Photosynthesis is a chemical reaction during which light energy is converted into chemical energy. Carbon dioxide (CO2) and water are transformed into glucose (C6H12O6) and oxygen. The reaction can be simplified as:
The process is carried out in chloroplast (an organelle of a plant cell) whose most important part is green dye, namely Chlorophyll a and b. The whole procedure is divided into two phases – light-dependent and the light-independent (Calvin cycle) reactions.
The importance of photosynthesis lays in its ability to bind inorganic carbon into organic compounds (carbon from CO2 is inserted into a saccharide). Only plants and some species of bacteria are capable of this transformation.
The first phase is carried out in the presence of sunlight, hence the name “light-dependent”. During this phase, light energy is transformed into chemical energy in the form of molecules of NADPH2 and ATP, with oxygen as the by-product.
At the beginning of the reaction, sunlight is absorbed by Chlorophyll a which is consequently excited and releases its electrons. The released electrons join with the NADP molecule. Chlorophyll b is also excited and it receives electrons from the dissociated water molecules (photolysis of water occurs – water molecules are dissociated by light energy). The accepted electrons are then transferred to Chlorophyll a; ATP molecules are formed during this process. Every two water molecules are dissociated into an oxygen molecule, which escapes into the atmosphere, and four hydrogen ions (H+). The created hydrogen ions are then joined with two NADP molecules and create two molecules of NADPH2. Molecules of NADPH2 have reduction characteristics. After this, one cycle of life-dependent phase is finished; however, this cycle constantly repeats itself. Chlorophyll, serving as a catalyst, doesn’t change and only water and light energy are consumed.
Light-independent reaction (Calvin cycle)
The second phase occurs in the dark and so we call it ligh-independent. In short, ATP and reductive NADPH2 interfere with carbon dioxide (CO2) which is then build into saccharides. The starting material is an activated pentsaccharide (ribulose 1,5-bisphosphate) which is gradually converted into hexasaccharide (glucose) by the addition of carbon dioxide.
For an experiment on natural dyes in leaves click here.
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