Page 225 - Color Atlas of Biochemistry

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216 Organelles

Functions and composition p. 140) and photosynthesis (see p. 128)—also
occur in membranes.
The most important membranes in animal 5. Interactions with other cells for the pur-
cells are the plasma membrane, the inner poses of cell fusion and tissue formation, as
and outer nuclear membranes, the mem- well as communication with the extracellular
branes of the endoplasmic reticulum (ER) and matrix.
the Golgi apparatus, and the inner and outer 6. Anchoring of the cytoskeleton (see
mitochondrial membranes. Lysosomes, peroxi- p. 204) to maintain the shape of cells and
somes, and various vesicles are also separated organelles and to provide the basis for move-
from the cytoplasm by membranes. In plants, ment processes.
additional membranes are seen in the plastids
and vacuoles. All membranes show pola-
B. Composition of membranes
rity—i. e., there is a difference in the composi-
tion of the inner layer (facing toward the Biological membranes consist of lipids, pro-
cytoplasm) and the outer layer (facing away teins,and carbohydrates (see p. 214). These
from it). components occur in varying proportions
(left). Proteins usually account for the largest
proportion, at around half. By contrast, carbo-
A. Functions of membranes
hydrates, which are only found on the side
Membranes and their components have the facing away from the cytoplasm, make up
following functions: only a few percent. An extreme composition
is seen in myelin, the insulating material in
1. Enclosure and insulation of cells and or-
ganelles. The enclosure provided by the nerve cells, three-quarters of which consists
of lipids. By contrast, the inner mitochondrial
plasma membrane protects cells from their
environment both mechanically and chemi- membrane is characterized by a very low pro-
portion of lipids and a particularly high pro-
cally. The plasma membrane is essential for portion of proteins.
maintaining differences in the concentration
When the individual proportions of lipids
of many substances between the intracellular in membranes are examined more closely
and extracellular compartments. (right part of the illustration), typical patterns
2. Regulated transport of substances, for particular cells and tissues are also found.
which determines the internal milieu and is The illustration shows the diversity of the
a precondition for homeostasis—i. e., the
maintenance of constant concentrations of membrane lipids and their approximate
quantitative composition. Phospholipids are
substances and physiological parameters.
Regulated and selective transport of substan- predominant in membrane lipids in compar-
ison with glycolipids and cholesterol.Triacyl-
ces through pores, channels, and transporters glycerols (neutral fats) are not found in mem-
(see p. 218) is necessary because the cells and
organelles are enclosed by membrane sys- branes.
Cholesterol is found almost exclusively in
tems. eukaryotic cells. Animal membranes contain
3. Reception of extracellular signals and substantially more cholesterol than plant
transfer of these signals to the inside of the membranes, in which cholesterol is usually
cell (see pp. 384ff.), as well as the production
of signals. replaced by other sterols. There is no choles-
terol at all in prokaryotes (with a few excep-
4. Enzymatic catalysis of reactions. Impor-
tant enzymes are located in membranes at the tions). The inner mitochondrial membrane of
eukaryotes is also low in cholesterol, while it
interface between the lipid and aqueous is theonlymembranethatcontainslarge
phases. This is where reactions with apolar amounts of cardiolipin. These facts both sup-
substrates occur. Examples include lipid port the endosymbiotic theory of the devel-
biosynthesis (see p. 170) and the metabolism
of apolar xenobiotics (see p. 316). The most opment of mitochondria (see p. 210).
important reactions in energy conver-
sion—i. e., oxidative phosphorylation (see

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