1. Photosystem I
Light strikes P700
Electron released
NADP reduced to NADPH (stored I stroma)
Photosystem II
Light strikes P680
Electron goes to plastiquinone (Pq), carrying H into lumen
Electron goes into ETC then eventually back into Photosystem I

2. Calvin Cycle (C3 Cycle) Dark Reactions
Occurs in the stroma of chloroplasts
Process is also known as carbon fixation
The carbon from CO2 is “fixed”, or attached, together to a larger molecule to make glucose
ATP and NADPH drive these reactions

3. Calvin Cycle (C3 Cycle) Dark Reactions
Start with ribulose bisphosphate (5-carbon) and add CO2
Two phosphoglycerate (PGA) molecules form
First stable compound formed from CO2 fixation
RuBP (5C)
CO2
6-C intermediate
PGA (3C)
PGA (3C)

4. Calvin Cycle (C3 Cycle) Dark Reactions
PGA becomes PGAL (phosphoglyceraldehyde) by using energy from ATP & NADPH
Two PGALs join to form glucose
Can be polymerized into starch for storage
PGA (3C)
PGA (3C)
PGAL (3C)
PGAL (3C)
Note that
glucose (6C)

5. Calvin Cycle (C3 Cycle) Dark Reactions
ADP and NADP that is produced is recycled back into the light reactions

6. Photorespiration
On hot, dry days plants close their stomata in order to prevent excess water loss.
Problem: gas exchange becomes limited
CO2 can’t come in
O2 can’t leave
CO2 becomes scarce, so rubisco binds O2 in place of CO2
The result is the consumption of ATP & release of CO2.

7. Instead of producing two 3-C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolate is produced.
 Remove phosphate group from phosphoglycolate to get glycolic acid  transferred to peroxisome and converted into glycine  transferred to mitochondria and converted into serine  used to make other organic molecules

8. To prevent this process, two specialized biochemical additions have been evolved in the plant world:
CAM

9. Hatch-Slack Pathway (C4 cycle)
C4 plants are common to environments that are hot, sunny, and semiarid
Sugarcane, corn, succulents

11. Hatch-Slack Pathway (C4 cycle)
Has extra steps before going into C3 pathway (Calvin Cycle)
C goes into oxaloacetate (4-carbon) instead of the phosphoglycerate (3-carbon)
The plant opens its stomata in the early morning to let CO2 in. The CO2 diffuses into the mesophyll cells where it is combined with a 3C compound called PEP (phosphoenolpyruvate)

12. Hatch-Slack Pathway (C4 cycle)
Photosynthesize faster because they don’t do photorespiration
Faster crop yield

13. Increasing rate of photosynthesis
C3- photosyntesize better in cooler weather
C4- photosyntesize better in warmer weather
Increasing rate of photosynthesis
If we compare the leaves of C3 and C4 plants under varying temperatures, C3 plants are much more efficient in photosynthesizing in cooler weather than C4 plants. C3 plants decrease their photosynthetic rates at higher temperatures because they perform photorespiration at those temperatures.
Leaf temperature (°C)

14. (atmospheric water vapor)
Increasing rate of photosynthesis
C3 and C4 plants both increase their rate of pho
Humidity
(atmospheric water vapor)

15. Crassulacean Acid Metabolism (CAM)
Occur in very hot, dry environments
Perform C3 pathway
Does carbon fixation at night & refixes during the day using rubisco (RuBP enzyme)
CAM plants use a similar process as C4 plants except they let CO2 inat night, turn it into malic or aspartic acid and then store it in the vacoules of their photosynthetic cells.
When the sun comes out, they are able to close their stomata and breakdown the malate (malic acid) to keep the internal concentration of CO2 high enough to prevent photorespiration. This enables the plants to keep their stomata closed in order to prevent dessication (from drying out)

16. Increasing rate of photosynthesis
C3- photosyntesize better in cooler weather
C4- photosyntesize better in warmer weather
Increasing rate of photosynthesis
CAM
Leaf temperature (°C)

17. Increasing rate of photosynthesis
CAM
Humidity

18. Sun and Shade Plants a b Increasing rate of photosynthesis Sun plants
C3 and C4 plants can be categorized as sun and shade plants. This depends on their photosynthetic efficiency under varying light intensity. “a” represents conditions of low light intensity. Which one can we say photosynthesizes better? Shade plants! As light intensity increases, sun plants do far better than shade plants as indicated by “b”. Up to a certain point, the intensity of light does not have a affect the rate of photosynthesis. This is because chlorophylls can be destroyed at very high intensities of light.
Shade plants
Increasing light intensity