Photorespiration, C4 and CAM Plants
I. Carbon dioxide enters a leaf through holes in the leaf’s surface called stomata.
Water is also lost through the stomates during the process of transpiration.
On hot, dry days, stomates close to conserve water, thereby reducing the amount of ]
carbon dioxide that is taken in to the leaf.
II. C3 plants are plants that use rubisco, an enzyme, to add CO2 to ribulose biphosphate (RuBP) to form a 3 carbon organic compound called 3 phosphoglycerate (PGA).
Examples: rice, soybeans, and wheat. These are the plants we studied on the coloring sheet.
During hot, dry days, C3 plants stomates close and and amount of carbon dioxide goes down while the amount of oxygen goes up. This begins a process called photorespiration.
1. Rubisco accepts oxygen and adds it to the Calvin cycle, instead of carbon dioxide. The result is glycolic acid.
2. This leaves the chloroplast and enters the mitochondria or peroxisome where it is broken down into carbon dioxide. This process is not beneficial to plants or animals, because no oxygen is being produced, only consumed.
3. Photorespiration can decrease photosynthesis output by as much as 50%.
III. C4 plants are plants with a unique leaf anatomy. Photosynthesis can occur in the bundle sheath cells (around the veins of the leaf) and in the mesophyll cells (loosely packed cells between the bundle sheath and the leaf surface.
The process begins in the mesophyll cells when…
1. Carbon dioxide is added to phosphoenolpyruvate (PEP) to form a four carbon molecule called oxaloacetate. The enzyme called PEP carboxylase does this. Compared to rubisco, PEP carboxylase has a much higher affinity for carbon dioxide, even on hot, dry days.
2. The mesophyll cells then transport their four carbon organic compound to the bundle sheath cells through plasmodesmata.
3. Once in the bundle sheath cells, the four carbon compounds now release the carbon dioxide, which is reassimilated into organic material by rubisco and the Calvin cycle.
This minimizes photorespiration and therefore increases photosynthetic output. Sugar cane and corn are examples of C4 plants.
IV. CAM plants are plants with water-storing abilities, such as pineapple and cactus. These plants are called succulents. They close their stomates during the day and open them at night to receive carbon dioxide.
1. During the night, the stomates open and carbon dioxide is fixed into the organic form. This is called crassulacean acid metabolism.
2. The mesophyll cells store the organic molecules in their vacuoles until the next morning.
3. When the stomates close in the day time, the light reactions proceed and the carbon dioxide is released from the organic acids so the Calvin cycle can take place.