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Instruction 6-4

Biodiversity | Ecosystem Changes | Fluctuation in Population Size | Water, Carbon and Nitrogen Cycle | Stability in an Ecosystem | Energy Pyramid | Accommodation and Adaptation

Water, Carbon and Nitrogen Cycle
CA GR.9-12 6.d.

In this lesson, we are going to discuss the three main cycles through any ecosystem. They are the water, carbon and nitrogen cycles. We will go into depth with each cycle.

Water Cycle

Within the water cycle, energy is supplied by the sun, which drives evaporation whether it is from the ocean surfaces or from treetops and leaves. The sun, with the help of wind, also supplies the energy, which drives the weather systems, which moves the water vapors, in the form of clouds, from one place to another, or else it would only rain over oceans.

Precipitation occurs when water condenses from a gaseous state in the atmosphere and then falls to earth.

Evaporation is the reverse process where liquid water becomes gaseous. Once water condenses, gravity takes over and the water is pulled to the ground. Gravity continues to operate, either pulling the water underground or groundwater across the surface (also called runoff), either way gravity goes on to pull water lower and lower until it reaches the oceans.

Frozen water (ice) may be trapped at the cooler regions of the Earth such as the poles, glaciers and on mountain as snow or ice, and may remain as such for very long periods.

Lakes, ponds, and wetlands form where water is temporarily trapped and stored.

The oceans are salty because any erosion of minerals that occurs as the water runs to the ocean will add to the mineral content of the ocean water. Water cannot leave the oceans except by evaporation, and evaporation leaves the minerals behind. Thus, rainfall and snowfall are comprised of relatively clean water, with the exception of pollutants that are picked up, as the waste falls through the atmosphere, an example of this would be acid rain.

Organisms play an important role in the water cycle. As you know, most organisms contain a significant amount of water, which is about 90% of their body weight. The water is not contained for a long time and moves out of the organism quickly in most cases. Animals and plants lose water through evaporation from the body surfaces and through evaporation from the lungs or other typeís gas exchange mechanisms.

In plants, water is drawn in at the roots and moves to the gas exchange organs, the leaves, where it evaporates quickly. This special case is called transpiration because it is responsible for so much of the water that enters the atmosphere. In both plants and animals, the breakdown of carbohydrates (sugars) to produce energy (respiration) produces both carbon dioxide and water as waste products. Photosynthesis reverses this reaction, and water and carbon dioxide are combined to form carbohydrates. Now you understand the relevance of the term carbohydrate; it refers to the combination of carbon and water in the sugars we call carbohydrates.

Carbon Cycle

Once you understand the water cycle, the carbon cycle is somewhat simple. From a biological perspective, the key events here are the complementary reactions of respiration and photosynthesis.

Respiration takes carbohydrates and oxygen and combines them to produce carbon dioxide, water, and energy. Photosynthesis takes carbon dioxide and water and produces carbohydrates and oxygen.

This might sound a little confusing but, the outputs of respiration are the inputs of photosynthesis, and the outputs of photosynthesis are the inputs of respiration. The reactions are also complementary in the way they deal with energy.

Photosynthesis takes energy from the sun and stores it in the carbon-carbon bonds of carbohydrates; respiration releases that energy. Both plants and animals carry on respiration, but only plants (and other producers) can carry on photosynthesis. Respiration performed by plants and animals is called exothermic reaction and it involves the breaking down of glucose (or other organic molecules) into carbon dioxide and water.

The chief reservoirs for carbon dioxide are in the oceans and in rock. Carbon dioxide dissolves readily in water. Once there, it may precipitate (fall out of solution) as a solid rock known as calcium carbonate (limestone). Corals and algae encourage this reaction and build up limestone reefs in the process. On land and in the water, plants take up carbon dioxide and convert it into carbohydrates through photosynthesis.

This carbon in the plants now has three possible fates. It can be liberated to the atmosphere by the plant through respiration; it can be eaten by an animal, or it can be present in the plant when the plant dies.

Animals acquire all their carbon in their food, and, because of this, all carbon in biological systems ultimately comes from plants (autotrophs). In the animal, the carbon also has the same three possible fates. Carbon from plants or animals that is released to the atmosphere through respiration will either be taken up by a plant in photosynthesis or dissolved in the oceans. When an animal or a plant dies, two things can happen to the carbon in it. It can either be respired by decomposers (or released to the atmosphere), or it can be buried intact and ultimately form coal, oil, or natural gas (fossil fuels).

Through combustion of organic material, which oxidizes the carbon, it contains, producing carbon dioxide (as well as other things, like smoke). Burning fossil fuels such as coal, petroleum products, and natural gas releases carbon that has been stored in the geosphere for millions of years. This is a major reason for rising atmospheric carbon dioxide levels. Through reactions of limestone. Limestone, marble and chalk are composed mainly of calcium carbonate. As deposits of these rocks are eroded by water, the calcium carbonate is broken down to eventually form, among other things, carbon dioxide and carbonic acid. Production of cement and lime is done by heating limestone, which produces a substantial amount of carbon dioxide.

Humans also have an impact on the carbon cycle because when we burn fossil fuels we release excess carbon dioxide into the atmosphere. This means that more carbon dioxide goes into the oceans, and more is present in the atmosphere. The excess of carbon dioxide in the atmosphere causes global warming, this is due to the carbon dioxide in the atmosphere allowing more energy to reach the Earth from the sun and trapping it, than it allows escaping from the Earth and out into space.

Nitrogen Cycle

The nitrogen cycle is one of the most difficult of the cycles to learn, simply because there are so many important forms of nitrogen, and because organisms are responsible for each of the inter-conversions. Remember that nitrogen is critically important in forming the amino portions of the amino acids, which in turn form the proteins of your body.

Proteins make up skin and muscle, among other important structural portions of your body, and all enzymes are proteins. Since enzymes carry out almost all of the chemical reactions in your body, it is easy to see how important nitrogen is. The principal reservoir of nitrogen is the atmosphere, which is about 78% nitrogen.

Nitrogen gas in the atmosphere is composed of two nitrogen atoms bound to each other. It is a non-reactive gas meaning it takes a lot of energy to get nitrogen gas to break up and combine with other things, such as carbon or oxygen.

Nitrogen gas can be taken from the atmosphere (fixed) in two basic ways. First, lightning provides enough energy to "burn" the nitrogen and fix it in the form of nitrate, which is a nitrogen with three oxygens attached. This process is replicated in fertilizer factories to produce nitrogen fertilizers.

The other form of nitrogen fixation is by nitrogen fixing bacteria, which use special enzymes instead of the massive amount of energy found in lightning to fix nitrogen. These nitrogen-fixing bacteria come in three forms: some are free-living in the soil; some form symbiotic, mutualistic with the roots of bean plants and other legumes; and the third form of nitrogen-fixing bacteria are the photosynthetic cyanobacteria (blue-green algae) which are found most commonly in water. All of these fix nitrogen, either in the form of nitrate or in the form of ammonia (nitrogen with three hydrogens attached).

Most plants can take up nitrate and convert it to amino acids. Animals acquire all of their amino acids when they eat plants (or other animals). When plants or animals die (or release waste), the nitrogen is returned to the soil. The usual form of nitrogen returned to the soil in animal wastes or in the output of the decomposers, is ammonia.

Ammonia is toxic, but fortunately, there are nitrite bacteria in the soil and in the water, which take up ammonia and convert it to nitrite, which is nitrogen with two oxygens. Nitrite is also somewhat toxic, but another type of bacteria, nitrate bacteria, will take nitrite and convert it to nitrate, which can be taken up by plants to continue the cycle. We now have a cycle set up in the soil (or water), but what returns nitrogen to the air? It turns out that there are denitrifying bacteria, which take the nitrate and combine the nitrogen back into nitrogen gas.

An example of the nitrogen cycle in a real world use would be the saltwater fish tank. Have you or some one you know ever tried to make a saltwater fish tank work properly and have fish thrive. It takes weeks to set up a salt-water tank, because you must have sufficient numbers of nitrite and nitrate bacteria present to detoxify the ammonia produced by the fish and decomposers in the tank. If you didnít, the ammonia levels in the tank will build up and kill the fish. This is usually not a problem in freshwater tanks for two reasons:

One, the pH in a freshwater tank is at a different level than in a saltwater tank. At the pH of a freshwater tank, ammonia is not as toxic. Secondly, there are more multicellular plant forms that can grow in freshwater, and these plants remove the ammonia from the water very efficiently. It is hard to get enough plants growing in a saltwater tank to detoxify the water in the same way. If you can get the nitrogen cycle working in your tank you will be able to sustain a wonderful little ocean ecosystem.

Experiments for Home and Classroom

Build a terrarium is going to be a small eco-system, just like the aquarium. In addition, this site has numerous other great ecology experiments to try.

Find out why does it get really hot inside a car parked in the sun. It's the greenhouse effect. Try this experiment to learn more about green house gases.

for Students, Parents and Teachers

Now let's do Practice Exercise 6-4 (top).


Next Page:  Stability in an Ecosystem (top)