Energy

• Energy is neither created nor destroyed -- only converted from one form to another. In order to grow, plants need to synthesize proteins, lipids, nucleic acids and other molecules; synthesizing these molecules takes energy. Plants must obtain this energy from somewhere, hence their dependence on sunlight.
  
  

Entropy

• Entropy can be thought of as a measure of disorder in a system. The second law of thermodynamics holds that the entropy of any closed system can never decrease, so only processes that bring about an increase in entropy -- either of the system or its surroundings -- occur in nature. Reactions like DNA or protein synthesis involve a decrease in entropy, so these reactions must be powered by other processes that increase entropy. Plants are able to drive these reactions using energy from sunlight.
  
  

Effects

• The net effect of photosynthesis is to convert energy from electromagnetic radiation into energy stored in chemical bonds, where it can be retrieved for later use. Animals depend on plants for the energy we need, so we also depend indirectly on sunlight as well.