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214 Organelles
Structure and components B. Membrane lipids
The illustration shows amodel of asmall
A. Structure of the plasma membrane section of a membrane. The phospholipids
are the most important group of membrane
Allbiologicalmembranes are constructed ac-
cording to a standard pattern. They consist of lipids. They include phosphatidylcholine
(lecithin), phosphatidylethanolamine, phos-
a continuous bilayer of amphipathic lipids ap- phatidylserine, phosphatidylinositol, and
proximately 5 nm thick, into which proteins sphingomyelin (for their structures, see
are embedded. In addition, some membranes p. 50). In addition, membranes in animal cells
also carry carbohydrates (mono- and oligo-
saccharides) on their exterior, which are also contain cholesterol (with the exception
bound to lipids and proteins. The proportions of inner mitochondrial membranes). Glycoli-
pids (a ganglioside is shown here) are mainly
of lipids, proteins, and carbohydrates differ found on the outside of the plasma mem-
markedly depending on the type of cell and brane. Together with the glycoproteins, they
membrane (see p. 216).
Membrane lipids are strongly amphipathic form the exterior coating of the cell (the gly-
molecules with a polar hydrophilic “head cocalyx).
group” and an apolar hydrophobic “tail.” In
membranes, they are primarily held together C. Membrane proteins
by the hydrophobic effect (see p. 28) and
weak Van der Waals forces, and are therefore Proteins can be anchored in or on membranes
mobile relative to each other. This gives mem- in various ways. Integral membrane proteins
cross right through the lipid bilayer. The sec-
branes a more or less fluid quality.
The fluidity of membranes primarily de- tions of the peptide chains that lie within the
pends on their lipid composition and on tem- bilayer usually consist of 20 to 25 mainly
perature. At a specific transition temperature, hydrophobic amino acid residues that form a
right-handed α-helix.
membranes pass from a semicrystalline state
to a more fluid state. The double bonds in the Type I and II membrane proteins only
alkyl chains of unsaturated acyl residues in contain one transmembrane helix of this
type, whiletypeIII proteins contain several.
the membrane lipids disturb the semicrystal-
line state. The higher the proportion of unsa- Rarely, type I and II polypeptides can aggre-
gate to form a type IV transmembrane pro-
turated lipids present, therefore, the lower the tein. Several groups of integral membrane
transition temperature. The cholesterol con-
tent also influences membrane fluidity. While proteins—e. g., theporins (seep.212)—pene-
trate the membrane with antiparallel β-sheet
cholesterol increases the fluidity of semicrys-
talline, closely-packed membranes, it stabil- structures. Due to its shape, this tertiary
structure is known as a “β-barrel.”
izes fluid membranes that contain a high pro-
portion of unsaturated lipids. Type V and VI proteins carry lipid anchors.
Like lipids, proteins are also mobile within These are fatty acids (palmitic acid, myristic
the membrane. If they are not fixed in place acid), isoprenoids (e. g., farnesol), or glycoli-
by special mechanisms, they float within the pids such as glycosyl phosphatidylinositol
(GPI) that are covalently bound to the peptide
lipid layer as if in a two-dimensional liquid;
biological membranes are therefore also de- chain.
Peripheral membrane proteins are associ-
scribedasbeing a “fluidmosaic.” ated with the head groups of phospholipids
Lipids and proteins can shift easily within
one layer of a membrane, but switching be- or with another integral membrane protein
(not shown).
tween the two layers (“flip/flop”)is not possi-
ble for proteins and is only possible with dif-
ficulty for lipids (with the exception of cho-
lesterol). To move to the other side, phospho-
lipids require special auxiliary proteins
(translocators, “flipases”).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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