How Plants Help Us Breathe
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Before we begin talking about the social issues involved with photosynthesis it would be helpful here to explain to students that the earth is considered a closed system. The earth is referred to as a closed eco system because all of the materials we need for life are here. We get no new infusions of material and so the earth must continually recycle these materials so that life can continue. We have already spoken about one of the cycles the carbon dioxide-oxygen cycle. It might be useful to also mention and illustrate two other important cycles.
What is the Nitrogen Cycle?
Nitrogen is also an important element in a living cell. There is nitrogen in the air but plants and humans cannot use it in the gaseous form. Plants get nitrogen when bacteria attach themselves to the roots of plants. The bacteria combine gaseous nitrogen and hydrogen to make a chemical compound called ammonia. Plants absorb the ammonia. Animals eat plants and get the nitrogen they rely on.14
What is the Water Cycle?
Water on the earth’s surface continually condenses in the sky and falls again as rain, snow or sleet. Living things need water to carry chemicals to cells and remove waste materials from cells. Plants release water through transpiration, a process in which water vapor is given off through leaves and into the air. Animals release water through exhalation, perspiration, and in waste products. This released water goes into the air and becomes part of the water cycle.15