The atoms and molecules on the Earth cycle among the living and nonliving components of the biosphere.

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 through the processes of cell respiration and other life activities.

The number of organisms any environment can support is called its carrying capacity.

The carrying capacity of an environment is limited by the available energy, water, oxygen, and minerals, and by the ability of ecosystems to recycle the remains of dead organisms through the activities of bacteria and fungi.

Living organisms have the capacity to produce populations of unlimited size, but available resources in their environments are finite.

This restricts the growth of populations and produces competition between organisms.