 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Matter, Energy, and Life
Be Alert BoxesBE ALERT FOR: Matter Versus Energy
It is very important to make a distinction between matter and energy. Matter is concrete. You can hold it, smell it, and weigh it. Energy is more abstract. It is defined as the potential to do work. This potential exists in a variety of forms, including electrical, solar, chemical, and heat. The problem is that energy doesn't have a physical presence in the sense that matter does.
Consider a spoonful of sugar. Sugar contains calories of energy usable by our body cells. If you were to dismantle the sugar molecules, you would end up with piles of carbon atoms, hydrogen atoms, and oxygen atoms. But there would be no pile of energies. In sugar, the energy is present as forces of attraction that hold the various atoms together.
What makes it a bit confusing is that the sugar molecules contain both matter and these abstract energy forces.
BE ALERT FOR: Productivity Units
Ecologists often refer to ecosystem productivity as the quantity of biological material, or biomass, produced in a given area per unit of time. This would lead you to expect expressions of productivity to be in units such as tons or kilograms.
It might be puzzling when productivity is also described in terms of energy units-calories produced instead of weights produced. There is a simple explanation for this. Each gram of organic substance contains a certain number of calories of stored chemical energy. For example, one gram of carbohydrate or protein contains four calories of chemical energy.
Suppose, thanks to its photosynthetic activity, a plant added 100 grams of dry weight (excludes water content) during one week. These 100 grams constitute an amount of new biomass, and so the plant's net productivity for the week could be expressed as 100 grams. However, since those grams are essentially carbohydrate and protein, the plant can also be considered to contain 4 X 100 = 400 new calories of energy. So, the net productivity could properly be expressed in either form: 100 grams of matter or 400 calories of energy.
BE ALERT FOR: The Nitrogen Cycle
Living things are composed principally of water. Organisms also contain the organic substances carbohydrates, lipids, proteins, and nucleic acids, which are crucial to the creation of cells and their life-sustaining activities. Life, in fact, is an endless game of creating, using, and transforming these substances. None of these molecules can be formed without atoms of carbon, nitrogen, phosphorus, and others. A basic ecosystem process is the provisioning of its members with an endless stream of these materials. This is accomplished through the system's material cycles.
You will quickly master the hydrologic and phosphorus cycles. The carbon and nitrogen cycles are a bit more challenging. A word about the nitrogen cycle in particular is needed. Nitrogen atoms exist as parts of several kinds of chemical substances. Part of your challenge is to sort those out and to recognize that nitrogen can be used by organisms only when it is present in certain of those chemical forms.
Also deserving special attention is the role of bacteria. General decomposition bacteria and other decomposers provide crucial closing-the-cycle activity in material cycles generally. In the nitrogen cycle, however, different species of bacteria perform a series of different and important steps. In fact, the theme of the nitrogen cycle could be, "Have you thanked a bacterium today?" You'll see what I mean.
|
|
2003 McGraw-Hill Higher Education