Organisms can be classified according to how they get energy. Organisms that use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds are called autotrophs (AWT-oh-TROHFS). Most autotrophs use the process of photosynthesis to convert light energy from the sun into chemical energy in the form of organic compounds, primarily carbohydrates.
The tree in Figure 6-1 is an autotroph because it converts light energy from the sun into organic compounds. The tree uses these compounds for energy. The caterpillar also depends on the tree for energy, as it cannot manufacture organic compounds itself. Animals and other organisms that must get energy from food instead of directly from sunlight or inorganic substances are called heterotrophs (HEHT-uhr-oh-TROHFS). The bird is also a het-erotroph. The food that fuels the bird originates with an autotroph (the tree), but it passes indirectly to the bird through the caterpillar. In similar ways, almost all organisms ultimately depend on autotrophs to obtain the energy necessary to carry out the processes of life.
Photosynthesis involves a complex series of chemical reactions in which the product of one reaction is consumed in the next reaction. A series of chemical reactions linked in this way is referred to as a biochemical pathway.
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