Introduction:
The photochemical phase, also known as the light-dependent reactions or the light reactions, is the first stage of photosynthesis. It takes place in the thylakoid membrane of chloroplasts and involves the absorption of light energy, the splitting of water molecules, the release of oxygen, and the formation of ATP and NADPH. However, it does not include the process of carbon dioxide fixation.

Explanation:
The photochemical phase can be divided into two main processes: the non-cyclic photophosphorylation and the cyclic photophosphorylation.

1. Non-cyclic photophosphorylation:
- The process begins with the absorption of light energy by pigments, such as chlorophyll, in the thylakoid membrane.
- This light energy is transferred to the reaction center, where it excites an electron.
- The excited electron is then passed through a series of electron carriers, creating an electron transport chain.
- As the electron moves down the electron transport chain, it releases energy, which is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient.
- The protons then flow back through ATP synthase, an enzyme embedded in the membrane, which uses the energy from the proton flow to synthesize ATP from ADP and inorganic phosphate. This process is called photophosphorylation.
- Simultaneously, water molecules are split by an enzyme called water-splitting complex, releasing oxygen gas and hydrogen ions.
- The hydrogen ions are transported across the thylakoid membrane and are later used in the formation of NADPH.

2. Cyclic photophosphorylation:
- In certain conditions, such as when there is a shortage of NADP+ or a high concentration of NADPH, an alternative pathway called cyclic photophosphorylation occurs.
- In this process, the excited electron from the reaction center is returned to the same pigment molecule, rather than being passed to NADP+.
- The electron is then recycled through the electron transport chain, generating ATP through photophosphorylation without the production of NADPH or oxygen.

Role of the photochemical phase:
- The photochemical phase is essential for capturing light energy and converting it into chemical energy in the form of ATP and NADPH.
- ATP is the primary energy currency of cells, while NADPH is a reducing agent used in the subsequent dark reactions (also known as the Calvin cycle or the light-independent reactions) to fix carbon dioxide and produce glucose.
- The oxygen released during the photochemical phase is a byproduct and is important for aerobic respiration in organisms that consume oxygen.

Conclusion:
In summary, the photochemical phase of photosynthesis includes processes such as light absorption, water splitting, oxygen release, ATP formation through photophosphorylation, and NADPH formation. However, it does not include the process of carbon dioxide fixation, which occurs in the subsequent dark reactions.