Waxes are esters of very long fatty acids (26 to 34 carbons) and
monohydroxy alcohols. They serve as waterproof protective coatings
on the outsides of leaves and fruit, and on skin, fur, and feathers
of animals. They are classed as lipids because their long hydrocarbon
tails make them insoluble in water, but soluble in organic solvents
such as benzene.
Fatty acid derivatives are not the only kind of lipids. Terpenes
are volatile oils in plants that can be thought of as being built
from units of isoprene, C5H8 (above). Terpenes
extracted from geranium, citronella, bay, lemon, mint, pine, and
many other sources, are highly aromatic in the true meaning of the
word, but are not necessarily "aromatic" in the special
way that chemists have appropriated the term to indicate delocalized
ring compounds.
A few typical terpenes are shown here, with dashed lines to show
how each molecule can be divided into isoprene-sized units. The
b-carotene molecule, and the phytol tail
on the chlorophyll molecule are long-chain terpenes.
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