Photosynthesis occurs in two phases. The first phase is the Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules that are used in the second phase.

The Light Independent Process (or Dark Reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present.

Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

The initial process in photosynthesis is the decomposition of water (H₂O) into oxygen (which is released) and hydrogen; direct light is required for this process. The hydrogen and the carbon and oxygen of carbon dioxide (CO₂) are then converted into a series of increasingly complex compounds that result finally in a stable organic compound, glucose (C₆H₁₂O₆), and water. This phase of photosynthesis utilizes stored energy and therefore can proceed in the dark.

Mesophyll cells in a leaf are the principal centers of photosynthesis. Carbon dioxide from the atmosphere enters the leaf by diffusion through the stomata. Water from the soil is taken up by the roots, sent up through the stem and finally to the leaves where it is distributed in the mesophyll tissue. During daytime, when sunlight falls on the leaf, the light energy is trapped by the chlorophyll of the upper layers of the mesophyll, specially the palisade cells. This energy is utilized in chemical processes involved in the manufacture of food (Glucose).

The overall chemical equation of this process is as follows:
6CO₂ + 12H₂O C₆H₁₂O₆ + 6H₂O + 6O₂

The 6 molecules of H₂O liberated at the end of the process are those that are reformed and not out of the original ones.

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