Objectives
Students will have opportunities to:
a.
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Trace the cycling of nitrogen, oxygen, carbon-dioxide and water through the biosphere with the aid of diagrams.
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b.
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Explain the flow of energy through an ecosystem using examples.
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Approximate time: one week
Nitrogen Cycle
All living things need nitrogen to help build proteins, DNA, and certain other body chemicals. However, in spite of being surrounded by nitrogen gas, most organisms cannot use this free nitrogen in the air. Most living things can use only nitrogen that is combined with other elements by nitrogen fixing bacteria. Useable nitrogen is also added to an ecosystem by lightning, which converts atmospheric nitrogen and oxygen into nitrate. Other sources of nitrogen are erosion of rocks rich in nitrates and the decomposition of organic matter.
The nitrogen cycle is the process by which nitrogen in the air is changed to a form which can be taken up and used by plants to make proteins. Organisms that eat plants use the plant proteins to make their own proteins. When plants and animals die, decomposers break down the proteins of dead organisms, changing them into ammonia which can be used by plants or returned to the air. Thus, nitrogen is returned to the atmosphere when it is once more released as a gas by denitrifying bacteria that break up organic molecules for their food releasing any nitrogen they contain as gas. (See diagram below.)
Decomposers are very essential to the ecosystem. Firstly, they rid the environment of dead plant and animal matter and secondly return substances such as nitrogen, carbon, phosphorous, sulphur and magnesium to the environment. These substances are then used by other plants to make food and the cycle continues. If the nutrients were not returned to the environment, organisms within that ecosystem could not survive for long. A few kinds of bacteria can take nitrogen directly from the air and form nitrogen compounds. This process is called nitrogen-fixation. Some of these nitrogen-fixing bacteria live in the soil. Others grow on the roots of plants known as legumes including beans, peas and peanuts. The bacteria therefore supply the plants with usable nitrogen.
Oxygen and Carbon-Dioxide Cycle
Plants use carbon-dioxide and produce oxygen during photosynthesis. Plants, animals and other organisms produce water and carbon-dioxide during respiration. N.B. See diagram below.
During photosynthesis, plants return oxygen to the atmosphere as a by-product which animals use up during respiration. Carbon-dioxide is also added to the air by the decomposition of organic matter, and the burning of coal, oil and gas. Plants balance this action by carrying on photosynthesis as well as respiration.
Water Cycle
Water is the molecule that supports life. Most of the water on earth is found in lakes, streams and oceans. Surface water in these lakes, streams and oceans is heated by the sun and turns to water vapor, a gas. This process is called
Evaporation
. The water vapor then rises up into the air. In the upper atmosphere, water vapor cools and changes into liquid droplets. Eventually, the droplets fall back to the surface as rain and snow. This process is called
Precipitation
. This circulation of water from the atmosphere to the earth and back to the atmosphere is called the water cycle. From pure, clean water to waste water, back to pure, clean water again, is still another kind of water cycle. Even the land part of the earth itself is part of a cycle. The soil that forms a thin layer on much of the land part of the earth was once rock. Through long years of grinding, wearing away by winds, or the action of animals and plants that lived on the rock, smaller and smaller pieces of rock were changed to soil. Soil is also composed of organic matter, decomposed animals and plants. Since the plants which supply our food depend on the soil, it is very important for people to conserve the supply. Just by growing things may deplete the quality and the supply, and the water washes much of the soil toward the sea. Thus, the human animal has interfered also with the soil and rock cycle.
Year after year, the cycles of nature continue. In addition to the cycles of their physical environments, the complicated food chain supports a balance of life on our planet Earth. The balance fluctuates, depending on the number of natural enemies and the conditions that favor certain complex interrelationships between plants and animals, and between prey and predator, exists in all kinds of ecosystems.
Supplies are very limited in this closed system and it makes a great deal of sense for us to turn to recycling as a means of survival. We must learn to use and reuse, on a larger and bigger scale to prevent an imbalance in the ecosystem.