1.2
Lipid Bilayers
Lipid molecules have a very low solubility in water as individual entities. When their concentration as
monomers exceeds a very low limit called the critical association concentration (typically <1 nM; see
Tanford 1980), they self-associate in such a way that their hydrophobic chains come together, leaving
only the polar heads exposed to water. The 3D organizations that lipids can adopt in aqueous solutions
depend, in particular, on their shape. In lipids such as phosphatidylcholine (PC; Fig. 1.1), the cross
section of the molecules, taken normal to their long axis, is about the same at the level of the polar head
and the acyl chains, making the molecule roughly cylindrical. When such molecules assemble side
by side, they therefore tend to form a flat sheet, one face of which is hydrophilic, the other hydrophobic. Two such sheets whose hydrophobic faces come together form a lipid bilayer, in which the
acyl chains are removed from the contact with water (Table 1.1, line ③; see e.g. Marčelja 1974).
Because the edges of a patch of bilayer expose hydrophobic groups, they tend to come together,
forming a closed vesicle. This simple mechanism is one of the essential features of life. Indeed, the
mere exposure of such lipids to water will create vesicles and, therefore, define closed aqueous
compartments. Whereas a lipid bilayer is very thin (typically 5–6 nm; see Fig. 1.3), its innermost
region is extremely hydrophobic and will not let ions or polar molecules diffuse rapidly from one
aqueous compartment to the other. It is therefore possible, given a minimal input of energy, to maintain
concentration differences between the inside and outside of lipid vesicles, a process that is fundamental
to biology, and on which is based a broad range of biological functions.
A lipid bilayer-based membrane, the plasma membrane, separates the aqueous solution inside
the cells (the cytosol
1 ) from the exterior medium. It is the only membrane in many bacteria, whether
eubacteria or archaebacteria, as well as in enveloped viruses, where it has been ripped off the plasma
membrane of the cell the virus budded from. In Gram-negative (Gram
À
) bacteria (so called because of
their characteristic appearance following a staining procedure developed by bacteriologist H.G. Gram),
a second membrane, the outer membrane, surrounds the cell, protecting the plasma membrane from
direct exposure to the outside medium and defining an intermembrane space (the periplasm), in which
resides the peptidoglycan, a mesh-like external cytoskeleton that protects the cell against osmotic
shocks. The outer membrane contains proteins that select which molecules are imported into the
periplasm and which are left out (see § 1.3). In eukaryotic cells, the inside of the cell (the cytoplasm) is
divided into a multitude of compartments separated from the cytosol by lipid bilayer-based membranes
(Fig. 1.2). Each of these compartments has its own membrane and luminal composition and set of
functions. A constant traffic of vesicles, which bud from one compartment and fuse with another,
ensures the transport, from one cell compartment to another or toward the plasma membrane and the
extracellular medium, of water-soluble molecules contained within the compartments, as well as that of
amphipathic molecules associated with their membrane.
Some of the compartments inside eukaryotic cells have a complex structure, being derived from
symbiotic organisms that have become integrated and have evolved to fulfill special functions. Thus,
mitochondria, in which respiration takes place, feature a double membrane, the outer one playing to
some extent the role of an outer bacterial membrane. The inner one, in which the respiratory complexes
that ensure electron transfer from reducing substrates to molecular oxygen reside, is convoluted into
tubes or sacculi (the cristae) so as to increase the surface available (cf. Fig. 1.2). Chloroplasts, which
are derived from photosynthetic bacteria, have a two-membrane exterior envelope, and their aqueous
1 The cytosol is usefully defined as “that portion of the cell which is found in the supernatant fraction after centrifuging an
homogenate at 105,000 Â g for 1 hour” (Clegg 1983), that is, essentially, a solution devoid of cytoskeleton, membrane
fragments, DNA, etc. but comprising most water-soluble proteins.
4
1 Membrane Proteins and Their Natural Environment
Lipid Bilayers
Lipid molecules have a very low solubility in water as individual entities. When their concentration as
monomers exceeds a very low limit called the critical association concentration (typically <1 nM; see
Tanford 1980), they self-associate in such a way that their hydrophobic chains come together, leaving
only the polar heads exposed to water. The 3D organizations that lipids can adopt in aqueous solutions
depend, in particular, on their shape. In lipids such as phosphatidylcholine (PC; Fig. 1.1), the cross
section of the molecules, taken normal to their long axis, is about the same at the level of the polar head
and the acyl chains, making the molecule roughly cylindrical. When such molecules assemble side
by side, they therefore tend to form a flat sheet, one face of which is hydrophilic, the other hydrophobic. Two such sheets whose hydrophobic faces come together form a lipid bilayer, in which the
acyl chains are removed from the contact with water (Table 1.1, line ③; see e.g. Marčelja 1974).
Because the edges of a patch of bilayer expose hydrophobic groups, they tend to come together,
forming a closed vesicle. This simple mechanism is one of the essential features of life. Indeed, the
mere exposure of such lipids to water will create vesicles and, therefore, define closed aqueous
compartments. Whereas a lipid bilayer is very thin (typically 5–6 nm; see Fig. 1.3), its innermost
region is extremely hydrophobic and will not let ions or polar molecules diffuse rapidly from one
aqueous compartment to the other. It is therefore possible, given a minimal input of energy, to maintain
concentration differences between the inside and outside of lipid vesicles, a process that is fundamental
to biology, and on which is based a broad range of biological functions.
A lipid bilayer-based membrane, the plasma membrane, separates the aqueous solution inside
the cells (the cytosol
1 ) from the exterior medium. It is the only membrane in many bacteria, whether
eubacteria or archaebacteria, as well as in enveloped viruses, where it has been ripped off the plasma
membrane of the cell the virus budded from. In Gram-negative (Gram
À
) bacteria (so called because of
their characteristic appearance following a staining procedure developed by bacteriologist H.G. Gram),
a second membrane, the outer membrane, surrounds the cell, protecting the plasma membrane from
direct exposure to the outside medium and defining an intermembrane space (the periplasm), in which
resides the peptidoglycan, a mesh-like external cytoskeleton that protects the cell against osmotic
shocks. The outer membrane contains proteins that select which molecules are imported into the
periplasm and which are left out (see § 1.3). In eukaryotic cells, the inside of the cell (the cytoplasm) is
divided into a multitude of compartments separated from the cytosol by lipid bilayer-based membranes
(Fig. 1.2). Each of these compartments has its own membrane and luminal composition and set of
functions. A constant traffic of vesicles, which bud from one compartment and fuse with another,
ensures the transport, from one cell compartment to another or toward the plasma membrane and the
extracellular medium, of water-soluble molecules contained within the compartments, as well as that of
amphipathic molecules associated with their membrane.
Some of the compartments inside eukaryotic cells have a complex structure, being derived from
symbiotic organisms that have become integrated and have evolved to fulfill special functions. Thus,
mitochondria, in which respiration takes place, feature a double membrane, the outer one playing to
some extent the role of an outer bacterial membrane. The inner one, in which the respiratory complexes
that ensure electron transfer from reducing substrates to molecular oxygen reside, is convoluted into
tubes or sacculi (the cristae) so as to increase the surface available (cf. Fig. 1.2). Chloroplasts, which
are derived from photosynthetic bacteria, have a two-membrane exterior envelope, and their aqueous
1 The cytosol is usefully defined as “that portion of the cell which is found in the supernatant fraction after centrifuging an
homogenate at 105,000 Â g for 1 hour” (Clegg 1983), that is, essentially, a solution devoid of cytoskeleton, membrane
fragments, DNA, etc. but comprising most water-soluble proteins.
4
1 Membrane Proteins and Their Natural Environment