What is C4 Photosynthesis?

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C4 photosynthesis is a specialized version of photosynthesis found in some plants (often those thriving in hot, dry environments) that improves efficiency by concentrating carbon dioxide (CO2) around the enzyme responsible for fixing it (RuBisCO). This reduces a process called photorespiration, which wastes energy and limits photosynthesis in regular “C3” plants.


Imagine C4 photosynthesis as a turbocharged engine compared to the standard C3 engine in most plants. Here’s how it works:

  • CO2 capture: Unlike C3 plants, which directly fix CO2 into a 3-carbon molecule in the mesophyll cells, C4 plants first capture CO2 into a 4-carbon molecule (malate or aspartate) in these cells. This initial capture involves an enzyme called PEP carboxylase, which has a much higher affinity for CO2 than RuBisCO, even at high temperatures and low CO2 concentrations.
  • CO2 concentration: The 4-carbon molecule then migrates to specialized bundle-sheath cells surrounding the veins in the leaf. These cells have a high concentration of RuBisCO enzymes.
  • CO2 release and fixation: Inside the bundle-sheath cells, the 4-carbon molecule is broken down, releasing CO2 in a concentrated area around RuBisCO. This high concentration minimizes the competition between CO2 and oxygen, significantly reducing photorespiration, a wasteful process where RuBisCO mistakenly binds with oxygen instead of CO2.
  • Calvin cycle: The now-abundant CO2 in the bundle-sheath cells efficiently enters the Calvin cycle (the “dark reactions” of photosynthesis) for conversion into sugar (glucose).

Benefits of C4 photosynthesis

  • Higher efficiency: C4 plants can photosynthesize more efficiently, especially in hot and dry environments where CO2 levels are often low. This translates to faster growth and better water use compared to C3 plants.
  • Adapted to harsh environments: C4 plants are often found in grasslands, deserts, and tropical regions where sunlight is intense and water is scarce. Their adaptations make them particularly successful in these challenging conditions.

Examples of C4 Plants

  • Corn
  • Sugarcane
  • Sorghum
  • Millet
  • Switchgrass

C4 photosynthesis is a fascinating example of how plants have evolved to optimize their energy-generating machinery for specific environments. By concentrating CO2 and minimizing photorespiration, C4 plants excel in conditions where other plants might struggle, making them crucial players in various ecosystems.

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