6.1a Energy flows through ecosystems in one direction, typically from the Sun, through

photosynthetic organisms including green plants and algae, to herbivores to carnivores

and decomposers.

6.1b The atoms and molecules on the Earth cycle among the living and nonliving com-

ponents of the biosphere. For example, carbon dioxide and water molecules used in

photosynthesis to form energy-rich organic compounds are returned to the environment

when the energy in these compounds is eventually released by cells. Continual input of

energy from sunlight keeps the process going. This concept may be illustrated with an

energy pyramid.

6.1c The chemical elements, such as carbon, hydrogen, nitrogen, and oxygen, that make

up the molecules of living things pass through food webs and are combined and recom-

bined in different ways. At each link in a food web, some energy is stored in newly

made structures but much is dissipated into the environment as heat.

6.1d The number of organisms any habitat can support (carrying capacity) is limited by

the available energy, water, oxygen, and minerals, and by the ability of ecosystems to

recycle the residue of dead organisms through the activities of bacteria and fungi.

6.1e In any particular environment, the growth and survival of organisms depend on

the physical conditions including light intensity, temperature range, mineral availability,

soil/rock type, and relative acidity (pH).

6.1f Living organisms have the capacity to produce populations of unlimited size, but

environments and resources are finite. This has profound effects on the interactions

among organisms.

6.1g Relationships between organisms may be negative, neutral, or positive. Some

organisms may interact with one another in several ways. They may be in a

producer/consumer, predator/prey, or parasite/host relationship; or one organism may

cause disease in, scavenge, or decompose another.