The original source of energy is the sun while most of the energy in the organisms is used in the form of energy. The energy from the sun is used by the green plants to make food by the process of photosynthesis and this food is used by the human beings and animals. The energy in animals and human is in the form of ATP.
Enzymes can be best described as catalysts in living systems. This is because enzymes accelerate chemical reactions without being used up in the process.