day
Enzymes fix CO2 into 4-C compounds and transport them to cells where CO2 is released and enters calvin cycle ( lose ½ as much H2O as C3)
Examples
Corn, sugar cane, crabgrass

14. CAM Pathways Open stomata at night; close during the day
Take in CO2 at night and fix into compounds
Release O2 during the day and enter the Calvin cycle
Example: cacti, pineapples

16. Rate of Photosynthesis
Light Intensity:
Increase rate of photosynthesis, then levels off ( max. rate of photosynthesis)
Higher intensity, excites more electrons in cholorphyll
@ same intensity, all available electrons are excited

17. Amount CO2
Amount of CO2:
Increases rate of photosynthesis to a point, then levels off

18. Temperature Higher temperature accelerates the chemical rxns.
certain temp. because the enzymes becomes ineffective and unstable
Stomata closes-limiting H2O loss and CO2 entry into the leaves

19. Concentration of O2
Higher O2 will decrease the rate of photosynthesis and increase the rate of photorespiration
Rubisco will bind with oxygen
Will send PGA into respiration, instead of finishing photosynthesis
Decreasing amount of organic compound produced