Plant Photosynthesis

alphahese tools have had such a dramatic impact on plant physiology that is worth to study them briefly. Radioactive elements are chemically identical with their stable isotopes and differ only in the mass of their nuclei. They decay at rate that is proportional to their instability by disintegrating with the ejection of radiations—usually alpha or beta particles. Since these are ionizing radiations, they can be detected by various instruments that detect ionization, particularly the Geiger-Mueller counter or the scintillation counter. The Geiger-Mueller counter detects the production of ionization in a special ion chamber, the Geiger-Mueller tube which has thin-window through which the ionizing radiations can enter. The scintillation counter detects and counts flashes of light emitted by a crystal or a special liquid on absorption of a radiation. The decay rate of isotopes is exponential, being proportional to the amount present. The stability or decay time of an isotope is measured in live isotopes (for example, ³²P half-life=14 days), correction must be made from day to day to allow for this decay. ¹⁴C has a half-life of about 6000 years so no correction need be applied.

Because radioactive atoms are chemically like their stable counterparts, they undergo the same chemical reactions. Except for some slight effects, which may result from the fact that ¹⁴C is slightly heavier than ¹¹C, radioactive carbon atoms behave in precisely the same manner as non-radioactive ones; thus they enter all the reaction sequences and label all the compounds that are involved in the normal metabolism of carbon. If ¹⁴CO₂ is supplied to a plant in light for a period 5 seconds only those compounds synthesized during that 5 seconds will become radioactive.

Moreover, if new (radioactive) carbon atoms is added to an existing (non-radioactive) carbon atom is added to an existing (non-radioactive) compound, only that atom will be radioactive. The presence and position in the molecule of the radioactive atom can be determined by chemically degrading the compound, carbon by carbon, and assaying each carbon independently for its radioactivity. Using this technique it is possible to detect the pathways of carbon in metabolism. These in turn must be correlated with the types of reactions involved and the nature of the enzymes that are present to achieve complete understanding of the metabolic system.

It was noticed by Calvin that only C₁ of PGA (first stable compound) was radioactive giving the idea that only C₁ has been contributed by CO₂. C₂ and C₃ of PGA have been contributed by some substance X. For detecting X, Chlorella cells were fed with ¹⁴CO₂ for more than 0 seconds. It was noticed that all the three carbon atoms of PGA i.e. C₁, C₂ and C₃ became radioactive. Because only PGA can receive radioactive carbon from CO₂, X has received radioactive carbon from PGA due to long term exposure (more than 60 seconds). Compounds thus receiving radioactivity were found to be 4C, 5C, 6C, 7C.

X + CO₂ ———> PGA

Thus X which combines with CO₂ to form PGA was found to be Ribulose 1,5 biphosphate.