NUTRITIONAL TYPE

category into which an organism may be placed according to the chemical nature of the nutrients it requires. Three separate classification schemes are commonly employed. One scheme is based on the chemical nature of the foods needed. Thus, organisms such as green plants and some bacteria that require only inorganic compounds are called autotrophic organisms, or autotrophs; heterotrophs, such as all animals and fungi and most bacteria, require organic as well as inorganic compounds. A second scheme is based on the nature of the energy source utilized by the organism. Those that trap radiant energy (light) and convert it to chemical energy in the form of adenosine triphosphate (ATP) are phototrophs, or photosynthetic organisms. Chemotrophs, or chemosynthetic organisms, utilize organic or inorganic compounds, such as oxygen, to satisfy their ATP requirements. A third scheme is based on the type of electron-donor material utilized to synthesize certain cell constituents. A lithotroph consumes inorganic compounds in this activity (e.g., in most plants, water is the ultimate source of electrons); in an organotroph, organic substances, such as carbohydrates, perform an analogous function. An organism may be classified according to more than one scheme. Thus, a green plant is photolithotrophic, certain bacteria are photoorganotrophic, and other bacteria are chemolithotrophic. Animals, most bacteria, and yeasts are chemoorganotrophic.