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

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

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)

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)

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

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.

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

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

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

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)

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

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)

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

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)

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

Increasing rate of photosynthesis CAM Humidity

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