Explanation of using twelve molecules of water as substrate in photosynthesis equation:
The equation for photosynthesis is:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Importance of water as a substrate:
- Water is a crucial substrate in the process of photosynthesis as it provides the necessary electrons for the light-dependent reactions to occur.
- During the light-dependent reactions, water molecules are split into oxygen, protons, and electrons by the enzyme complex known as photosystem II.
- The electrons released from the water molecules replace the electrons lost by chlorophyll in the reaction center, allowing the process of electron transport chain to proceed.
- This electron transport chain ultimately leads to the production of ATP and NADPH, which are essential for the Calvin cycle (light-independent reactions) to produce glucose.

Reason for using twelve molecules of water:
- Each water molecule provides two electrons for the electron transport chain.
- In order to produce one molecule of glucose (C6H12O6), a total of 12 electrons are required.
- Therefore, 6 molecules of water are needed to provide 12 electrons to complete the process of photosynthesis and produce one molecule of glucose.

Conclusion:
Water molecules as a substrate play a crucial role in photosynthesis by providing electrons for the production of ATP and NADPH. Using twelve molecules of water ensures that an adequate supply of electrons is available to complete the process and generate glucose.