The "flow of energy" refers to how solar energy and inorganic nutrient energy are converted at the autotroph/producer level of the trophic sphere into food and energy for consumers. What this really means is that flow of energy explains (1) how sunlight becomes life sustaining food for plants, humans and animals and (2) how inorganic minerals become life sustaining food for plants, humans and animals. In other words, sitting in the sun and eating a rock will not keep you, your cat, your frog or your African violet alive. Something happens to sunlight and to inorganic minerals somewhere in Earth that transforms them into life sustaining food for every living thing. What happens and where? The answer to this is identified by the term flow of energy. What energy flow? The flow of energy is the flow of solar energy and of inorganic nutrient energy.
Solar heat, which is the same as energy, from the Sun reacts with inorganic nutrients within plants and algae to convert carbon-carbon bonds to glucose sugar (carbohydrates) and oxygen. The process, a complex chain of reactions, is called photosynthesis.
Inorganic nutrients, which lack carbon-carbon bonds, in the soil and water surrounding the plant or algae are drawn in from the soil's and water's "inorganic nutrient pool" and provide the nutrients that solar energy works upon to produce organic nutrients through photosynthesis. These organic nutrients initiate the food chain for consumers in the higher trophic levels.
The plant and algae producer level is called the autotrophic level. Trophic means feeding: trophic levels are feeding levels and the first, the primary producer level, is the autotrophic level that feeds itself from solar energy acting upon inorganic nutrients.
Consumers at rising trophic levels eat the plants or eat the animals that eat the plants. Through the trophic process, (1) inorganic nutrients are cycled and converted to food and (2) heat, or, energy is lost by being used up or expelled. Nutrients are cycled. Heat is not; it is lost.
Primary producers (plants, algae) are consumed by various consumers at primary, secondary, tertiary and higher levels.
- Primary consumers are herbivores and consume only plants and/or algae; these range from bees to cows.
- Secondary consumers are carnivores because they consume the herbivores that have consumed plants. These are exclusively meat eaters and range from crows to lions.
- Tertiary consumers are omnivores because they consume on the secondary level by consuming herbivores that have consumed plants, like cows, and because they also consume on the primary herbivore level by directly consuming plants. Humans are the best known omnivores.
Whatever a consumer's trophic level, from autotroph to tertiary-trophs, all consumers decompose and are consumed by the detritivores level that consumes the detritus of decomposition. Detritivores are a critical part of the flow of energy because it is they that break down carbon-carbon bonds back to the originating inorganic nutrients. These inorganic nutrients are returned to the soil or water and acquired by primary producers that then convert the inorganic nutrients to organic carbohydrates through solar energy activating photosynthesis. The cycle has returned to where it began.
Now the two avenues of primary producers' energy absorption, solar energy and inorganic nutrients, describe the closed cycle of the flow of energy that begins and ends with inorganic nutrients yet also begins with and is sustained by solar heat energy.