Water
Man
Water
Man
One of the basic laws of physics tells us that matter is never created or destroyed. Atoms found in the molecules of carbon dioxide and water are not destroyed when they are used in photosynthesis. They are either used to build sugars, or in the case of some of the oxygen, released to the air. No atoms are destroyed when cellular respiration takes these same sugars. Aerobic respiration breaks the sugars all the way back down to carbon dioxide and water, while fermentation produces carbon dioxide and some kind of alcohol. Carbon is often stored in fossil fuels which are breakdown products of dead plants or animals. Individual water molecules may be tied up for thousands of years in the polar ice caps, only to be released during warmer times. This is how the two most important things to life, carbon and water are always "recycled".
Other elements that are necessary for life also move through cycles as they pass through living systems. Nitrogen is probably the most imortant of these "lesser" elements. There is lots of nitrogen available to ecosystems, as the air we breathe is 71% pure nitrogen. However we have to have the help of bacteria that live in the roots of certain plants (members of the pea family, or legumes). These bacteria are able to take pure gaseous nitrogen and "fix" it into compounds we can use. Then, when organisms die and decompose, their nitrogen is returned to the atmosphere.
Other atoms we have tracked through living systems include
calcium and phosphorous. While not used in the same quantities as the others,
they are still important to living systems.
