Photosynthesis: Green plants (herbivorous plants) prepare carbohydrates or starch or glucose (C6 H12 O6) in the presence of sunlight using the cell organelle called Chloroplast.
Further it requires only sunlight and water for preparing carbohydrates.
Carbohydrates are the source of energy to humans and herbivorous animals(living world because in the food chain even the carnivores are indirectly dependent on plants) thus one can say the basics of energy source for the living world are plants . Plants , inturn accquire the energy by the process of photosynthesis. 
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Because it is through that food is processed by plants for them.