The last article dealt with an introduction to photosynthesis, its place of occurrence and the chemicals involved. How exactly does sun’s radiation induce the formation of glucose from simple chemical species like water and carbon dioxide? Plants and planktons are a major source of oxygen on earth. Oxygen is a product of photosynthesis along with glucose. Experimentally it was observed that, although both CO2 and H2O have oxygen atom in them, H2O is the source of oxygen released in the process. How is water the source of oxygen released and not the carbon dioxide gas? To understand these questions lets take a very close look at photosynthesis.
Photosynthesis occurs inside chloroplasts where a series of complex chemical species present initiate the break down of water and assist the formation of an energetic molecule called as NADPH. This NADPH helps in the conversion of CO2 to glucose. These are two different parts of photosynthesis. The series of chemicals assisting breakdown of water has two light tapping centres. These two centres are called as photosystem-I(PS-I) and photosystem-II(PS-II) which are two different types of chlorophyll molecules. A chlorophyll molecule has a magnesium ion inside its ring like structure. This metal atom is the one that taps sun’s energy. The two photo-systems best absorb when light has a wavelengths 700 and 680nm for PS-I and PS-II respectively. As what we have learnt about Einstein’s theory on photoelectric effect during our A Level Physics tuition class, it says that when light falls on the metal atom present in chlorophyll, when it is of desired wavelength, chlorophyll molecule loses an electron. Simultaneously water molecule loses electron present in the H atom and gets converted to oxygen releasing hydrogen ions. The electron lost from the water molecule replenishes the metal atom. At the same time on PS-I electron is released and consequently assists the formation of NADPH.
The electron released from the magnesium of PS-II through the chain of complex chemical species, mentioned above, reaches PS-I and replenishes it. In the process water molecule is broken down with the release of oxygen and the formation of NADPH. This is the light dependent part of photosynthesis. In next part of it carbon dioxide is converted into glucose. 6 molecules of carbon dioxide enter the process at various stages of the reaction and form glucose with the assistance of NADPH. Thus light energy trapped in the form of NADPH gets finally converted to glucose. This glucose acts a source of energy and also gets converted to complex sugars like starch that act as reserves of energy. Thus light energy is converted into chemical energy through photosynthesis.