Write an essay outlining the experimental approach used to
elucidate the C3 (or Calvin) cycle and discuss the limitations of this mechanism in our current atmosphere.
The C3 cycle is the process by which plant cells use sunlight to alter carbon atoms, eventually synthesising sugar, which is used as an energy source. This process plays a crucial role in plant life and shapes the earths ecosystem fundamentally, and was elucidated through an experiment carried out at Berkeley University. While the C3 cycle is a fundamentally important underpinning of life on earth, it is not an entirely efficient process in the modern atmosphere, and may be being slowly replaced by new evolutionary developments, more fitting for current O2/CO2 concentrations.
Experiments carried out at Berkeley University were the first to elucidate the process of the C3 cycle. The experiment used the organism unicellular algae Chorella, which was held in a glass container whilst being irradiated by light in order to drive photosynthesis. Then the organism was injected with radio labelled 14CO2, before being killed at various timed intervals. The dead cells from these intervals were then separated, allowing organic compounds to be extracted, which are then treated using radio labelled chromatography. In this process, organic compounds extracted from the dead cells are placed on photographic paper, as the radio labelled C14 causes fogging on this film, the carbon can be tracked across the various time intervals, allowing for the identification of what molecule it becomes at what stage of the C3 cycle. It is because of this experiment that we now understand the three phases of the C3 cycle; the first in which carbon enters the cycle and is catalysed by rubisco; the second phase in which ATP and NADPH reduce phosphate groups as well as phase three in which ATP and leftover G3P are used to create rubisco and begin the cycle again.
In a primitive atmosphere, the C3 cycle was far more effective than in our current atmosphere. Photosynthesis in the most basic terms, operates when plants take in CO2 and expel O2. In a primitive atmosphere abundant with CO2 this process proceeds mostly unimpeded, while in the modern atmosphere, an atmosphere rich in O2 and poor in C02, plants often take in 02 and produce CO2. This phenomenon mimics the respiration of mammals, and in plants is called photorespiration. Photorespiration occurs roughly one fifth of the time in C3 plants, and due to this inherent flaw of rubisco the majority of plants in the modern atmosphere are highly inefficient.
Some plants however have overcome this flaw, due to the recent development (in evolutionary terms) of the C4 cycle. C4 plants still use the C3 cycle, but in addition use the C4 mechanism to bypass rubisco through the PEP carboxylase enzyme. This enzyme fixes CO2 far more efficiently than rubisco, making these plants much more efficient in their photosynthesis. Plants which utilise C4 have an obvious advantage over C3 plants, but are still in the minority. This may be due to their recent development, and these plants will, over time, naturally out compete and replace C3 plants.