This design is an advantage to C4 plants because when the stomata are
closed and oxygen levels are high, the problem of photorespiration where the reaction of the rubisco enzyme with CO2 is competitively inhibited by oxygen, is diminished because the Calvin cycle is insulated from the light reactions. As a result we hypothesized that photosynthesis in C4 plants should have higher rates of photosynthesis than C3 plants when CO2 levels are low.
We placed two different C3 plants, Juniper leaves and Gesneriacead, and a C4
plant, Corn, each into a chamber individually and attached the probe measuring
concentration of CO2. In each plant we left the lamps behind water bottles, to
maintain constant temperature, until the plant reached a point of equilibrium
where the rate of photosynthesis equals the rate of respiration. There was
significant difference between the results of each type of plant. The juniper
leaves had a point of equilibrium of 98ppm and Gesneriacead stopped absorbing
CO2 at 88ppm. Corn on the other hand had a point of equilibrium of 22ppm. This
proves that when CO2 levels are low, C4 plants have a higher rate of
photosynthesis and more success absorbing CO2 due to their specialized systems
reducing photorespiration. We determined that our results were consistent with
the hypothesis because the C4 plant has a better rate of photosynthesis.
Our results were consistent with our predictions and therefore we considered
our experiment reliable and we fail to reject our hypothesis but we could have
done more trials with other C3 and C4 plants to make the results more valid.
Results of Cohen et al. 2001 on the CABLE web site also concluded that C4
plants use CO2 more efficiently than C3 plants.
Many plants which live in dry conditions have evolved an alternative carbon fixation pathway to enhance the efficiency of rubisco so that they don’t have to keep their stomata open as much, and thus they run less risk of dying due to dehydration. These plants are called C4 plants, because the first product of carbon fixation is a 4-carbon compound (instead of a 3-carbon compound as in C3 or “normal” plants). C4 plants use this 4-carbon compound to effectively “concentrate” CO2 around rubisco, so that rubisco is less likely re react with O2.
In order for plants to take in CO2, they have to open structures called stomata on their leaves, which are pores that allow gas exchange. Plants also lose water vapor through their stomata, which means that they can become dehydrated in dry conditions as they photosynthesize. Because photorespiration (reaction of rubisco with oxygen instead of carbon dioxide) drastically reduces the efficiency of rubisco, which is already a very slow-working enzyme, this means plants in dry conditions are at risk of death by dehydration even as they’re trying to make their own sugar from photosynthesis to stay alive.
PEP carboxylase is located in the mesophyll cells, on the leaf exterior near the stomata. There is no rubisco in the mesophyll cells. CO2 entering the stomata is rapidly fixed by PEP carboxylase into a 4-carbon compound, called malate, by attaching the CO2 to PEP. The malate is then transported deeper into the leaf tissue to the bundle sheath cells, which are both far away from the stomata (and thus far away from oxygen) and contain rubisco. Once inside the bundle sheath cells, malate is decarboxylated to release pyruvate and CO2; the CO2 is then fixed by rubisco as part of the Calvin cycle, just like in C3 plants. Pyruvate then returns to the mesophyll cells, where a phosphate from ATP is used to regenerate PEP. Thus in C4 plants, C4 carbon fixation has a net added cost of 1 ATP for every CO2 delivered to rubisco; however, C4 plants are less likely to die of dehydration compared to C3 plants in dry conditions.
What Biol 1510 students need to remember about C4 is that these plants have added a CO2 concentration mechanism to feed rubisco and the Calvin cycle; the mechanism uses PEP carboxylase to initially make a 4-carbon compound, that then releases CO2 to rubisco in leaf cells that are exposed to little oxygen. While this mechanism reduces the oxygenase activity of rubisco, it has an extra energy cost in the form of another ATP per mole CO2 fixed.