C₃ and C₄ Plants

C₃ Plants

• The initial fixation of carbon in the Calvin cycle occurs when rubisco adds CO₂ to RuBP.
  • The product of this process is a three-carbon compound, 3-phosphoglycerate.
  • Accordingly, all plants that do this are called C₃ plants.
• Famous C₃ plants used in agriculture:
  • Rice
  • Wheat
  • Soybeans

Figure 10.16, page 181, Campbell's Biology, 5th Edition

The Problem with Being C₃

• On hot, dry days, the stomata of C₃ plants are closed to prevent water loss.
• Thus, they produce less food on these days.
  • The declining amount of CO₂ starves the Calvin cycle.

Figure 8.1, Purves's Life: The Science of Biology, 7th Edition

C₃ Plants and Photorespiration: A Challenge to Photosynthesis

• Rubisco, the enzyme that is responsible for carbon fixation (combining CO₂ with RuBP), can also add O₂ to RuBP.
  • When the O₂ concentration within a leaf becomes greater than the CO₂ concentration, rubisco will add O₂ to RuBP instead of CO₂.
• The resulting molecule cannot be used in the Calvin cycle.
• In fact, it is toxic to the plant, so it is exported out of the chloroplast and broken down into CO₂ again in the peroxisomes.
  • This process is called photorespiration because it resembles respiration in that it consumes O₂ and produces CO₂, but it occurs during photosynthesis.
  • It is bad, because it decreases the photosynthetic output of the plant by consuming ATP and producing CO₂ instead of glucose.

Solutions to the Problem C₃ Plants Experience in Hot and Dry Environments

Two evolutionary solutions have emerged:

• C₄ Plants
• CAM Plants

C₄ Plants

• C₄ photosynthesis is an add-on feature to C₃ photosynthesis.
  • It is named C₄ because in this process, the first byproduct of fixing CO₂ is a four-carbon molecule instead of a three-carbon molecule.
• Thousands of species: sugercane and corn are the most famous.
• Their key feature: the unique anatomy of their leaves that allows this process to occur
• Have two types of photosynthetic cells:
  • Mesophyll cells
    • Loosely arranged around the bundle-sheath cells
    • Pre-Calvin cycle reaction occurs in these cells.
    • CO₂ is fixed into a four-carbon molecule.
    • The four-carbon molecule is shuttled to the…
  • Bundle-sheath cells
    • Tightly packed around the veins in the leaf
    • The Calvin cycle can only occur in these cells.

Figure 10.17, page 183, Campbell's Biology, 5th Edition

C₄ Photosynthesis

The Details

• Mesophyll cells
  • Carbon fixation: CO₂ is added to phosphoenolpyruvate (PEP), a 3-carbon molecule, by the enzyme PEP carboxylase.
  • The first product of this reaction is oxaloacetic acid (OAA), which is a four-carbon molecule—hence the name “C₄ photosynthesis.”
  • OAA is converted to malate (four-carbon), which is shuttled to the bundle-sheath cells.
• Bundle-sheath cells
  • Malate is converted to pyruvate and CO₂.
  • The CO₂ enters the Calvin cycle, as we have already studied.
  • Pyruvate is phosphorylated with an ATP molecule and then shuttled back into the mesophyll cells to continue the cycle.

The Purpose

• Problem:
  • In order for photosynthesis to occur, the stomata must remain open so that CO₂ can enter.
  • However, with the stomata open, water escapes from the plant.
• Solution:
  • To increase the efficiency of photosynthesis in plants exposed to hot and dry conditions, C₄ photosynthesis evolved.
  • C₄ plants can keep their stomata closed, avoid photorespiration, and thus increase photosynthetic efficiency.
• Process:
  • CO₂, in the form of malic acid, is shuttled to the bundle-sheath cells and then converted back to CO₂.
  • The bundle-sheath cells are surrounded by mesophyll cells, so very little O₂ in the intercellular space reaches the bundle-sheath cells.
  • Rubisco can now fix this CO₂ without competition from O₂ and thus less chance for photorespiration.
  • PEP carboxylase and rubisco both fix CO₂, but PEP carboxylase has a higher affinity for CO₂.

CAM Plants

• CAM stands for crassulacean acid metabolism.
  • A different mode of carbon fixation
  • A second way for plants to deal with hot and arid conditions
• Named after the plant family in which it was first discovered (Crassulaceae), which are succulents (water-storing plants):
  • Ice plants
  • Cacti
  • Pineapples
• Strategy: open stomata at night and close stomata during the day.
  • Benefit: reduced water loss during the day, since the stomata are closed.
  • Problems: can't get CO₂ for the Calvin cycle, since the stomata are closed!

CAM Photosynthesis

• CO₂ is fixed into a four-carbon molecule, oxaloacetic acid (OAA), by the enzyme PEP carboxylase, which then converts OAA to malic acid.
  • Very similar to C₄ photosynthesis
• During the night, the malic acid accumulates and is stored in the vacuoles.
• During the day, the malic acid is shuttled from the vacuole to the chloroplast.
  • Here, CO₂ is created from the malic acid and then enters the Calvin cycle.
• Unlike C₄ photosynthesis, this process occurs in only one cell: the mesophyll cell.

C₄ versus CAM

Figure 10.18, page 184, Campbell's Biology, 5th Edition; unknown source