Light Independent Reactions

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

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

14. Animation: http://highered.mcgraw-hill.com/sites/0070960526/student_view0/chapter5/animation_quiz_1.html • 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. 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 CO2are subsratesto 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: • 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 CO2is 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 CO2is 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