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

RuBP (5C) CO2 6-C intermediate PGA (3C) PGA (3C)

  Start with ribulose bisphosphate (5-carbon) and add CO2 Two phosphoglycerate (PGA) molecules form  First stable compound formed from CO2 fixation

Calvin Cycle (C3 Cycle)

Dark Reactions

PGA (3C) PGAL (3C) PGA (3C) PGAL (3C) glucose (6C)

  PGA becomes PGAL (phosphoglyceraldehyde) by using energy from ATP & NADPH Two PGALs join to form glucose  Can be polymerized into starch for storage

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.

 To prevent this process, two specialized biochemical additions have been evolved in the plant world:  C4  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)

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

Leaf temperature (°C)

C3 C4 Increasing rate of photosynthesis

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)

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

Increasing rate of photosynthesis Humidity C3 C4 CAM

Sun and Shade Plants

Increasing rate of photosynthesis a b Sun plants Shade plants Increasing light intensity