Page 55 - Color Atlas of Biochemistry

P. 55

46 Biomolecules

Overview are derivatives of the polyunsaturated fatty
acid arachidonic acid (see p. 390).

A. Classification
B. Biological roles
The lipids are a large and heterogeneous
group ofsubstances ofbiologicaloriginthat 1. Fuel. Lipids are an important source of en-
are easily dissolved in organic solvents such ergy in the diet. In quantitative terms, they
as methanol, acetone, chloroform, and ben- represent the principal energy reserve in ani-
zene. By contrast, they are either insoluble or mals. Neutral fats in particular are stored in
only poorly soluble in water. Their low water specialized cells, known as adipocytes.Fatty
solubility is due to a lack of polarizing atoms acids are released from these again as needed,
such as O, N, S, and P (see p. 6). and these are then oxidized in the mitochon-
Lipids can be classified into substances that dria to form water and carbon dioxide, with
are either hydrolyzable—i. e., able to undergo oxygen being consumed. This process also
hydrolytic cleavage—or nonhydrolyzable.Only gives rise to reduced coenzymes, which are
a few examples of the many lipids known can used for ATP production in the respiratory
be mentioned here. The individual classes of chain (see p.140).
lipids are discussed in more detail in the fol- 2. Nutrients. Amphipathic lipids are used
lowing pages. by cells to build membranes (see p. 214). Typ-
Hydrolyzable lipids (components shown in ical membrane lipids include phospholipids,
brackets). The simple esters include the fats glycolipids, and cholesterol. Fats are only
(triacylglycerol; one glycerol + three acyl res- weakly amphiphilic and are therefore not
idues); the waxes (one fatty alcohol + one acyl suitableas membranecomponents.
residue); and the sterol esters (one sterol + one 3. Insulation. Lipids are excellent insula-
acyl residue). The phospholipids are esters tors. In the higher animals, neutral fats are
with more complex structures. Their charac- found in the subcutaneous tissue and around
teristic component is a phosphate residue. various organs, where they serve as mechan-
The phospholipids include the phosphatidic ical and thermal insulators. As the principal
acids (one glycerol + two acyl residues + one constituent of cell membranes, lipids also in-
phosphate) and the phosphatides (one glyc- sulate cells from their environment mechan-
erol + two acyl residues + one phosphate + ically and electrically. The impermeability of
one amino alcohol). In the sphingolipids,glyc- lipid membranes to ions allows the formation
eroland oneacylresidue arereplaced by of themembranepotential (see p.126).
sphingosine. Particularly important in this 4. Special tasks. Some lipids have adopted
group are the sugar-containing glycolipids special roles in the body. Steroids, eicosa-
(one sphingosine + one fatty acid + sugar). noids, and some metabolites of phospholipids
The cerebrosides (one sphingosine + one fatty have signaling functions. They serve as hor-
acid + one sugar) and gangliosides (one sphin- mones, mediators, and second messengers
gosine + one fatty acid + several different (see p. 370). Other lipids form anchors to at-
sugars, including neuraminic acid) are repre- tach proteins to membranes (see p. 214). The
sentatives of this group. lipids also produce cofactors for enzymatic re-
The components of the hydrolyzable lipids actions—e. g., vitamin K (see p. 52) and ubiq-
are linked to one another by ester bonds.They uinone (see p.104). The carotenoid retinal, a
are easily broken down either enzymatically light-sensitive lipid, is of central importance
or chemically. in the process of vision (see p. 358).
Non-hydrolyzable lipids. The hydrocarbons Several lipids are not formed indepen-
include the alkanes and carotenoids.The lipid dently in the human body. These substances,
alcohols are also not hydrolyzable. They in- as essential fatty acids and fat-soluble vita-
clude long-chained alkanols and cyclic sterols mins, are indispensable components of nutri-
such as cholesterol, and steroids such as es- tion (see pp. 364ff.)
tradiol and testosterone. The most important
acids among the lipids are fatty acids. The
eicosanoids also belong to this group; these

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