Underlying this food chain is a flow of energy in the form of nutrients from one to another. Energy and useful terms. Energy is the capacity to do work. Work is the transfer of energy. Energy can be in different states.
Kinetic energy is the energy of motion, and potential energy is stored, like in a gallon of gas.
The science of energy is called thermodynamics , since it's easiest to measure energy by the amount of heat used or created. Hence, plants use the sun's energy for photosynthesis and to make chemical bonds in food molecules, which animals can then eat and break the chemical bonds for their energy needs. Hence, when an animal eats a plant, some of that energy within the plant material is lost. After some thinking, you should realize how these two laws of thermodynamics affect the makeup of food chains, e.g., why there are more plants than herbivores, or more herbivores than carnivores. Often, especially with biological reactions, one needs an activation energy to start the process.