Sunlight is crucial for photosynthesis, the process by which plants synthesize their food. Here's how it's required:

1. Energy Source:
   Sunlight provides the energy needed to convert carbon dioxide (CO₂) and water (H₂O) into glucose (sugar) and oxygen (O₂). This energy is a catalyst for the whole process.
2. Chlorophyll Activation:
   Chlorophyll, the pigment in plants that captures light energy, absorbs sunlight most efficiently in the red and blue regions of the spectrum. When chlorophyll absorbs sunlight, it becomes energized.
3. Photochemical Reactions (Light-Dependent Reactions):
   • The light energy captured by chlorophyll drives the light-dependent reactions of photosynthesis, which occur in the thylakoid membranes of the chloroplasts.
   • During these reactions, light energy is used to split water molecules (photolysis) into hydrogen ions (H⁺), electrons, and oxygen.
   • The electrons are passed along an electron transport chain, generating ATP (adenosine triphosphate) and NADPH, which are energy-carrying molecules used in the next stage of photosynthesis.
   • Oxygen is released as a byproduct.
4. Carbon Fixation (Light-Independent Reactions or Calvin Cycle):
   • The ATP and NADPH produced during the light-dependent reactions provide the energy and reducing power needed to convert CO₂ into glucose during the Calvin cycle, which occurs in the stroma of the chloroplasts.
   • This process is also called carbon fixation because inorganic carbon (CO₂) is "fixed" into an organic molecule (glucose).
5. Overall Equation:
   The overall equation for photosynthesis is: 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

In summary, sunlight is essential because it provides the energy that drives the entire photosynthetic process, from the initial capture of light energy by chlorophyll to the final synthesis of glucose.