Energy moves in a one-way direction through the biosphere. It starts from the solar energy trapped by green plants through photosynthesis, then moves to the primary consumers, or herbivores, who eat the green plants, travels up the food chain to the secondary consumers, or carnivores, moves to the tertiary consumers, who eat the secondary consumers, and finally it ends up fueling the metabolisms of the detritus feeders. Each of the levels that the sun's original energy flows through is referred to a trophic level. The entire energy-flow system is sometimes called a "trophic pyramid" or "energy pyramid."
The biosphere refers to the totality of all of the various ecosystems on Earth. It can be thought of as a self-regulating closed system encompassing the entire zone of life on Earth. Unlike the various forms of matter moving through the biosphere, which are recycled, the solar energy that was originally trapped through photosynthesis is used up as it travels through the biosphere's various trophic levels. Since energy cannot be destroyed, it is actually being used by living organisms and converted into metabolic energy and kinetic energy, such as movement, keeping warm and catching food.
As energy moves through the trophic levels, there is less of it available for the next level. This can be viewed as an example of heat loss. The use of the sun's original energy to fuel metabolic processes at the cellular level, which is called respiration, can be compared to the burning of a fossil fuel to generate heat.