Environmental Engineering Reference
In-Depth Information
Nucleic acids (DNA and RNA) are linear unbranched assemblies of nucleotides
(pyrimidines and purines), and compared to the other three biopolymers they are
present in biomass in only trace amounts (always less than 0.1% of dry-matter
mass). They occur in highest concentration in meat and i sh and in relatively high
concentration in some cereals and vegetables (Imai don and Sosulski 1990). Their
role is qualitative rather than quantitative as they store, transmit, and process genetic
information and direct protein synthesis; no less important, they take part in cellular
signaling and act as cofactors in enzymatic reactions, and adenosine triphosphate
is the principal carrier of biochemical energy in all organisms.
Proteins , composed of amino acids , have several fundamental roles: as ubiquitous
enzymes they catalyze biochemical reactions; as hormones they stimulate and
coordinate specii c activities; as antibodies they help to guard organisms against
antigens; as transport media (hemoglobin, cytochromes) they shunt molecules
among tissues; as storage proteins (milk's casein, eggs' albumin) they act as deposits
of amino acids; as structural proteins (keratin, collagen, elastin) they form many
longlasting cover (hair, feathers) and connective (tendons, ligaments) tissues; and as
contractile proteins (actin, myosin) they enable muscle motion. Proteins are the
dominant constituents of all metabolizing heterotrophic tissues, but their presence
in phytomass is relatively limited (with the highest concentration in seeds), and they
are virtually absent from woody matter, as well as from many vegetables and fruits.
To synthesize their body proteins, humans must ingest nine essential amino acids .
All food proteins contain these amino acids, but only the proteins of heterotrophic
origin have them in ratios that are optimal for human growth and maintenance of
metabolizing tissues; plant proteins are always dei cient in one essential amino acid
(cereals in lysine, legumes in cysteine), and hence a healthy strict vegetarian diet
requires a combination of different plant foods. This reality explains the importance
of legumes in all traditional (overwhelmingly vegetarian) agricultural societies, as
well as the rapid increases in consumption of animal foods (and the concurrent
decline in legume intake) that have characterized the universal dietary transition
brought about by higher incomes.
Lipids are composed of long chains of fatty acids and are used as building blocks
of cell membranes and internal organs, in cellular signaling, in the supply of fat-
soluble vitamins, and as the most important energy stores (adipose tissue) in het-
erotrophs: their energy density of 39 kJ/g is unmatched by any other biopolymer.
They are present in fairly high concentrations in some plant tissues (particularly
in seeds and nuts as polyunsaturated, monounsaturated, and saturated oils) and
in even higher amounts in large animal bodies (mostly as subcutaneous and
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