The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed. This can be demonstrated within a classic food web where light energy from the sun is harnessed as radiant energy by plants, converted into chemical energy, and stored as complex carbohydrates. The vegetation is then consumed by animals and during the digestion process, the sugars release energy as heat. The sugars also produce chemical energy that either gets used up doing work, stored in macromolecules like glycogen and fatty acids or are consumed by a predator. The waste products and dead organic matter from animals are then decomposed by bacteria and fungi and returned to the soil to provide food for plants to grow and the cycle continues.

The First Law of Thermodynamics

During the process of photosynthesis, the photons are used to make complex carbohydrates that the plants use to live and grow, and oxygen is released into the atmosphere.

The plants eventually become food for animals—herbivores—and during the digestion process, the sugars are broken down and energy is released—either as heat or to provide chemical energy from glucose, to drive cellular processes that allow the animal to survive and reproduce. It can also be stored in macromolecules as chemical energy. For example, glycogen can be stored in the liver or muscles and can be quickly accessed in times of high energy demand. It can also be stored as an energy reserve in the form of fatty acids. Finally, the animal may pass on the energy to a predator—a carnivore, that will use it in a similar way.

Waste products from each step are also returned to the air, water, or soil and used to maintain the food web. Decomposers like bacteria and fungi eat dead organic matter and return it to the soil so that it can be used again by plants to grow.