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  • Photosynthesis, literally, means 'synthesis of food with the help of light'.
  • Photosynthesis is one of the most fundamental biological functions. By this process the chlorophyll, contained in the chloroplasts of green plants, traps the energy of sunlight and transforms it into chemical energy which is stored in the chemical bonds of plant food stuff. In simple words it can be defined as " the process of conversion of light energy to chemical energy". Essentially photosynthesis is the combination of carbon dioxide and water, to form various carbohydrates with the evolution of oxygen. Photosynthesis occurs in green plants, photosynthetic protists and in certain bacteria (Monera). Photosynthetic pigments are present on the granum of the chloroplast in the respective photosystem I and II which perform light reaction. This chapter deals with the mechanism of photosynthesis, other pathways of energy production and factors affecting photosynthesis.

Historical Perspective

The broad outlines of the photosynthetic process were known in the eighteenth and nineteenth centuries. It was realized that photosynthesis involved three principal ingredients, namely water, carbon dioxide and light and also that plants produced not only food but also oxygen gas. Water comes primarily from the soil (for terrestrial plants) and must travel from roots to the leaves; carbon dioxide is taken in from the atmosphere by means of tiny apertures called stomata in the leaves. It was also learnt that light was absolutely necessary for the production of oxygen and food. In the first decade of the nineteenth century, it was discovered that carbon dioxide uptake and oxygen release were closely related and that both depended on light. It took almost a century and half to determine whether the oxygen that is evolved comes from carbon dioxide or from water. This was finally understood by using the heavy isotope of oxygen (18O). Green plants were allowed to photosynthesise under two experimental conditions. In the first situation, they were supplied with water containing the heavy oxygen isotope 18O and with carbon dioxide containing only the common isotope of oxygen 16O. In the second situation carbon dioxide was labelled with 18O and the water contained only 16O. Oxygen gas was collected from each group of plants and the presence of 18O was analysed. It was found that oxygen containing 18O was abundantly produced only when plants were given 18O labelled water. The plants which were given 18O labelled carbon dioxide did not produce 18O labelled oxygen at all, conclusively showing that the oxygen evolved during photosynthesis came from water and not form carbon dioxide. With this information and taking into account the number of carbon dioxide molecules needed to form a simple sugar such as glucose the overall photosynthesis can be summarized by the following equation
6 CO2 + 12 H2O C6H12O6+ 6 O2 + 6 H2O
However, the process of photosynthesis does not occur in a single reaction. It is a complicated process involving many steps. Before going into the details of photosynthesis let us examine the structure of the photosynthetic machinery, the chloroplasts.

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