Functional Characteristics of Living Things
Precise definitions of life, especially when dealing with viruses and other microorganisms are often difficult. Although there is a wide variety of life on Earth, especially with regard to outward structure and form (morphology), all living organisms, both plant and animal, share a few fundamental functional characteristics that together define life processes.
The long-term survival of any species is, of course, dependent upon the ability to reproduce. Although selected individuals and population groups may be temporarily prevented from reproduction or chose not to reproduce, the capacity to reproduce--and to produce subsequent generations that are also capable of reproduction--is essential to the survival of every species.
In some form, all living things show an ability respond to environmental stimulation. The response may result in gross movement, but a response may also be reflected only in a subtle, internal change in the organism's physiology.
Living organisms at all levels must be able to assure the integrity of their structure and physiology by maintaining some sort of boundary to the external environment. This boundary ranges from cell membranes (e.g., the outer cell membrane provides a protective barrier and interface with the extracellular environment) to large systems such as the skin and epithelial covering of the human integumentary system that are designed to retain tissue moisture and provide a barrier to infection.
Boundary maintenance is also critical to the maintaining homeostatic balance. Without boundaries, organisms would be unable to maintain stability in their internal environments or to be selective in their responses to physiological stimuli. For example, species show a wide variety of physiological heat reduction mechanisms when confronted with elevated temperatures in the external environment.
Responsiveness to external stimulus is often characterized as an organism's irritability.
Organisms must also obtain nutrients from the external environment. Schemes range from simple nutrient absorption to aggressive hunting and digestion of foods that provide needed nutrients. Regardless of how they physically obtain nutrients, all organisms must possess the appropriate metabolic biochemical apparatus to accomplish digestion and to convert and utilize the chemical energy contained in nutrients.
Nutrients can contribute to an organism's growth and development in one of three basic ways. The nutrients become raw materials for growth. For example, calcium is a raw material need to promote and maintain bone in the human skeletal system. Essential amino acids, a building block of protein, are those that the human body cannot reproduce via the metabolic breakdown other nutrients. Accordingly, essential amino acids must be available in the diet. Nutrients can also enter into a variety of biochemical pathways, associating with enzymes and reacting with other substances as part of an organism's metabolism. Finally, nutrients may facilitate reactions within an organism's particular metabolic pathways.
Organisms must also be able to excrete waste. This elimination of metabolic waste is an essential part of the homeostatic mechanisms designed to maintain levels of nutrients, water, and oxygen within tolerable limits and within a temperature range conducive to the reactions taking place within metabolic pathways. Including in the elimination of waste is the elimination of excess fluid.
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