Answer...
Energy enters the food chain as sunlight from the sun.
Chloroplasts in plants (autotrophs) which capture sunlight and use sunlight as an energy source to make ATP which in turn is used to power the building of orgnaic molecules like sugar.
The energy stored in molecules is passed from one trophic level to the next in stages (each stage is one trophic level in the food chain.
Each trophic level contains about 10% of the energy of the trophic level beneath it (the level which it consumes and gets its energy from.)
So little energy is transferred because the energy stored is the molecules at one trophic level is:
a) partly lost when it is changed into heat during cell respiration;
b) not consumed (like hair or skin or bone) by the next trophic level
c) material is eaten but not digested or is excreted and therefore not assimilated;
In general, in every ecosystem, energy stored in material which is at the "top" of the food chain then passes to decomposers and detritivores and finally to saprotrophs as dead organic matter.
In this way, nutrients (like N, C, P, and S) cycle within ecosystems as they are passed from one organism to the next; in other words nutrients are recycled. For example, carbon "begins" as carbon dioxide, a gas produced by aerobic cellular respiration of animals. Plants use this carbon and "fix" it into organic molecules like glucose. In turn that glucose can be made into starch. This starch is stored in leaves or roots until it is eaten by animals. The animals break down the glucose into pyruvates and use them to produce energy which in turn is used to make proteins which are used structurally in the animal as keratin in skin, insulin for hormones, lipids in cell membranes, etc. When these animals die they are first eaten by scavengers. When scavengers die they decompose as bacteria and other organisms break them down and use their nutrients to build other molecules as well as using their molecules as an energy source.
Some nutrients are leached "originally" from rocks by mechanical, chemical and organic weathering. These minerals are dissolved by fungi like lichens, abosrbed and ultimately used by plants before being transferred up the food chain!