The Energy Cycle


At any given moment, trillions of atoms and molecules are circulating between the living and the non-living world.  Life’s processes (i.e. reproduction, growth, etc.) require that a constant supply of energy and nutrients be introduced into the body.  The sun begins the process through organisms like plants and other photosynthetic producers.  The higher organisms eat the lower organisms, break down their matter and rearrange the molecules to make their own matter.  When any organism dies, the remains are broken down and put back into the cycle as inorganic molecules.


The sun is the source of all of our planet’s energy.  Sunlight is absorbed and used by the producers to create matter from smaller matter.  Energy from the sun is used to drive the process of photosynthesis, constantly renewing the energy lost to the environment as heat from life’s processes.  The autotrophs, also known as the producers, include plants, many bacteria, plant-like protists and even some fungi.  These organisms

perform photosynthesis and create the basic organic matter from inorganic matter.  In most cases the energy from the sunlight is captured by chlorophyll.  The

use of nutrients nutrients (i.e. H2O, nitrates, etc.) allow autotrophs to produce matter for higher trophic levels.  The producers break down some of the matter that they manufacture to produce energy (yes plants do have metabolic processes other than photosynthesis!).


Consumers, or heterotrophs, eat matter and digest it to form smaller pieces of matter.  These smaller pieces of matter are then used to make new matter unique to the organism.  There are several levels of consumers.  The order depends on how many organisms have been eaten before.  A 1st order consumer eats only plants, while a 2nd order consumer eats something that has eaten plants.  If a 2nd order consumer gets eaten, the organism that eats it becomes a 3rd order consumer, and so on and so forth.  The different categories of consumers include: h

erbivores (plants), carnivores (meat) and omnivores (both).  Each of these organisms eat organic matter to produce energy and small pieces of matter.  The energy is used to continue life’s processes (i.e. reproduction, growth, etc.) while the matter is used to build and rebuild structure.

At the “bottom” of the food chain are the s

cavengers and decomposers. These organisms break down dead matter to return it to the soil for use by producers so that the cycle can begin again. 



Scavengers generally eat dead meat while decomposers will eat anything that has died.  The dead organic matter is broken down to produce energy and small pieces of inorganic matter to return [what isn’t used] to the environment for the producers.  The energy is again used to perform life’s processes.

The only thing that should need to be renewed is the energy for another cycle to be performed.  It is also important to note that the energy is not destroyed, it is simply lost as heat to the environment.  The energy cycle, or food chain, doesn’t necessarily follow the same pathway each time (i.e. a tiger does not always eat gazelle anymore than you would eat a peanut butter sandwich at every meal).  For this reason, it is generally more useful to use a food web to describe the food-consumer relationships in an area.   A food web shows [all of] the organisms in an area and connects those that are eaten to those that eat them. 





This generic food chain shows how energy moves between organisms.  Notice that no matter what dies, there is a path for the matter to get back into the food chain through the scavengers and decomposers.


This food web does not show all of the organisms that it should, but it does demonstrate how organisms have more than one food source.  The arrows show the direction of matter/energy flow.  Since the producers do not eat the scavengers and decomposers, the large arrow shows that the matter and energy from their level is simply transferred to the producer level.  The arrows on the right show that each level eventually dies and the scavengers and decomposers break them down.