Nutrition is the process by which organisms obtain energy to maintain life functions, and matter to create and maintain structure. Both energy and matter are obtained from nutrients.In autotrophic nutrition organisms make their own organic nutrients from an external supply of relatively simple organic raw materials (inorganic nutrients) and energy. If the source of energy is sunlight the process is called photosynthesis. This is the process of green plants, algae and photosynthetic bacteria(cyanobacteria). It is one of the most important of all biological chemical processes because almost all living things depend on photosynthesis either directly of indirectly for their organic nutrients
In green plants photosynthesis takes place in the cells containing chloroplasts.Most chloroplasts occur in the mesophyll cells of green leaves, but some are also found in parenchyma cells below the epidermis of herbaceous stems. In the chloroplasts the energy of sunlight is trapped in the pigment .chlorophyll. Light energy absorbed by chlorophyll is converted into chemical energy of specific organic compounds (mainly sugar and starch, but also some amino acids and lipids), all of which possess much greater energy than the starting compounds of carbondioxide and water.Oxygen is a waste product.
From the definition of photosynthesis we can expect that the process needs light, chlorophyll in the chloroplast, carbondioxide and water. Since photosynthesis is a biochemical process requiring many enzymes, a suitable temperature is also necessary. These are essential conditions for photosynthesis in a plant cell.
F F Blackman's study of limiting factors led him to propose that photosynthesis involved two simultaneous steps: a photochemical reaction ( the light step) and a series o biochemical steps involving many enzymes ( the dark step)
The light step occurs in the grana membranes of the chloroplasts.It leads to the synthesis of ATP and NADPH and the release of oxygen gas from the splitting of water.
The excitation of a chlorophyll electron.........Light energy absorbed by the antenna chlorophyll molecules cause the displacement of and 'excited' (high-energy) electron, and the splittting of water. A vacancy ( a positive charged 'hole') in the reaction centre is left by the excited electron.
The splitting of water........The electron vacancy in the reaction centre is powerful enough in energy terms to split water and fill this hole with electrons from the water. The energy to split water molecules is the light energy that raises electrons in chlorophyll to and excited state. Light energy becomes chemical energy as a result of a combination of processes in the form of ATP and NADPH.
In the dark step , carbondixoide is converted to carbohydrate. The dark step occurs in the stroma of the chloroplasts and involves many reactions ,each catalysed by a different enzyme. The dark step requires the products of the light step (ATP and NADPH), but does not itself require the presence of light, this is why it is referred to as a dark reaction. Of course the dark step and light step occur simultaneously in chloroplasts each requiring the products of the other.
The major products of photosynthesis are a simple carbon sugar called glucose and oxygen which is an essential by-product.Infact, this is the singualr most important source of oxygen...
Illustration using the Calvin cycle.
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