PATHLIGHT PREPARATORY Unit 3: Ecosystems-What Are They? & How Do They Work?

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Lessons: 3.1, 3.2, 3.3, 3.4, and 3.5

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Whole mass of air surrounding the earth.


Innermost layer of the atmo- sphere. It contains about 75% of the mass of earth’s air and extends about 17 kilometers
(11 miles) above sea level.

greenhouse gases

Natural effect that releases heat in the atmosphere near the earth’s surface. Water vapor, carbon dioxide, ozone, and other gases in the lower atmosphere (troposphere) absorb some of the infrared radiation (heat) reflected by the earth’s surface. Their molecules vibrate and transform the absorbed energy into longer-wavelength infrared radiation in the troposphere.


Second layer of the atmosphere, extending about 17–48 kilometers (11–30 miles) above the earth’s surface. It contains small amounts of gaseous ozone (O3), which filters out about 95% of the incoming harmful ultraviolet (UV) radiation emitted by the sun.


Earth’s liquid water (oceans, lakes, other bodies of surface water, and underground water), frozen water (polar ice caps, floating ice caps, and ice in soil, known as permafrost), and water vapor in the atmosphere.


Earth’s intensely hot core, thick mantle composed mostly of rock, and thin outer crust that contains most of the earth’s rock, soil, and sediment.


Zone of the earth where life is found. It consists of parts of the atmosphere (the troposphere), hydrosphere (mostly surface water and groundwater), and lithosphere (mostly soil and surface rocks and sediments on the bottoms of oceans and other bodies of water) where life is found.


Biological science that studies the relationships between living organisms and their environment; study of the structure and functions of nature.




Living organisms.

trophic level

All organisms that are the same number of energy transfers away from the origi- nal source of energy, usually sunlight, that enters an ecosystem.

producers (autotrophs)

Organism that uses solar energy (green plants) or chemical energy (some bacte- ria) to manufacture the organic compounds it needs as nutrients from simple inorganic compounds obtained from its environment.


Complex process that takes place in cells of green plants. Radiant energy from the sun is used to combine carbon dioxide (CO2) and water (H2O) to produce oxygen (O2), carbohydrates (such as glucose, C6H12O6), and other nutrient molecules.

consumers (heterotrophs)

Organism that cannot synthesize the organic nutrients it needs and gets its organic nutrients by feeding on the tissues of producers.

primary consumers

Organism that feeds on all or part of plants (herbivore) or on other producers.


Plant-eating organism. Examples include deer, sheep, grasshoppers, and zooplank- ton.


Animal that feeds on other animals.

secondary consumers

Organism that feeds only on primary consumers.

tertiary consumers

Animals that feed on animal-eating animals. They feed at high trophic levels in food chains and webs.


Animal that can use both plants and other animals as food sources. Examples include pigs, rats, cockroaches, and humans.


Organism that digests parts of dead organisms and cast-off fragments and wastes of living organisms by breaking down the complex organic molecules in those materials into simpler inorganic compounds and then absorbing the soluble nutrients. This process returns most of these chemicals to the soil and water for reuse.

detritus feeders

Organism that extracts nutrients from fragments of dead organisms and their cast-off parts and organic wastes.


Consumer organism that feeds on detritus, parts of dead organisms, and cast- off fragments and wastes of living organisms.

aerobic respiration

Complex process that occurs in the cells of most living organisms,
in which nutrient organic molecules such as glucose (C6H12O6) combine with oxygen (O2) to produce carbon dioxide (CO2), water (H2O), and energy.

food chain

Series of organisms in which each
eats or decomposes the preceding one.

food web

Complex network of many inter- connected food chains and feeding relationships.

pyramid energy cycle (flow)

Diagram representing the flow of energy through each trophic level in a food chain or food web. With each energy transfer, only a small part (typically 10%) of the usable energy entering one trophic level is trans- ferred to the organisms at the next trophic level.

Gross Primary Productivity (GPP)

Rate at which an ecosystem’s producers capture and store a given amount of chemical energy as biomass in a given length of time.

Net Primary Productivity (NPP)

Rate at which all the plants in an ecosystem produce
net useful chemical energy; equal to the differ- ence between the rate at which the plants in an ecosystem produce useful chemical energy (gross primary productivity) and the rate at which they use some of that energy through cellular respiration.

biogeochemical cycle

Natural processes that recycle nutrients in various chemical forms from the nonliving environment to living organisms and then back to the nonliving environment. Examples include the carbon, oxygen, nitrogen, phosphorus, sulfur, and hydrologic cycles.

hydrologic cycle (water cycle)

A cycle that collects, purifies, and distributes the earth’s fixed supply of water from the environment to living organisms and then back to the environment.


Process in which water is absorbed by the root systems of plants, moves up through the plants, passes through pores (stomata) in their leaves or other parts, and evaporates into the atmosphere as water vapor.

carbon cycle

Cyclical movement of carbon in different chemical forms from the environment to organisms and then back to the environment.

phosphorus cycle

Cyclic movement of phosphorus in different chemical forms from the environment to organisms and then back to the environment.

sulfur cycle

Cyclic movement of sulfur in various chemical forms from the environment to organisms and then back to the environment.

nutrient cycling

The circulation of chemicals necessary for life, from the environment (mostly soil and water) through organisms and back to the environment.