F-430 A tetrapyrrole structure containing nickel, a
component of the ENZYMEmethyl-coenzyme M reduc-
tase, which is involved in the formation of methane in
methanogenic bacteria. The highly reduced macrocyclic
structure, related to PORPHYRINs and CORRINs, is
termed a CORPHIN.
See alsoMETHANOGENS;OXIDOREDUCTASE.

facilitated diffusion A process by which carrier pro-
teins, also called permeases or transporters or ion chan-
nels, in the cell membrane transport substances such as
glucose, sodium, and chloride ions into or out of cells
down a concentration (electrochemical) gradient; does
not require the use of metabolic energy.
See also ACTIVE TRANSPORT; VOLTAGE-GATED
CHANNELS.

facultative anaerobe A facultative anaerobe is a
microorganism that makes ATPby aerobic respiration if
oxygen is present but, if absent, switches to fermenta-
tion under anaerobic conditions.

facultative organism Any organism that changes a
metabolic pathway to another when needed.

facultative saprophyte Any organism that is usually
parasitic but can also live as a SAPROPHYTE.

facultative symbiont Any organism that chooses a

symbiotic relationship with a host only if the relation-

ship presents itself but is not physiologically required

to do so for survival.

Fahrenheit, Daniel Gabriel(1686–1736) German

Instrument Maker, Physicist Daniel Gabriel Fahren-

heit, a German instrument maker and physicist, was

123

F

Bacillus cereusshowing hemolysis on sheep-blood agar.B.

cereusis a gram-positive beta hemolytic bacteria that can live in

an environment with or without the presence of oxygen (i.e., a

facultative anaerobe).(Centers for Disease Control/Courtesy of

Larry Stauffer, Oregon State Public Health Laboratory)