118 Plant Photorespiration
Learning Outcomes
- Differentiate between C3, C4, and CAM plant approaches to photorespiration
High crop yields are pretty important—for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by 20 percent and the yield of soybeans by 36 percent in the United States, for instance, you might be curious to know what it was[1].
As it turns out, the factor behind those (real-life) numbers is photorespiration. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs O2 rather than CO2. It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.
Some plants, unlike wheat and soybean, can escape the worst effects of photorespiration. The C4 and CAM pathways are two adaptations—beneficial features arising by natural selection—that allow certain species to minimize photorespiration. These pathways work by ensuring that Rubisco always encounters high concentrations of CO2 making it unlikely to bind to O2.
Now, let’s take a closer look at the C3, C4 and CAM pathways and see how they do (or don’t!) reduce photorespiration.
C3 plants
A “normal” plant—one that doesn’t have photosynthetic adaptations to reduce photorespiration—is called a C3 plant. The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this “standard” mechanism of carbon fixation are called C3 plants, for the three-carbon compound (3-PGA) the reaction produces[2]. About 85 percent of the plant species on the planet are C3 plants, including rice, wheat, soybeans and all trees.
C4 plants
In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called bundle-sheath cells.
To see how this division helps, let’s look at an example of C4 photosynthesis in action. First, atmospheric CO2 is fixed in the mesophyll cells to form a simple, 4-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind O2. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of CO2. The CO2 is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C3 photosynthesis.
This process isn’t without its energetic price: ATP must be expended to return the three-carbon “ferry” molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric CO2. However, because the mesophyll cells constantly pump CO2 into neighboring bundle-sheath cells in the form o