6-2: Calvin Cycle

1. 6-2: Calvin Cycle

2. What comes next? In the second set of reactions of photosynthesis, plants use the energy that was stored in ATP and NADPH during the Light Reactions to produce organic compounds in the form of SUGARS These organic compounds are then consumed by autotrophs + heterotrophs alike for energy The most common way that plants produce organic compounds is called the Calvin Cycle

3. Carbon Fixation • Calvin cycle is a series of enzyme-assisted chemical reactions that produces a 3-C sugar • CO2 “fixed” into organic compounds • Known as Carbon Fixation • 3 CO2 molecules must enter to make 3-C sugar • Occurs within the stroma of chloroplast

4. Steps of Calvin Cycle • CO2 diffuses into the stroma from cytosol. An enzyme combines CO2 + RuBP (5-C molecule). The 6-C molecules immediately splits into 2 3-C molecules called 3-PGA

5. Let’s look @ STEP 1

6. Steps of Calvin Cycle • 3-PGA is converted into another 3-C molecule called G3P in two parts. • First: 3-PGA receives a phosphate group from ATP • Second: receives a proton (H+) from NADPH, which releases the phosphate group • ADP, NADP+, and phosphate can go back to the Light Reactions to make more ATP + NADPH

7. Let’s look @ STEP 2

8. Steps of Calvin Cycle • One of the G3P molecules leaves Calvin Cycle and is used to make organic compounds (a carbohydrate) that is stored for later use • The remaining G3P molecules are converted back to RuBP through addition of phosphate groups from ATP molecules. RuBP then starts the Calvin Cycle over again.

9. Let’s look @ STEP 3

10. Calvin Cycle Misc. • Pathway named for American scientist Melvin Calvin • Calvin Cycle is most common pathway for carbon fixation to take place • C3 plants • Plants that fix carbon exclusively through the Calvin Cycle • C3 = (C-C-C) = 3-C compound initially formed in process • Examples: wheat, barley, potatoes

11. Alternative Pathways • Under hot + dry climates, plants rapidly lose water to the air • Stomata (or stoma) – a small pore usually located on the underside of a leaf • Used in gas exchange

12. Alternative Pathways (cont.) • Plants can partially close their stomata to reduce the water loss • Major passageways through which CO2 enters and O2 leaves a plant • Low CO2 levels and high O2 levels inhibit the Calvin Cycle from working • Plants must find a way to deal with those conditions

13. C4 Pathways • C4 plants • Plants that fix CO2 into a 4-C compound • During hottest part of day, stomata are partially closed • Certain cells of C4 plants have enzymesthat fix CO2 when CO2 levels are down + O2 levels are up • Compounds transported to other cells where they can enter the Calvin Cycle to produce carbohydrates • Examples: corn, sugar cane, crabgrass • C4 plants do not lose as much wateras C3 plants do – Why? • Stomata are partially closed during day

14. CAM Pathway • CAMplants include pineapples + cacti • At night, the stomata open • During the day, the stomata are closed • CO2 that is taken in at night is fixed into a variety of organic compounds • During the day, CO2 is released by the compounds and enters the Calvin Cycle • CAM plants grow slowly • Lose less water than either C3+ C4 plants

15. Summary of Photosynthesis • Two Stages: • Light Reactions • AKA – Light-Dependent Reactions, Light Harvesting Reactions • Calvin Cycle • AKA – Light-Independent Reactions, Dark Reactions • Ongoing cycle - Biochemicalpathways • Simplest Equation for Photosynthesis • CO2 + H2O + light energy (CH2O) + O2 • Glucose is not a direct product of photosynthesis!!!

16. Factors that Affect Photosynthesis • Light Intensity • More light stimulates more electrons in Photosystems of ETC • Light reactions happen more rapidly • Peaks at a point where there are nomoreelectrons to excite

17. Factors that Affect Photosynthesis • Carbon Dioxide • More CO2 allows “fixing” in Calvin Cycle to take place faster but reaches a saturation point as well

18. Rate of Photosynthesis • Temperature • Increases initially but declines at peak • Why? • Because at some higher temperatures, enzymes stop working + become ineffective • Also, the stomata close limiting water loss + CO2 entry

19. Calvin Cycle Video Recap

20. The Photosynthesis Song It’s Catchy!