C3, C4 and CAM plants

NOTE: This post applies for classes that meet on Thursday and Friday as well. C block will not meet on Friday this week because of Parent's Weekend.

Today we discussed alternative routes for plants to conduct the Calvin Cycle when there is an absence of CO2, extreme heat and/or extreme sunlight.

Below is a summary for each type that should help clear up any confusion. This information was taken from the Pima Community College Website.

C3 Photosynthesis : C3 plants.

• Called C3 because the CO2 is first incorporated into a 3-carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..
• Most plants are C3.

C4 Photosynthesis : C4 plants.

• Called C4 because the CO2 is first incorporated into a 4-carbon compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
• Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
• Adaptive Value:
  • Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
  • Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.
• C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism

• Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.
• Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
• Adaptive Value:
  • Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
  • May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).
• CAM plants include many succulents such as cactus and agaves and also some orchids.