An individual entity that has all of the characteristics of life described above and possessing the same hereditary information in all its cells.
A group of individuals of the same species living in the same environment and actively interbreeding.
A group of interacting populations occupying the same environment.
To understand the last several items, it is necessary in addition to define environment: the physical and chemical surroundings in which individuals live. Strictly speaking, the environment does not have boundaries. In practice, of course, it is common to restrict the discussion to a particular region, such as a single forest, a particular glen within that forest, or just the soil in a glen in a forest.
The first half of this book is organized roughly along the lines of the hierarchy. A more detailed description of the items in the hierarchy follows.
Metabolism This is the view of life at the chemical level. The study of chemical reactions in organisms is called biochemistry. The reactions can be classified into (1) energy conversion reactions, (2) synthesis and breakdown of cell material, and (3) reactions associated with specific functions such as reproduction or motility.
The energy conversion reactions include those that capture energy from sunlight (photosynthesis) and convert it into food or that convert food energy into a simple form that is available for other reactions (respiration, fermentation). The synthesis reactions include production of the components of the major constituents of cells, such as proteins, carbohydrates, and lipids (which include fats), or minor ones (in terms of mass), such as nucleic acids, hormones, and vitamins. Reactions that break molecules down into simpler forms may facilitate the removal of waste products.
All of the metabolic reactions are connected to each other by a web of metabolic pathways, which consists of sequences of biochemical reactions. A particular compound can often participate in several reactions, thus becoming a link between those pathways.
Organelles Some of the specialized functions carried out by these substructures include protein synthesis, photosynthesis (the conversion of light energy to chemical energy), and respiration (the release of chemical energy into a form usable by cells). The organelles that perform these functions are the ribosomes, the chloroplasts, and the mitochondria, respectively. There are many other organelles, especially in higher organisms. Not all cells have all types of organelles.
Cells All living things are made of cells. Some are composed of a single cell and are called unicellular. Organisms made of more than one cell are called multicellular. A key structure of all cells is the plasma membrane. This separates the cell contents from its environment and forms the boundary of many of the cell's organelles. There are two primary types of cells. The prokaryote is the simpler of the two. It has no internal membrane structures, and the only organelle is the ribosome. Bacteria are typical prokaryotes. The other type is the eukaryote, which has internal membrane-bound structures such as mitochondria and chloroplasts. All the plants and animals are eukaryotic organisms, as are some single-celled organisms such as protozoans and algae.
Tissues These are present only in multicellular organisms, of course. Examples include muscle tissue, bone tissue, or nerve tissue. Tissues should not be confused with organs. Although they may be composed predominantly of one type of tissue, organs may also be formed from a variety of tissues.
Organs This is a complete unit such as an entire muscle, a bone, or the brain. Many organs serve a single function. Others organs have multiple functions, such as the pancreas and the adrenal gland. Both of these glands produce more than one hormone, which have unrelated functions.
Organ Systems Some organs work in concert with each other and form a system. Examples are the nervous system, the circulatory system, the respiratory system, the skeletal system, and the endocrine system.
Organism An organism is a living thing with a single genome (the set of hereditary information contained in all its cells' nuclear regions), which exists, for at least part of its life cycle, in the environment and separately from others of its kind. Some organisms, such as the algae and the fungi in lichens, must live together, but they have different genomes. Others live in colonies but exist separately at an earlier stage of their lives. An example of the latter is the jellyfish, which is really a colony of organisms that started out as free-swimming larvae. The individual cells of a jellyfish may come from different parents, yet it is difficult not to think of a jellyfish as a single organism. This is another example of the difficulty in making distinctions in biology.
Population Examples of populations include all of the striped bass that breed in the Hudson River (although they spend most of their lives in the Atlantic Ocean), and the Nitrosomonas europaea bacteria in a biological wastewater treatment plant.
Sometimes the definition of population is extended to any group of organisms sharing a characteristic, such as the population of invertebrates (animals lacking an internal skeleton) in the soil, or the population of deer ticks that carry the Lyme disease bacteria. Thus, in this usage, a population may either incorporate more than one species or may be a subset of a species. Although such usage is common, it should be avoided. It would be more proper to label multiple-species groups as populations (note the plural) and to call the subset of a species a subpopulation.
Community Strictly speaking, a community comprises all of the organisms in a region. Thus, an earthworm may be considered a member of a number of communities, depending on the region defined: It may be a member of the soil community, the forest community, or the community of organisms on a particular island in a lake.
The word community is sometimes used in a looser sense, similar to the second usage of the word population described above. For example, some may speak of the community of soil bacteria when they are concerned about the interactions among those members, even though there are other species in the same environment. Again, it would be more precise to use the term populations.
Ecosystem The ecosystem is the most inclusive unit studied in biology. Again, the term may be restricted to local systems, such as a particular lake, or an island and nearby waters. Strictly speaking, since all of these systems are interconnected, there is really only one known ecosystem—the Earth's. This all-inclusive ecosystem is also known as the biosphere.
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