All organisms need to transform and interconvert a
vast number of organic compounds to enable them
to live, grow, and reproduce. They need to provide
themselves with energy in the form of ATP,
and a supply of building blocks to construct their
own tissues. An integrated network of enzymemediated
and carefully regulated chemical reactions
is used for this purpose, collectively referred
to as intermediary metabolism, and the pathways
involved are termed metabolic pathways. Some of
the crucially important molecules of life are carbohydrates,
proteins, fats, and nucleic acids. Apart
from fats, these are polymeric materials. Carbohydrates
are composed of sugar units, whilst proteins
are made up from amino acids, and nucleic acids
are based on nucleotides. Organisms vary widely
in their capacity to synthesize and transform chemicals.
For instance, plants are very efficient at synthesizing
organic compounds via photosynthesis
from inorganic materials found in the environment,
whilst other organisms such as animals and
microorganisms rely on obtaining their raw materials
in their diet, e.g. by consuming plants. Thus,
many of the metabolic pathways are concerned
with degrading materials taken in as food, whilst