the *electron transport chain. The
two-carbon acetyl coenzyme A (acetyl
CoA) reacts with the four-carbon oxaloacetate to form the six-carbon citrate. In a series of seven reactions,
this is reconverted to oxaloacetate
and produces two molecules of carbon dioxide. Most importantly, the
cycle generates one molecule of
guanosine triphosphate (GTP – equivalent to 1 ATP) and reduces three
molecules of the coenzyme *NAD to
NADH and one molecule of the coenzyme *FAD to FADH 2 . NADH and
FADH 2 are then oxidized by the electron transport chain to generate
three and two molecules of ATP respectively. This gives a net yield of 12
molecules of ATP per molecule of
acetyl CoA.
Acetyl CoA can be derived from
carbohydrates (via *glycolysis), fats,
or certain amino acids. (Other amino
acids may enter the cycle at different
stages.) Thus the Krebs cycle is the
central ‘crossroads’ in the complex
system of metabolic pathways and is
involved not only in degradation and
energy production but also in the
synthesis of biomolecules. It is
named after its principal discoverer,
Sir Hans Adolf *Krebs.
Kroll process A process for producing certain metals by reducing the
chloride with magnesium metal. It
can be used for titanium, e.g.
TiCl 4 + 2Mg → Ti + 2MgCl 2 .
It is named after William Kroll, who
devised the process in 1940.
krypton Symbol Kr. A colourless
gaseous element belonging to group
0 (the *noble gases) of the periodic
table; a.n. 36; r.a.m. 83.80; d. 3.73
g m
–3
; m.p. –156.6°C; b.p. –152.3°C.
Krypton occurs in air (0.0001% by volume) from which it can be extracted
by fractional distillation of liquid air.
Usually, the element is not isolated
but is used with other inert gases in
Ûuorescent lamps, etc. The element
has Üve natural isotopes (mass num309
krypton
k
-ketoglutarate
α
4C
4C
4C
4C
4C
6C
6C
GTP
CO 2
CO 2
5C
P
3C
2C
NADH + H +
NADH + H +
NADH + H +
FADH 2
glycolysis
pyruvate
acetyl CoA
oxaloacetate
citrate
isocitrate
NAD +
NAD +
malate
fumarate
FAD
succinate
succinyl CoA
NAD +
GDP
Krebs cycle
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two-carbon acetyl coenzyme A (acetyl
CoA) reacts with the four-carbon oxaloacetate to form the six-carbon citrate. In a series of seven reactions,
this is reconverted to oxaloacetate
and produces two molecules of carbon dioxide. Most importantly, the
cycle generates one molecule of
guanosine triphosphate (GTP – equivalent to 1 ATP) and reduces three
molecules of the coenzyme *NAD to
NADH and one molecule of the coenzyme *FAD to FADH 2 . NADH and
FADH 2 are then oxidized by the electron transport chain to generate
three and two molecules of ATP respectively. This gives a net yield of 12
molecules of ATP per molecule of
acetyl CoA.
Acetyl CoA can be derived from
carbohydrates (via *glycolysis), fats,
or certain amino acids. (Other amino
acids may enter the cycle at different
stages.) Thus the Krebs cycle is the
central ‘crossroads’ in the complex
system of metabolic pathways and is
involved not only in degradation and
energy production but also in the
synthesis of biomolecules. It is
named after its principal discoverer,
Sir Hans Adolf *Krebs.
Kroll process A process for producing certain metals by reducing the
chloride with magnesium metal. It
can be used for titanium, e.g.
TiCl 4 + 2Mg → Ti + 2MgCl 2 .
It is named after William Kroll, who
devised the process in 1940.
krypton Symbol Kr. A colourless
gaseous element belonging to group
0 (the *noble gases) of the periodic
table; a.n. 36; r.a.m. 83.80; d. 3.73
g m
–3
; m.p. –156.6°C; b.p. –152.3°C.
Krypton occurs in air (0.0001% by volume) from which it can be extracted
by fractional distillation of liquid air.
Usually, the element is not isolated
but is used with other inert gases in
Ûuorescent lamps, etc. The element
has Üve natural isotopes (mass num309
krypton
k
-ketoglutarate
α
4C
4C
4C
4C
4C
6C
6C
GTP
CO 2
CO 2
5C
P
3C
2C
NADH + H +
NADH + H +
NADH + H +
FADH 2
glycolysis
pyruvate
acetyl CoA
oxaloacetate
citrate
isocitrate
NAD +
NAD +
malate
fumarate
FAD
succinate
succinyl CoA
NAD +
GDP
Krebs cycle
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