Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Trophic Pyramids and Levels
Cycles in the biosphere and Energy Flow through Ecosystems
Energy flows through ecosystems in one direction, from photosynthetic organisms to herbivores to carnivores and decomposers.
The cycling of nutrients in any ecosystem is essential to maintain a balance in that ecosystem. Ecosystems are dependent upon resources that are used by organisms and the recycling of wastes disposed by them. Essentially the same atoms and molecules are being used over and over again. Nutrient cycling of organic and inorganic substances takes place in all ecosystems. Nutrients critical for maintaining homeostasis in any ecosystem are carbon, nitrogen, and phosphorus.
The carbon cycle begins with atmospheric CO2 , which is absorbed by plants. CO2 is fixed through photosynthesis into glucose, which is used by all organisms in respiration to produce ATP. In respiration, CO2 is released as a waste product and sent back to the atmosphere. This cycle sets up the basic food chain for all ecosystems. CO2 is released into the atmosphere through decomposition and combustion as well, adding to the reservoir of CO2 necessary to continue this cycle.
The nitrogen cycle is based upon the action of the decomposers in soil. Decomposers have the ability to convert nitrogen wastes and dead organic matter into a usable form for plants. In addition, a special group of nitrogen-fixing bacteria can convert nitrogen gas from the atmosphere into nitrates.
The phosphorous cycle is a sedimentary cycle; that is, rock containing small amounts of phosphorous is eroded and phosphorous then becomes available to plants. Animals take in phosphorous from the food web, use it in making ATP and DNA, and through excretion, release it as a waste. Bound-up phosphorous is released into the soil by decomposers.
Water, essential for all life, must also be cycled. The cycling of water occurs through evaporation, condensation, and precipitation. The sun causes evaporation from the oceans and lakes as well as transpiration from plants. Water is carried to the atmosphere condensed and falls in the form of rain or snow. The amount of precipitation helps define the type of ecosystem that will exist in a particular geographic region.
The fuel for ecosystems is the sunís energy. This radiant energy is captured by plants and converted into a usable form of chemical energy, namely glucose. Energy flow in ecosystems occurs when the primary producers (plants) carrying this energy are eaten by herbivores (animals that eat only plants), which in turn are eaten by carnivores (animals that eat only animals). This sets up what is known as a food chain. Food chains, however, are usually rare. In most ecosystems food chains usually become food webs. A web occurs when one or more levels of the food chain interconnect with other levels for their food supply. Food webs are usually complex and can define niche adaptations in ecosystems.
Biotic, abiotic, biosphere, natural resources, organic compound and inorganic compound, decomposition, condensation, precipitation, microorganisms, photosynthesis, carbon cycles, energy flow, producers, consumers, herbivores, carnivores, omnivores, food chains, food webs, habitat, niche, decomposers
Predator-prey dynamics, carbon cycle, ecosystem, trophic structure
Phosphorous cycle, carbon cycle, nitrogen cycle, human waste disposal and fossil fuel consumption
Decomposers, trophic levels, heat budgets, energetics
Conservation of matter, first and second laws of thermodynamics
Trophic pyramids, food web, hydrologic cycle, carbon cycle, nitrogen cycle, phosphorous cycle