F-430 A tetrapyrrole structure containing nickel, a
component of the ENZYMEmethyl-coenzyme M REDUC-
TASE, that is involved in the formation of methane in
METHANOGENICbacteria. The highly reduced macro-
cyclic structure, related to PORPHYRINs and CORRINs, is
termed a CORPHIN.

face-centered cubic A cubic unit cell having
atoms, molecules, or ions at the corners and in the

center of each face; the unit cell produced by cubic

close packing.

facilitated diffusion A process by which carrier

proteins, also called permeases or transporters, or ion

channels in the cell membrane transport substances

such as glucose, sodium, and chloride ions into or out

of cells down a concentration (electrochemical) gradi-

ent; does not require the use of metabolic energy.

See also VOLTAGE-GATED CHANNELS; ACTIVE

TRANSPORT.

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.

Fahrenheit, Daniel Gabriel(1686–1736) German

Instrument maker, physicist Daniel Gabriel Fahren-

heit, a German instrument maker and physicist, was

born in Danzig, Germany (now Gdansk, Poland), in

1686, the oldest of five children. Fahrenheit’s major

contributions lay in the creation of the first accurate

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F