Introduction
The primary source of energy for all living things on the earth is the sun. The energy received from the sun travels 150 million kilometers to reach the earth. Although this energy comes in two forms: light and heat, the heat energy cannot be captured directly by the plants or animals. However, the heat energy does warm up the non-living surroundings of plants and animals. Only plants can capture light energy directly through the process of photosynthesis, in which plants convert the light energy into stored energy. The green plants can manufacture their own food and so plants are called as autotrophs or self-nourishing living beings. Photosynthesis is possible because green plants contain an energy- capturing substance called chlorophyll.

Analysis
Plant cells contain an organelle called the chloroplast. The chloroplast allows plants to harvest energy from sunlight. Specialized pigments in the chloroplast (including the common green pigment chlorophyll) absorb sunlight and use this energy to complete the chemical reaction:

6 CO2 + 6 H2O + energy (from sunlight) C6H12O6 (glucose) + 6 O2

The glucose that is produced is the food of the plants that is used to make other compounds. Similarly, glucose is also stacked in the plant body for further use and chemical manipulation. This glucose also forms the food for the herbivores. [Vermaas, 2004]

The whole process of photosynthesis may be divided into two stages: the light reaction and the dark reaction. The light reaction requires light while the dark reaction can occur without light. In the light reaction, electron and proton transfers occur in the presence of light and in the dark reaction, the biosynthesis of organic compounds from carbon dioxide occurs. In order to capture the light particles, the leaves of the plant have pigments called chlorophyll and various electron capturing proteins that can effectively help in the reactions that involve electron and proton exchange reactions. The excited electrons that are captured, provide the energy for the reaction centers to carry on with other chemical reactions [Whitmarch and Govindjee, 2004]

The light reaction is essentially an electron transfer reaction where excited electrons (the energy for exciting the electrons comes from the sunlight and involves various photosynthetic systems called Photosystem I and Photosystem II) are used to carry out reduction and oxidation reactions to give rise to new compounds that are further used in other reactions. For example, the electrons from water are used to reduce NADP+ to NADPH. When light strikes the Photosystem II pigments, electrons are bumped off from the porphyrin ring of chlorophyll, which ultimately reaches the Photosystem I sites. The movement of the electrons through what is called as an electron transport chain creates a lot of chemical reactions that involve transfer of electrons between compounds. Finally the electrons are used to create NADPH.

The energy that is stored in NADPH is used to reduce carbon. Similarly the light reactions also give rise to large number of ATP molecules which are formed by attaching a phosphate group to ADP. ATP is also called as the “energy currency” of the cell since it releases a large amount of energy when needed.

This step can be summarized as

H2O + light ? ½ O2 + 2H+ + 2e
2NADP+ + 2H+ + 2e- ? 2NADPH

The electrons in this reaction are also used to repair the porphyrin ring that lost electrons in the process. Hence it may be seen that the whole process involves the reuse of electrons whose movements across electron transfer systems initiates various steps in the photosynthetic cycle.

In the dark cycle the energy that is stored in NADPH and ATP is used for creating organic compounds. The dark reactions begin when the Carbon from Co2 is attached to a 5-carbon sugar compound called Ribulose bi-phosphate. After carboxylaion, the compound is broken into two molecules of 3-phosphoglycerate. The 3-phosphoglcerate is later converted to 3-phosphoglyceraldehyde. This reaction needs much energy and the energy is provided by the ATP that may be broken down in the process. The whole cycle in which the Co2 is converted to glucose was worked out by Calvin and hence is called the Calvin cycle. This cycle generates glucose as well as Ribulose bi phosphate that are circulated back into the system to aid in the photosynthetic process.

The Calvin cycle may be summarized as follows

(RuBP) + carbon dioxide ? 3-phosphorogycerate (catalysed by Rubisco)
3 C5H8P2O11 + 3 CO2 ? 6 C3H3P1O6 + H2O
3-phosphoroglycerate + ATP ? 1,3 bisphosphoroglycerate + ADP
6 C3H3P1O6 + 6 ATP ? 6 C3H3P2O10 + 6 ADP
1,3 bisphosphoroglycerate + NADPH ? 3-phosphoglyceraldehyde+ NADP+ + Pi
6 C3H3P2O10 + 6 NADPH ? 6 C3H5P1O6 + 6 NADP+ + 6 Pi
3-phosphoglyceraldehyde ? ribulose 5-phosphate + Pi
5 C3H5P1O6 ? 3 C5H8P1O7 + 2 Pi
ribulose 6-phosphate + ACP? ribulose 1,5 bisphosphate + ADP
3 C5H8P1O7 + 3 ATP ? 3 C5H8P2O11 + 3 ADP

[Wikipedia, 2004]