In the anabolic stream of metabolism, larger
compounds are formed from smaller ones.
Chemical reductions play a predominant role
in these reactions. An example is the
formation of complex carbohydrates from
lactate or carbon dioxide.
Catabolism is the opposite type of process,
the breakdown of more complex compound
into smaller ones. Chemical oxidation
breaking down more and different structural compounds. Eventually the substance of
every organism ends up in its surrounding again, either in the form of energy, for instance
as the effect of muscular action, the fluorescent light of certain fishes, etc., or as
substance, as excretory products or when the living organism finally dies.
different types of compounds? Are different types of energy
stored in the different types of bonds?
mental processes like thinking, emotions, or intentions? Is that
also released from structures in the organism that are broken
the other. Usually several key reactions of the anabolic pathway need different enzymes
and/or produce different intermediate compounds than those used in catabolism.
Anabolism and catabolism can take place in the same cell, but they often take place in
provides an example of a metabolic cycle
between organs. The so-called Cori cycle
comprises glycolysis in the exercising
muscle and gluconeogenesis in the liver.
In strenuous exercise, glucose in the
skeletal muscle is broken down
anaerobically to lactate (glycolysis) for
extra directly available energy. The blood
transports lactate to the liver. In the liver,
glucose can be built back up from lactate
(gluconeogenesis). This can be stored as
glycogen or transported back to the muscle
by the blood to be used as an energy
source for muscle contraction again.
alternate cyclically between organs in the
The aerobic part of metabolism begins in the mitochondrion. In the eight steps of the
citric acid cycle, which takes place in the mitochondrion, the metabolites of carbohydrates,
proteins and lipids are finally oxidized to carbon dioxide and water. This catabolic process
results in the formation of large amounts of biochemical energy in the form of adenosine
triphosphate (ATP). To complete this process, some energy-carrying compounds that are
formed in the citric acid cycle have to go through oxidative phosphorylation in the inner
energy to ATP with the help of the reduction
of O2 to H2O. These are for instance NADH
(the reduced form of nicotinamide adenine
dinucleotide) and FADH2 (the reduced form
of flavin adenine dinucleotide). The citric
acid cycle is also the starting point for
gluconeogenesis. And it provides intermediates
for the synthesis of proteins and
lipids and for the heme group of
hemoglobin. In the citric acid cycle anabolic
and catabolic pathways connect. Cycles
interconnect with other cycles with the citric
acid cycle at the center.
Acetyl-CoA and the citric acid cycle
Acetyl-CoA is the molecule that starts the citric acid cycle by binding to its ”end” product
oxaloacetate. All metabolites of carbohydrates and lipids enter this cycle as acetyl-CoA,
The metabolic process of the human organism normally starts with breakdown products
from the intestines that become available for anabolic or catabolic reactions. Anabolic
reactions build up the substances of the organism. Anabolic reactions enable the
organism to ‘get substance’ and take form. The organism becomes visible like a standing
wave in a creek.
Larger compounds contain bond energy that holds together the compounds’ structure.
This energy can be made available by catabolic reactions for those functions of the
organism, which require directly available energy. The metabolic flow of living organisms
ends with the catabolism of its substance, which frees energy for the organism’s functions
(such as anabolic biochemical processes, bio-electrical, bio-mechanical and active
The citric acid cycle, which takes place in mitochondria, is at the center of oxidative
breakdown and can also be the starting point for the reductive synthesis of large
compounds in the organism. Acetyl-CoA plays a major role at this center of metabolic
The metabolic cycle in plants begins with the breakdown of water. Green plants obtain
their energy from breaking down water with the help of solar energy. The oxidation of
water provides the energy required in the plant to build up its organism. Plants in turn are
an important part of the food cycle in nature. They supply animals and humans with