Photosynthesis and Photorespiration in C3 and C4 Plants


Benjamin R. Carron

CU, Boulder, Fall 2005


The purpose of our experiment was to determine the different CO2 concentrations when photosynthesis levels off in C3 and C4 plants.  This reflects how effectively and efficiently different plants can utilize CO2 present in the atmosphere.  The difference in the way these two types of plants conduct photosynthesis lies in their methods of CO2 fixation.  At the beginning of the Calvin cycle in both types of plants, CO2 is fixed by the enzyme rubisco.  The problem with rubisco is that it has an affinity for molecular oxygen (O2) as well as CO2.  In C4 plants, an intermediate pathway filters out O2 from coming into contact with rubisco.  In C3 plants, CO2 is pumped directly into the Calvin cycle, leaving rubisco exposed to O2.  Rubisco in a C3 will take up O2 rather than CO2 to undergo photorespiration.  Photorespiration occurs under low concentrations of CO2 and high concentrations of O2.  Neither ATP nor NADH2 are products of photorespiration, so it is a somewhat wasteful process.  Because C3 plants undergo photorespiration, they are much less efficient in photosynthesis.  If photorespiration occurs in C3 plants but not C4 plants, then photosynthesis will level off at a lower concentration of CO2 in C4 plants and a higher concentration of CO2 in C3 plants.

            To test this hypothesis, samples of both C3 and C4 plants were obtained.  The C3 plant was juniper needles and the C4 plant was palm grass.  One plant was placed in a chamber, the top of which was sealed with a CO2 gas sensor.  This gas sensor was connected to a computer that would graphically monitor the [CO2] within the chamber.  On either side of the chamber was a clear container filled with water to avoid significant temperature changes from the lights that were faced toward the container.  The lights provided the plants with photosynthetic light energy.  The [CO2] was then monitored until it leveled off at a specific [CO2] value.  Since C3 plants undergo photorespiration, we predicted that the [CO2] would be greater for junipers when photosynthesis leveled off.  The [CO2] at which photosynthesis leveled off would be smaller for palm grass because it does not perform photorespiration. 

            Our data shows that photosynthesis levels off at a greater [CO2] for C3 plants compared to a lower [CO2] for C4 plants.  The mean [CO2] when photosynthesis leveled off for the juniper needles was 348ppm.  The mean [CO2] when photosynthesis leveled off for the palm grass was 206.67ppm. A t-Test was performed to find that P=0.0286.  Since the P value is <0.05, the data is significantly different.  This means that our results are valid and show accurate trends.

            Our results are consistent with our hypothesis.  The experimental data supports our prediction that the C4 plant would have a lower [CO2] than the [CO2] present around the C3 plant when photosynthesis leveled off.  Despite our results being consistent with our hypothesis, there were several potential experimental problems.  When one plant was taken out of the chamber and replaced with another type of plant, the time interval during which the chamber remained unsealed was never constant.  This may have had an effect on the initial amounts of CO2 and O2 present within the chamber.  Also, for the third trial, new samples of each plant were used and tested in a new chamber with a new CO2 sensor and computer.  This definitely affected the results because this CO2 sensor was calibrated differently.  The results of this trial were consistent with our hypothesis, but they were not consistent with the results of the first two trials.  The final [CO2] for both plant types in trial three were much lower than the final [CO2] in the first two trials.

If the experiment was repeated, it would be essential to use the same supplies (plants, chamber, CO2 sensor, computer, etc.) for each trial.  Also, there would be a constant time interval during which the chamber would be unsealed between each trial. 

The results from Moritz. 2004 on the CABLE website showed that C3 photosynthesis levels off at a higher [CO2] than photosynthesis in C4 plants.  Also, Cohen et al. 2001 showed a hypothesis and results similar those of the Moritz. 2004 experiment and my own.  With the information gathered from this experiment, I am inclined to perform a new experiment with different types of C3 and C4 plants in addition to CAM plants.  CAM plants are comparable to C4 plants because they fix CO2 through an intermediate pathway before entering the Calvin cycle.  However, CAM plants have different patterns for their timing of the Calvin cycle and CO2 uptake.  I would like to develop a new experiment to include additional trials and all three types of plants.  This would provide further, more detailed information on the processes that occur between different types of plants and their methods of photosynthesis.