Introduction
Photosynthesis is a very important process in plants, allowing the plants to synthesize carbon dioxide (CO2) into glucose for food and energy. There are two reactions of photosynthesis: the light and the dark reactions. The basis of the light reactions is to convert light energy into chemical energy, and then the dark reactions take over converting CO2 into glucose by combining with ribulose 1,5-biphosphate. Ribulose 1,5-biphosphate carboxylase/oxygenase (RuBisCO) catalyzes the first step in carbon fixation (3). Rubisco is a very common enzyme in the leaves of plants, and may be the most abundant protein on Earth; it supports life on this planet. About 30-50% of total soluble protein in the chloroplast comes from Rubisco (4).
Plants can either be characterized as C3, C4, or Crassulacean acid metabolism
(CAM) plants. C2 and C3 plants fixate carbon during the day. CAM plants,
rather than having carbon fixation during the day, improve water uptake
efficiency due to stress conditions by switching CO2 uptake at
night, thus having less water loss. Normally, those plants that use the CAM
cycle are in dry, arid environments.
The plants that were used for this experiment were the Oak tree (Quercus alba) and the common Ice Plant (Carpobrotus edulis, formerly classified as Mesembryanthemum crystallinum). The object of this experiment was to determine the levels of the protein Rubisco in the leaves and shoots of these two plants, and make a comparison to how much chloroplast DNA there was in the samples. The Oak leaves use the C3 photosynthesis cycle, and the Ice Plant uses also C3, but is also able to switch to the CAM cycle under a water deficiency, or a salinity increase (2). The Ice Plant is known to lower the pH of the soil, which could have an effect on the amount of Rubisco present in the shoots. It is known that the pH rises due to the proton gradient (~7.0-8.0) across the thylakoid membrane, which then triggers an increase in the magnesium ions. Rubisco has a high optimal pH and only becomes activated by the addition of carbon dioxide and magnesium (1). If the Ice Plant lowers the pH, then it can be assumed that not as much Rubisco will be produced. The inference was made that both the plants were C3, and were chosen on the basis that the Rubisco production would be different between the two plants.
In
addition, another hypothesis was that the Rubisco protein levels would differ
during the day and night because the plants would fixate the carbon during the
day and not during the night. Because of this, it was decided to use samples
of both the Oak and the Ice Plant leaves collected at night and during the
day (Oak collected during the day and night is called DO and NO respectively;
the Ice Plant collected during the day and night are titled DI and NI
respectively). Due to Rubisco being used mainly during the day, it was generalized that
the Rubisco levels will be higher during the day than during the night.
Comparing the Oak leaf to the shoot of the Ice Plant, we can hypothesize that
the Rubisco levels of the Oak leaf would be higher than the Ice Plant. The Ice
Plant would still contain levels of Rubisco, but because Rubisco has a high
optimal pH, not as much would become activated.
Therefore, those plants collected at night will have lower levels of Rubisco than those collected during the day. The Ice Plant will have lower levels than the oak leaves due to the pH, and the Ice Plant collected at day may possibly even have lower levels than the Oak collected during the night.
©2006 All photographs are copyright of Christina Adams and Kristin Von Moll.