Although no definition of life is completely satisfactory, biologists agree that the ability to absorb and convert energy is a basic characteristic of life. Thus, the energy required for the growth and organized maintenance of individual cells, entire organisms, and even communities is a fundamental of biology. The study of the energy flow and energy transformations among living systems is called bioenergetics.
The need for energy is a fundamental characteristic of living things. Energy is the capacity to do work or to cause change. The energy living organisms use to do work is called free energy. Examples of free energy are the energy plants use for growing and producing food or the energy you use for exercise and thinking. Free energy becomes available in an organism when energy stored in one form changes to another form during chemical reactions. Organisms store energy in the organic molecules from which organisms are made. Such energy is known as chemical energy, and the portion of this chemical energy available to do work is free energy. Living cells need a constant source of free energy to do three basic types of work: chemical, transport, and mechanical. Chemical work includes constructing and breaking down large complex molecules, such as proteins and nucleic acids, in chemical reactions. Organizing these molecules into the larger structural components of cells, such as those of muscle and skin, is also chemical work. Transport work involves the movement and concentration of the raw materials, or nutrients, necessary to construct complex molecules and to increase cellular organization during growth. Mechanical work includes movement, such as the muscle contraction that enables you to kick the ball.
How do organisms obtain energy and nutrients? Some organisms, known as heterotrophs (hetero=other, troph=feed), obtain energy and nutrients from other organisms, either living or dead. Animals, fungi (mushrooms and molds), and most bacteria are heterotrophs. Other organisms obtain energy and nutrients from nonliving sources such as the sun, soil, and air; they are autotrophs (auto=self). Autotrophs include plants, certain bacteria, and other organisms that capture energy from the sun and use it to synthesize organic compounds from inorganic materials adsorbed from their surroundings. The system of chemical reactions through which autotrophs capture energy from sunlight and use it to synthesize organic compounds from carbon dioxide and water is photosynthesis.
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