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Two types of components make up the biosphere and its ecosystems: One type, called abiotic, consists of nonliving components such as water, air, nutrients, rocks, heat, and solar energy.

aerobic respiration

Producers, consumers, and decomposers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells this energy is released by aerobic respiration, which uses oxygen to convert glucose (or other organic nutrient molecules) back into carbon dioxide and water.

anaerobic respiration, or fermentation

Some decomposers get the energy they need by breaking down glucose (or other organic compounds) in the absence of oxygen. This form of cellular respiration is called anaerobic respiration, or fermentation.

aquatic life zones

Scientists divide the watery parts of the biosphere into aquatic life zones, each containing numerous ecosystems.


a thin spherical envelope of gases surrounding the earth's surface.

biogeochemical cycle

(literally, life-earth-chemical cycles) or nutrient cycles—prime examples of one of the four scientific principles of sustainability


the dry weight of all organic matter contained in its organisms.


large regions such as forests, deserts, and grasslands, with distinct climates and certain species (especially vegetation) adapted to them.


consists of the parts of the earth's air, water, and soil where life is found.


Two types of components make up the biosphere and its ecosystems: The other type, called biotic, consists of living and once living biological components—plants, animals, and microbes.

carbon cycle

Carbon is the basic building block of the carbohydrates, fats, proteins, DNA, and other organic compounds necessary for life. It circulates through the biosphere, the atmosphere, and parts of the hydrosphere, in the carbon cycle.

cell theory

The idea that all living things are composed of cells is called the cell theory and it is the most widely accepted scientific theory in biology.


All organisms (living things) are composed of cells: the smallest and most fundamental structural and functional units of life.


A few producers, mostly specialized bacteria, can convert simple inorganic compounds from their environment into more complex nutrient compounds without using sunlight, through a process called chemosynthesis.

community, or biological community

consists of all the populations of different species that live in a particular place.


All other organisms in an ecosystem are consumers, or heterotrophs ("other-feeders"), that cannot produce the nutrients they need through photosynthesis or other processes and must obtain their nutrients by feeding on other organisms (producers or other consumers) or their remains.


primarily certain types of bacteria and fungi, are consumers that release nutrients from the dead bodies of plants and animals and return them to the soil, water, and air for reuse by producers. They feed by secreting enzymes that speed up the break down of bodies of dead organisms into nutrient compounds such as water, carbon dioxide, minerals, and simpler organic compounds.


Detritus feeders, or detritivores, feed on the wastes or dead bodies of other organisms, called detritus ("di-TRI-tus," meaning debris).

detritus feeders, or detritivores

feed on the wastes or dead bodies of other organisms, called detritus ("di-TRI-tus," meaning debris). Examples include small organisms such as mites and earthworms, some insects, catfish, and larger scavenger organisms such as vultures.

ecological efficiency

The percentage of usable chemical energy transferred as biomass from one trophic level to the next is called ecological efficiency.


(from the Greek words oikos, meaning "house" or "place to live," and logos, meaning "study of") is the study of how organisms interact with their living (biotic) environment of other organisms and with their nonliving (abiotic) environment of soil, water, other forms of matter, and energy mostly from the sun.


is a community of different species interacting with one another and with their nonliving environment of soil, water, other forms of matter, and energy, mostly from the sun.

eukaryotic cell

is surrounded by a membrane and has a distinct nucleus and several other internal parts called organelles, which are also surrounded by membranes.

food chain

A sequence of organisms, each of which serves as a source of food or energy for the next, is called a food chain.

food web

organisms in most ecosystems form a complex network of interconnected food chains called a food web

genetic diversity

In most natural populations, individuals vary slightly in their genetic makeup, which is why they do not all look or act alike. This variation in a population is called genetic diversity.


consists of the earth's intensely hot core, a thick mantle composed mostly of rock, and a thin outer crust.

greenhouse gases

The remaining 1% of the air includes water vapor, carbon dioxide, and methane, all of which are called greenhouse gases, because they trap heat and thus warm the lower atmosphere. Almost all of the earth's weather occurs in this layer.

gross primary productivity (GPP)

is the rate at which an ecosystem's producers (usually plants) convert solar energy into chemical energy as biomass found in their tissues.


The place where a population or an individual organism normally lives is its habitat.

hydrologic cycle, or water cycle

collects, purifies, and distributes the earth's fixed supply of water.


consists of all of the water on or near the earth's surface.

limiting factor principle

A variety of abiotic factors can affect the number of organisms in a population. Sometimes one or more factors, known as limiting factors, are more important in regulating population growth than other factors are. This ecological principle is called the limiting factor principle: Too much or too little of any abiotic factor can limit or prevent growth of a population, even if all other factors are at or near the optimal range of tolerance.

limiting factors

A variety of abiotic factors can affect the number of organisms in a population. Sometimes one or more factors, known as limiting factors, are more important in regulating population growth than other factors are.

natural greenhouse effect

Without this the earth would be too cold to support the forms of life we find here today.

net primary productivity (NPP)

is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration.

nitrogen cycle

Fortunately, two natural processes convert or fix N2 into compounds useful as nutrients for plants and animals. One is electrical discharges, or lightning, taking place in the atmosphere. The other takes place in aquatic systems, soil, and the roots of some plants, where specialized bacteria, called nitrogen-fixing bacteria, complete this conversion as part of the nitrogen cycle.


such as pigs, foxes, cockroaches, and humans, play dual roles by feeding on both plants and animals.

phosphorus cycle

Phosphorus circulates through water, the earth's crust, and living organisms in the phosphorus cycle.


Most producers capture sunlight to produce energy-rich carbohydrates (such as glucose, C6H12O6) by photosynthesis, which is the way energy enters most ecosystems.


is a group of individuals of the same species that live in the same place at the same time.

primary consumers, or herbivores

(plant eaters), are animals such as rabbits, grasshoppers, deer, and zooplankton that eat producers, mostly by feeding on green plants.


sometimes called autotrophs (self-feeders), make the nutrients they need from compounds and energy obtained from their environment.

prokaryotic cell

is also surrounded by a membrane, but it has no distinct nucleus and no other internal parts surrounded by membranes.

pyramid of energy flow

The more trophic levels there are in a food chain or web, the greater is the cumulative loss of usable chemical energy as it flows through the trophic levels. The pyramid of energy flow illustrates this energy loss for a simple food chain, assuming a 90% energy loss with each transfer.

range of tolerance

Each population in an ecosystem has a range of tolerance to variations in its physical and chemical environment. Individuals within a population may also have slightly different tolerance ranges for temperature or other factors because of small differences in genetic makeup, health, and age.

secondary consumers, or carnivores

(meat eaters), are animals such as spiders, hyenas, birds, frogs, and some zooplankton-eating fish, all of which feed on the flesh of herbivores.


For a group of sexually reproducing organisms, a species is a set of individuals that can mate and produce fertile offspring.


The next layer, stretching 17-50 kilometers (11-31 miles) above the earth's surface, is the stratosphere. Its lower portion contains enough ozone (O3) gas to filter out most of the sun's harmful ultraviolet radiation.

sulfur cycle

Sulfur circulates through the biosphere in the sulfur cycle.

third- and higher-level consumers

are carnivores such as tigers, wolves, mice-eating snakes, hawks, and killer whales (orcas) that feed on the flesh of other carnivores.


Over land, about 90% of the water that reaches the atmosphere evaporates from the surfaces of plants through a process called transpiration.

trophic level

Ecologists assign every organism in an ecosystem to a feeding level, or trophic level, depending on its source of food or nutrients.


a thin spherical envelope of gases surrounding the earth's surface. Its inner layer, the troposphere, extends only about 17 kilometers (11 miles) above sea level at the tropics and about 7 kilometers (4 miles) above the earth's north and south poles.

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