Ecology, the study of relationships between organisms and their environment, involves investigating how organisms acquire and use energy, nutrients, and water from their environment. Required conditions for life include chemical elements, availability of liquid water, and moderate temperatures.
Matter—the substance of which things are made—is not created or destroyed under normal circumstances. It is recycled endlessly through organisms and their environment. This principle is known as the conservation of matter.
Elements are identified by their atomic number, the number of protons in their nuclei. Atoms can have variable numbers of neutrons (isotopes) or electrons (ions). Four elements, oxygen, carbon, hydrogen, and nitrogen, make up more than 96 percent of most living organisms.
Acids are substances that readily release hydrogen ions in solution; bases are substances that readily bond with hydrogen atoms. Acids and bases play important roles in life processes, and they are common sources of environmental problems, such as acid mine drainage. Buffers, substances that absorb or release hydrogen ions, neutralize acids and bases.
Ionic or covalent bonds join atoms to produce compounds or molecules. Breaking bonds releases energy, including that used by living cells.
Water's remarkable properties make it both the medium for life processes and a key factor in our global climate.
Four major classes of organic compounds, hydrocarbons, carbohydrates, proteins, and nucleic acids, provide structure, function, and energy for cells.
Energy cannot be created or destroyed (the first law of thermodynamics), but it is constantly degraded to less useful forms—entropy increases—as it is used (the second law of thermodynamics).
Photosynthesis is the conversion of solar energy to chemical bonds in organic molecules, using carbon dioxide (from air) and water. Chlorophyll, the green pigment in plants, provides the energy for nearly all living things on earth. Photosynthesis involves a complex series of reactions, some light-dependent and others not dependent on light. Cellular respiration is the process of breaking down organic molecules to release energy.
Species are often (but not always) defined as all organisms that can breed and produce fertile offspring in nature. A population is all the members of a species that live in an area, and a biological community comprises all the populations in an area. An ecosystem is a biological community and its physical environment. Ecosystems are generally open systems: they exchange materials and energy with their surroundings.
Energy and nutrients flow through food webs or trophic levels in an ecosystem. Productivity is a measure of how rapidly an ecosystem accumulates biomass, or biological material. Organisms in a food web can be identified as primary producers or consumers; they can also be identified as herbivores, carnivores, omnivores, scavengers, detritivores, or decomposers. The concept of ecological pyramids is that there are many more individuals, and more biomass, at lower trophic levels that at higher trophic levels.
Materials, such as water, carbon, nitrogen, sulfur, and phosphorus, also cycle through organisms and their environment. Substances are stored in rocks, air, water, and living things for varying lengths of time. One of the major impacts of humans on our environment has been to alter the rates of material cycling.
