Home Up Food Wastes Environment Reproduction


A fundamental requirement for life is a source of energy.  Since energy can neither be created nor destroyed, and since living organisms continually expend energy during growth, reproduction, and movement, living organisms must continually acquire energy elsewhere.

The primary source of energy on Earth is the Sun.  Radiations in the form of heat and light continually bombard the Earth.  Certain organisms such as photosynthetic bacteria, algae, and plants use some of this energy to manufacture organic compounds.  In very general terms, energy from the Sun is stored in the chemical bonds made by some bacteria, algae, and plants.  An easy way to think of this is to postulate that all living creatures are constructed of chemical compounds. One such compound diagramed below is the sugar, glucose.


The letter "C" is shorthand for the Carbon atom.  The letter "H" stands for Hydrogen, and the letter "O"  stands for Oxygen.  Thus glucose is a compound made up of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms.  If our eyes were good enough to see a glucose molecule, it would more closely resemble the diagram below.  The dashes between the atoms represent a chemical bond.

Energy is stored within the bonds, and released when the bonds are broken.  In general, each bond stores 3,500 to 5,000 calories.  These are the little calories, not the dietary ones.  1,000 little calories are the equivalent of 1 dietary calorie.  Organisms that photosynthesize (plants, etc.) use the energy from sunlight to build the bonds.

Organisms that cannot photosynthesize must get their energy from eating the  bodies of those organisms that can.  In the dark, organisms that can photosynthesis in the light must get their energy from their stored food just like animals do.  This stored energy exists in the form of carbohydrates, lipids, proteins,  nucleic acids. and some other compounds.

Animals eat the stored food in plants and utilize the energy contained therein for their own energy needs.  Animals may also eat other animals and eventually dead organisms are eaten by bacteria, molds, etc.

The sum total of all the chemical and physical processes which take place within all living organisms is termed Metabolism.  All of the above is to point out that most energy found in living organisms on Earth is directly or indirectly derived from sunlight.

The reason I haven't said that all energy in living organisms is derived from sunlight is because recently scientists have found bacteria-like organisms deep in the Earth that apparently live on the energy provided by the Earth's hot core.  There are hot water "smokers" at the bottom of the oceans that spew hot water into the sea and certain organisms live off the energy derived from this hot water.  Of course, perhaps the Earth's hot core may be indirectly caused by the gravitation, etc. of the Sun.

A few widely different types of life forms are illustrated below.  All of them and all other life forms must have a source of food~energy.


Green Algae

Large Marine Algae

Maple Tree

Budding Scientist


White Tail Deer

Mountain Lion





50 lb. Northern Pike (E. lucius)

Slugs drowning in a can of beer.

Plants eating sunlight.


eating a 24 lb. Northern Pike