Chapter Summary

Certain conditions, including the availability of required chemical elements, a steady influx of solar energy, mild surface temperatures, the presence of liquid water, and a suitable atmosphere, are essential for life on earth. Only the earth in our solar system has all these conditions. Life requires and makes possible highly organized exchanges of matter and energy between organisms and their environments.

Matter is the observable material of which the universe is composed. It generally exists in three interchangeable phases: gas, liquid, and solid. Matter is made up of atoms, which are composed of particles called protons, neutrons, and electrons. The energy of bonds that holds atoms together forms the basis for energy transfers in the bodies of living organisms and, therefore, in the biosphere.

A steady influx of solar radiation provides the heat and light energy needed to support life in the biosphere. Water, which covers approximately three-fourths of the earth’s surface, is a remarkable substance. Because of its unique characteristics, it stabilizes the biosphere’s temperature and provides the medium in which life processes occur. The earth’s atmosphere provides gases necessary for life, helps maintain surface temperatures, and filters out dangerous radiation.

Ecosystem dynamics are governed by physical laws, including the law of conservation of matter and the first and second laws of thermodynamics. The recycling of matter is the basis of the cycles of elements that occur in ecosystems. Unlike matter, energy is not cycled. Energy always flows through systems in a one-way process in which some energy is converted from a high-quality, concentrated form to a lower-quality, less useful, dispersed form. We describe this increase in disorder as entropy, a fundamental limit to life. In ecosystems, solar energy enters the system and is converted to chemical energy by the process of photosynthesis. The chemical energy stored in the bonds that hold food molecules together is available for metabolism of organisms.

A species is all the organisms of the same kind that are genetically similar enough to breed in nature and produce live, fertile offspring. The populations of different species that live and interact within a particular area at a given time make up a biological community. An ecosystem is composed of a biological community together with all the biotic and abiotic factors that make up the environment in a defined area. Although ecosystem boundaries may be rather arbitrary, the holistic or systems approach to biology has provided rich insights into who lives where, when, how, and why.

Matter and energy are processed through the trophic levels of an ecosystem via food chains and food webs. At each energy transfer point less energy is available to work because of the laws of thermodynamics, so energy must be supplied to an ecosystem continuously. The relationships between producers and consumers in an ecosystem, often depicted as pyramids, demonstrate this principle. Most of the energy that enters an ecosystem comes, ultimately, from the sun.

The biosphere is a source of large quantities of essential elements. In a given ecosystem, these elements are constantly used and reused by living organisms. Water, carbon, nitrogen, sulfur, and phosphorus, for instance, are recycled in ecosystems through complex biogeochemical cycles.