Life, whether in the form of a single-celled organism or a human being, might be defined as the capacity to harness energy for the purposes of constructing and maintaining vital tissues and sustaining metabolic processes. From the moment of conception (or budding in asexual life forms), energy must be acquired or generated to permit survival; depending on the organism, it is needed to fuel growth, cellular diversification, physiologic processes and mobility.
Bacteria and other unicellular organisms are able to "ingest" nutrients through their cell walls while photosynthetic algae and plants utilize solar energy to fuel their metabolism and growth. Fungi recycle nutrients from decaying plants and animals and some deep sea invertebrates feast on bacterial mats that form on the flanks of sulfide-spewing hydrothermal vents. Complex marine, freshwater and terrestrial food chains consist of producers (plants or algae), primary consumers (herbivores) and secondary consumers (carnivores); of course, a series of carnivorous activity leads to the top predator in each chain (e.g. mosquito, dragonfly, frog, snake, hawk).
Energy flows through these food chains, stored for periods of time in certain tissues (fat, glycogen, starch, bulbs) but eventually recaptured by scavengers and fungi when injuries, illness, predation or natural aging lead to the death of plants and animals. Prior to death, living organisms transfer energy to the environment via heat production and in the organic waste that they produce; they may also lose vital nutrients to parasites. While this cycle of energy and life is easily observed and readily accepted in nature, we humans often reject the fact that our own lives, from conception to death, reflect this universal process. We are, in fact, transient conduits of energy.