Photosynthesis
The word photosynthesis is taken from the Greek words photo (light) and synthesis (building up) which means beaulding up with light. Photosynthesis is a complex and natural process, where green plants, algae, and certain bacteria convert sunlight (light energy), carbon dioxide, and water into glucose (chemical energy) and oxygen.
Carbon dioxide + Water + Light energy → Glucose + Oxygen
6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O
The process is accompanied by a liberation of oxygen. The volume of oxygen liberated has been found to be equal to the volume of carbon dioxide absorbed. But it is to be noted that all the oxygen liberated in the process is released exclusively from water (H2O) and not from carbon dioxide (CO2), as first proved by Hill in 1937 and later by others by using radioactive oxygen, O18, in water (H2O18). Oxygen escapes from the plant body through the stomata. This formation of carbohydrates, commonly called carbon-assimilation, is the monopoly of green plants only, chlorophyll being indispensable for the process.
By this process not only are simple carbohydrates formed but also a considerable amount of radiant (light) energy absorbed from sunlight is stored up as potential chemical energy in the organic substances formed. It must be noted that photosynthesis takes place only in the green cells and, therefore, mainly in the leaf and to some extent also in the green shoot.
Mechanism of Photosynthesis
The intermediate chemical stages in the process stilI remain a mystery. Numerous researches carried out over a long period have failed to trace the diffeJient chemical reactions involved in the production of carbohydrates from carbon dioxide (CO2) and water (H2O), and this has led to a great deal of speculation.
Photosynthesis takes place in a series of chemical reactions, some are photochemical requiring light energy and some are chemical or enzymic requiring a particular temperature. Chlorophyll no doubt is indispensable for photosynthesis, but it is not known what exact role it plays in the process except that (a) it absorbs radiant (light) energy and possibly also transfers this energy to the photosynthetic products, and (b) it acts as a catalytic agent, itself undergoing no change during the photosynthetic process.
External factors like light, carbon dioxide and temperature are most essential for proper functioning of the chloroplasts. Several enzymes also play a part in successive stages of the process.
In recent years with the discovery of radioactive elements, particularly radioactive carbon, C14, it has been possible to trace at least some of the compounds through which the carbon dioxide passes on its way to the final products formed during the process of photosynthesis. This is called the 'tracer' method. Thus in 1950 Benson and Calvin by using C14O2 (with radioactive carbon in it) succeeded in tracing it through some of the intermediate stages of photosynthesis. They found that when the period of photosynthesis, i.e. the period of exposure to light, was shortened to a few seconds a detectable quantity of phosphoglyceric acid was formed. Phosphoglyceric acid is, therefore, the first stable intermediate product formed in photosynthesis. It is a 3-carbon compound and possibly it is formed from an unknown 2-carbon compound.
The radioactive carbon used in the experiments could be traced in the phosphoglyceric acid and finally in the sugar formed in the process. But exactly how sugar appears is not clear. It is possible that the union of two such 3-carbon compounds produces a 6-carbon compound, i.e. sugar, The overall reaction may be represented as:
6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O
Photosynthesis as a whole resolves itself into two types of reactions-light and dark. The process of photosynthesis begins with the absorption of light energy by chlorophyll and utilization of this energy in breaking up water (H2O) into oxygen and hydrogen. The splitting of water is possibly the only light reaction. Oxygen escapes, while hydrogen is stored in the chloroplasts in combination with some unknown compound which acts as an acceptor of hydrogen. By this process the light energy is converted into potential chemical energy.
Next, hydrogen is transferred to CO2 (which now acts as an acceptor of hydrogen) to form a 3-carbon compound-phosphoglyceric acid, as stated before. Finally the phosphoglyceric acid becomes transformed into sugar. All the reactions from the reduction of CO2 (i.e. addition of hydrogen) to the formation of sugar and starch are dark reactons. Nearly the whole of CO2 taken up in photosynthesis enters into the composition of sugar. Sugar is almost immediately converted into starch.
End Products in Photosynthesis
Oxygen and starch are the final products formed in photosynthesis. Oxygen escapes from the leaf but starch accumulates in it. Starch may be detected in the following way. In the evening collect one or more leaves and bleach them with methylated spirit. Then dip them into iodine solution. They are seen to turn bluish-black in colour indicating the presence of starch grains. Starch is insoluble in water. At night it is converted into sugar by the action of an enzyme known as diastase and translocated to storage organs. In the storage tissues sugar is reconverted into starch by the leucoplasts.
Conditions Necessary for Photosynthesis
Light intensity, temperature and carbon dioxide concentration of the air are the three most important external conditions for photosynthesis and its rate.
Light: This is the most important condition for photosynthesis. Formation of carbohydrates cannot take place unless light is admitted to the chloroplasts. The rate of photosynthesis also varies according to the intensity of light.
Carbon dioxide: Carbon dioxide of the air is the source of all the carbon for the various organic products formed in the plant, such as sugar, starch, etc., and, therefore, the process is in abeyance it carbon dioxide is not available to the plant. Under favourable conditions of light and temperature if carbon dioxide concentration rises from 0·03 per cent in the air to 0·1 per cent or even more, carbohydrate formation greatly increases.
Water: Water is indispensable for photosynthesis because water and carbon dioxide undergo chemical changes leading to the formation of carbohydrates under the influence of chloroplasts and in the presence of sunlight. It is, however, a fact that less than I per cent of the water absorbed by the roots is utilized in photosynthesis.
Temperature: Photosynthesis takes place within a wide range of temperature. It goes on even when the temperature is below 'the .freezing point of water, but the maximum temperature lies at about 45°C. The optimum temperature, i.e. the most favourable temperature for photosynthesis, may be stated to be 35°C.
Chlorophyll: This is essential for photosynthesis; the plastids are powerless in this respect without the presence of chlorophyll. For the same reason non-green parts of plants cannot photosynthesize (see experiment 4). Fungi and sapro., phytic and parasitic phanerogams have altogether lost this power, being devoid of chlorophyll.
Potassium: Potassium helps synthesis of carbohydrates and, therefore, in its absence starch grains are not formed. Potassium does not enter into the composition of carbohydrates but acts as a catalyst helping in their synthesis.