1. Light Independent Reactions
SBI4U1

2. CALVIN CYCLE A.K.A. Calvin Benson Cycle, Light Independent Reaction
“Dark Reaction”
Carbon Fixation
Discovered by “Melvn Calvin”

3. These rxns occur in presence or absence of light
Overall BIG Picture:
Calvin cycle converts CO2 from the atmosphere to carbohydrates (glucose)
Requires ATP and NADPH ( from light dependent rx.)
These rxns occur in presence or absence of light
Light independent (previously called dark rxns)

4. Occurs in the stroma

5. Why is the Calvin cycle known as C3 Photosynthesis?
The first compound formed in photosynthesis has 3 carbons! ( Hence plants that undergo this type of photosynthesis are called C3 plants!
This compound is “PGA”
Note*
For the next few slides, please have your diagrams of Calvin cycle infront of you! 

6. STEP 1) CARBON FIXATION

7. 1. Carbon Fixation (C3 photosynthesis)
Chemical bonding of 5-carbon RuBP and
1-carbon CO2
It is very unstable, so it immediately splits into TWO 3-carbon molecules called PGA
Plant that use this method are calle C3 plants
Summarized Equation:
CO2 + RuBP  unstable C6  2PGA

8. STEP 2) REDUCTION

9. 2. Reduction PGA is in a low energy state
Therefore, ATP is used to create 1,3-BPG
Following this, NADPH is oxidized to form G3P
Of the G3P produced:
some will exit cycle to form sugars (e.g. Glucose)
Remaining ( most) will move to the 3rd stage
( Regeneration)

10. STAGE 3) REGENERATION OF RuBP

11. 3. Regenerating RuBP Most G3P molecules go on to make more RuBP
Energy from ATP breaks and reforms the bonds to make the 5 C RuBP
Calvin cycle must be completed 6X to create molecule of glucose.

12. 6CO2 + 18ATP + 12NADPH + water 2G3P + 16Pi + 18ADP + 12NADP+
Net Equation:
6CO2 + 18ATP + 12NADPH + water 2G3P + 16Pi + 18ADP + 12NADP+

13. ACCOUNTING The accounting is complicated
3 turns of Calvin cycle = 1 G3P
3 CO2  1 G3P (3C)
6 turns of Calvin cycle = 1 C6H12O6 (6C)
6 CO2  1 C6H12O6 (6C)
18 ATP + 12 NADPH  1 C6H12O6
any ATP left over from light reactions will be used elsewhere by the cell

14. Animation:
The G3P produced can be used for the synthesis of other molecules for plants
Majority transported out of chloroplasts into cytoplasm
Where it used to produce sucrose, starch, or cellulose

15. Recall: Anatomy of a Leaf…

16. Stomata: opening surface of a leaf that allows exchange of gases (CO2 in and H2O out) Guard Cells: control the opening and closing of stomata. They surround the stomata

17. When the weather is hot/dry, guard cells close stomata
When the weather is hot/dry, guard cells close stomata. It does this as a response to conserve water loss ( transpiration)
As a result…If stomata is closes  CO2decreases in the leaves & O2 increases

18. Adaptations to Photosynthesis

19. What is Photorespiration?
Rubisco…the most abundant enzyme on earth!
Well….both O2 and CO2 are subsrates to Rubsico and can bind to its active site. Therefore they compete for it’s active site.
Photorespiration= O2 reacts with Rubisco, as a result 2 new products are formed: 2-carbon compound (phosphoglyerate) and a 3-carbon phosphoglycerate

20. In summary… In cool/moist conditions: In hot/dry conditions:
Stomata opens CO2 in leaves are sufficient
CO2 binds to Rubisco
Calvin Cycle can begin
In hot/dry conditions:
Stomata closes O2 is higher than CO2
O2 binds to Rubisco
Calvin cycle cannot begin

21. Under normal conditions C3 plants lose 20% of their energy to fix one CO2 molecule
Most plants are C3 plants (e.g. Wheat, rice, spinach, cotton, grasses)
Some plants have evolved mechanisms to reduce photorespiration: C4 and CAM Plants

22. C4 Plants
Inhabit hot, dry, environments
E.g. Corn, sugarcane, millet

23. Structure of C4 Plants ( Pg. 169)
C4 plants use energy to “pump” carbon dioxide into the bundle-sheath cells, where it becomes concentrated.

24. In the outer layer of mesophyll cells, CO2 is fixed by addition of a 3C phosphoenolpyruvate (PEP)
Produces 4C oxaloacetate
Converted to 4C malate
Transported to bundle sheath cell
Converted to pyruvate and CO2
Pyruvate is converted to PEP and CO2 enters the Calvin cycle (taking place in the bundle sheath cell)
Since CO2 is high = Calvin cycle is efficient

26. CAM Plants Water storing plants E.g. Cacti and pineapples
Thrive in hot, arid conditions
CO2 fixation is separated from Calvin cycle by time of day, rather than cell types

27. CAM PLANTS
-The opening and closing of stomata in CAM plants, are opposite from most plants.
- stomata are open at night and closed in the daytime. ---When CO2 is removed from the four-carbon compound malate in the daytime, it cannot leave the cell because the stomata are closed.
- In cool climates, CAM plants are very inefficient b/c they use energy to drive the reactions that store carbon dioxide.

29. Comparing Aerobic Respiration & Photosynthesis ( Similarities/Differences)
Rxns that capture light energy and convert it to organic material are related to aerobic respiration rxns
Products of aerobic respiration are starting substrates for photosynthesis
Products of photosynthesis are starting substrates for aerobic respiration
*Know Table 4.1 ( pg 170) Hint..Hint

30. Learning Expectations...
Relationship b/t light dependent and independent rxns
Purpose of the Calvin cycle
3 stages in the Calvin cycle
Net equation for Calvin cycle
Adaptations to photosynthesis
Difference between C3, C4, and CAM plants