1. The Calvin Cycle & Alternative Carbon fixers
Chapter 10.3 and 10.4
The Calvin Cycle & Alternative Carbon fixers

2. The Calvin Cycle Characteristics
Starting material is regenerated after molecules enter and leave the cycle.
The Calvin Cycle is anabolic, building sugar from smaller molecules and
consuming energy.
Carbon enters the cycle in the form of CO2 and leaves as sugar.
The cycle spends ATP and consumes NADPH to make sugar.
For the synthesis of a single sugar molecule the cycle must take place 3 times.
Calvin cycle takes place in the stroma of the chloroplast.
The fixation of carbon to a sugar occurs in 3 phases the produce a 3 carbon sugar glyceraldhyde -3-phosphate (G3P)

3. Calvin Cycle

4. Phase 1: Carbon Fixation
Each of the CO2 molecules that enters the cycle is attached to a sugar Ribulose Bisphosphate or abbreviated RuBP.
The enzyme that catalyzes this first step is named rubisco.
The Product of the reaction is a six-carbon intermediate that is unstable and immediately splits into half forming two 3-phosphoglycerate molecules.

5. Phase 2: Reduction
Each molecule then receives an additional phosphate becoming 1, 3-bisphosphoglycerate (consumption of 6ATP)
Then a pair of electrons from NADPH reduces it to G3P- a sugar.
One molecule of Glyceraldehyde-3-Phosphate exits the cycle and the other 5 are recycled to regenerate RuBP.

6. Phase 3: Regeneration of RuBP
5 molecules of G3P are rearranged into 3 molecules of RuBP.
To accomplish this the cycle uses 3 ATP.
For 1 molecule of G3P the cycle uses 9 ATP and 6 NADPH.
ATP and NADPH are regenerated during the light reactions in the thylakoids

7. Steps of Calvin Cycle

8. Chapter 10.4 C3, C4, and CAM
Since moving to land 475 million years ago plants have been adapting to prevent the problem of dehydration.
Stomata are main avenues of both gas exchange and transpiration.
Hot dry days cause stomata to close, which will reduces the photosynthetic yield

9. Photorespiration - Ex: rice, wheat, and soybeans
C3 Plants are plants that fix CO2 through rubisco to form 3 phosphogylcerate
- Ex: rice, wheat, and soybeans
Photorespiration-when CO2 becomes scarce, rubisco adds O2 instead of CO2.
-peroxisomes (enzymes) and mitochondria rearrange and split the product to form CO2
- process consumes ATP and no sugar is produced.
- unknown if process is beneficial
- drains as much as 50% of carbon fixed by Calvin cycle

10. C4 Plants
C4 Plants named for the 4-carbon compounds that form from alternative carbon fixation
Ex: sugarcane, corn, some grasses
Two distinct layers of leaf: Bundle-sheath cells and mesophyll cells.
The mesophyll cells are loosely wrapped around Bundle-sheath cell.
Bundle-sheath cells are arranged tightly around the veins of the leaf.
- creates spatial separation that will cause carbon fixation and Calvin cycle to occur in different layers

11. Carbon fixation
Carbon fixation of C4 plants utilizes different molecules
- phosphoenolpyruvate (PEP) takes place of RuBP
- PEP carboxylase replaces rubisco in initiating reaction and adding CO2 to PEP.
* allows CO2 to stay concentrated in the leaf even during hot and dry conditions
*C4 photosynthesis will reduce photorespiration and enhance sugar production

12. C4 Plants

13. CAM Plants Ex: jade, cacti, and pineapples
These plants close their stomata during the day to conserve water and open at night to take up CO2
While the stomata is open these plants take CO2 and incorporate into organic acids.
The mesophyll cells store the acids until morning and incorporate them into sugars.
Similar to C4 plants with CO2 changing into organic intermediates
Different in that the same cell handles the 2 steps but at different times of the day.
Ex: jade, cacti, and pineapples