Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards

Environmental Cycles

Movement of Elements and Compounds among Earth’s Reservoirs
The earth is a system containing essentially a fixed amount of each stable chemical atom or element. Each element can exist in several different chemical reservoirs. Each element on earth moves among reservoirs in the solid earth, oceans, atmosphere, and organisms as part of geochemical cycles.

Movement of matter between reservoirs is driven by the earth’s internal and external sources of energy. These movements are often accompanied by a change in the physical and chemical properties of matter. Carbon, for example, occurs in carbonate rocks such as limestone, in the atmosphere as carbon dioxide gas, in water as dissolved carbon dioxide, and in all organisms as complex molecules that control the chemistry of life.

Further Description:

Earth is a system containing essentially fixed amounts of each stable chemical element, except radioactive elements and the stable end products of radioactive decay processes. Water as a compound exists almost in a fixed amount on Earth. Each of these substances can exist in and move cyclically among several different chemical "reservoirs" in the solid earth, hydrosphere, atmosphere, and biosphere.

Movement of matter between "reservoirs" is driven by Earth’s internal and external sources of energy. These movements are often accompanied by physical and chemical changes. Carbon, for example, occurs in rocks as limestone, in the atmosphere as a gas, in water as dissolved carbon dioxide, in coal and petroleum as hydrocarbons, in mines as diamond, and in all living things as organic molecules. Water occurs in various physical states and chemical mixtures as it cycles on and through the solid earth, atmosphere, hydrosphere, and biosphere. Rocks, too, progress through a cycle in which different minerals are produced in different chemical and physical milieus.

Chemical cycles are common in nature and tend to recycle resources through the atmosphere, hydrosphere, solid earth, and biosphere. Elements from a nonliving environment (abiotic), such as rocks, air, and water, are cycled into the bodies of living organisms and then back into the nonliving environment. For example, nitrogen that once may have been part of a dinosaur may eventually form part of human DNA and still later become part of a tomato. The main geochemical cycles are carbon, phosphorous, nitrogen, water, and oxygen.

The carbon cycle involves the processes of photosynthesis and respiration. Carbon dioxide plays an important role in photosynthesis. Plants use energy from light to split water molecules; they then use carbon dioxide to synthesize carbohydrates. One of the products of this reaction is oxygen. Photosynthesis is the major source of oxygen in Earth’s atmosphere. For some 1.5 billion years before green plants were on Earth, algae and bacteria provided the photosynthesis needed to build Earth’s oxygen levels to the point that respiration of both plants and animals could occur.

Respiration, which occurs in both plants and animals, breaks up food molecules to obtain energy. It is the opposite process to photosynthesis, and at the cellular level it is connected to photosynthesis as a consequence of evolution. Photosynthesis and respiration form a major mechanism for the cycling of carbon. Part of the carbon cycle also involves the storage of carbon in the form of fuels and the storage of excess carbon dioxide in saltwater as carbonate. Carbon found in fuels is released through combustion. Carbonate is a main ingredient in the shells of ocean creatures and a large group of sedimentary rocks.

Nitrogen is an element that is often a limiting factor for plant growth. Although atmospheric nitrogen is abundant, it is not in a form that plants can readily access. The nitrogen molecule found in the atmosphere must be split and recombined with atoms to form molecules that are soluble in water. This is called nitrogen fixation. Typically nitrogen is fixed in the form of ammonium or nitrate ions. Some of this fixing occurs in the atmosphere due to lightning. Most nitrogen is fixed by bacteria. When plants and other organisms die, through leaching and the activities of other bacteria the nitrogen is returned to the atmosphere ready to begin the cycle again.

The water cycle is powered by the sun and cycles water through the oceans, glaciers, groundwater, freshwater rivers, and lakes. Through the processes of evaporation, transpiration, condensation, precipitation, infiltration, and runoff, water is cycled through our system and very little new water is added.

Oxygen also is involved in a cycle in which water is split through the process of photosynthesis to produce oxygen and carbon dioxide is used to synthesize carbohydrates. This oxygen is then used by animals and plants through respiration and by oxidation processes like burning to produce carbon dioxideCwhich, in turn, is needed for photosynthesis. Oxygen is capable of combining with almost all other elements and forms many oxides. These oxides can be found in rocks and minerals.

Phosphorous is involved in a lesser known chemical cycle. As these elements and compounds move through their cycles, physical and chemical changes occur. Water easily changes state, and many of the elements form related ions and complex compounds. Rocks are involved in many of these cycles. In a rock cycle, rock is transformed from one rock type into another. All of these cycles ultimately are powered by the sun.

Concepts Needed:

Grade 9

Chemical cycles, hydrologic cycle, transpiration, perspiration

Grade 10

Carbon cycle, precipitation, limestone, fossil fuels, coal, petroleum, porosity, permeability, traps, water budget, energy requirements

Grade 11

Condensation, evaporation, freezing, precipitation, latent heat of condensation

Grade 12

Cyclones, energy transfer

Empirical Laws or Observed Relationships:

Chemical reactions: oxidation-reduction, carbon cycle, rock cycle, nitrogen cycle, hydrologic cycle

Theories or Models:

First and second laws of thermodynamics, models of cycles

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Micro-Unit Description:

Environmental Cycles
Based on their knowledge of chemistry and biology, students should understand various cycles and their associated energy transformations. They should examine how weather is affected, as well as perspiration cooling of animals, the transpiration effects on plants, and the conditions that drive storms.

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