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Figure 2.14 gives a schematic representation of a membrane, showing embedded proteins and the lipid bi‐layer that makes up the bulk of the functional membrane. Membranes may be thought of as a sheet of phospholipids, individually represented in the figure as a spherical head with two kinked tails that face toward the middle of the lipid bi‐layer. The two sets of tails form the inner, hydrophobic, portion of the lipid bi‐layer. The backbone of the phospholipid is a glycerol molecule (Figure 2.14). Two fatty acid chains are attached to the glycerol to form the inwardly facing kinky tails, or hydrophobic, portion of the molecule, and a head group is attached to the remaining carbon on the glycerol molecule via a phosphate group (Figure 2.14). The result is a lipid molecule with a hydrophilic head and hydrophobic tails. Phospholipids are named for the head group they contain: one with choline as its head group would be named phosphatidylcholine.

To preserve its function as a protective barrier and transport center, a membrane must strike a balance between being too fluid (sol) and too solid (gel). Too fluid, and it would not be able to act as a barrier or to maintain cellular integrity, and embedded proteins would not have the structural backbone needed to aid in transport across it. Too solid, and transport proteins, which have to change conformations to function, would be constricted, and the lipid membrane itself would be more like a crystalline lattice and more prone to leakage. The phospholipids making up the fabric of the membrane are not one single molecular species but are several that, together with aggregate proteins, are organized into domains covering the membrane surface. Structure and function of the membrane differ between domains.

Figure 2.14 Membrane structure. (A) Schematic representation of a membrane showing the lipid bi‐layer structure with embedded proteins; (B) chemical structure and a three‐dimensional representation of a phospholipid molecule showing the glycerol and phosphate components of the hydrophilic head group and hydrophobic tails that face inward in membranes; (C) saturated vs. unsaturated fatty acids, (a) three molecules of stearic acid (18 carbons, no double bonds, melting temperature 69.6 °C) that pack tightly because of their linear geometries, (b) the addition of a single molecule of oleic acid (18 carbons, one double‐bond, melting temperature 56 °C) to a lipid membrane prevent tight packing of the molecules due to the bending of the tail of the unsaturated molecule.

The degree of fluidity of any membrane is determined by the melting point of the fatty acid chains in its phospholipids, and there is a great deal of diversity among them (Table 2.2). You will notice that the fatty acids with the lowest melting points (e.g. linoleic and linolenic) have multiple double bonds. As regards melting points, the “double‐bondedness,” or saturation state, of a fatty acid is its most critical feature. Why? Because fatty acids with double bonds introduce kinks into the tails of the phospholipids that prevent them from packing tightly (Figure 2.14). The kinks effectively weaken the weak bond interactions between the fatty acid chains, thus lowering the melting point of the lipid. It follows that species living in the cold, that need to maintain membrane fluidity at low temperatures to allow their nerves and muscles to function properly, have lipids rich in double bonds. That is to say, they are unsaturated: the ratio of C to H in the chains is less than it would be if all the bonds between the carbons were single bonds.

The idea that membranes need to maintain an optimum fluidity to allow transport proteins to function properly is the concept of homeoviscosity (Cossins and Bowler 1987; Hochachka and Somero 2002). Mammals and birds have lipids that melt at much higher temperatures than an Antarctic fish. As observed above, the melting point of a membrane lipid correlates inversely with the degree of unsaturation of its fatty acid chains. In fact, a plot of percent‐unsaturation vs. adaptation temperature for a group of species from differing thermal environments shows a highly coupled drop in percent‐unsaturation of lipids with adaptation temperature (Figure 2.15). Thus, the degree of unsaturation of species’ lipids can be added to our growing list of characteristics that change with a species’ thermal environment.

Interestingly, just as we see changes in lipid characteristics with a species’ habitat temperature, biochemical mechanisms that are usually considered evolutionary adaptation also exist for short‐term change in membrane lipids to accommodate more rapid changes in temperature. Figure 2.16 shows a fairly rapid change in lipid classes of trout gill membranes when they were moved acutely from 5 to 20 °C and vice versa. In each case, the ratio of phosphatidylcholine (PC) to phosphatidyl ethanolamine (PE) changed profoundly over about five days. You may rightly wonder why changing the head group of a membrane lipid would make much difference. The answer is that PE tends to have fatty acid chains with a greater degree of unsaturation than does PC, affording a greater degree of fluidity at lower temperature (Hochachka and Somero 2002).

Other mechanisms exist for adjusting the fluidity in biomembranes over the short term (hours to days to weeks) in addition to the change in lipid classes just described. Such a capability is particularly important to temperate species that must accommodate changes in temperature associated with seasonal cycles. In most instances, a need for change can be achieved through changes in the biosynthesis of lipids. An example is using enzymes that introduce double bonds into fatty acid chains to make them more suitable for use at cold temperature. Such enzymes are termed desaturases, and they can be up‐regulated quickly (Hochachka and Somero 2002).

It is most important to appreciate that not only do species’ membrane lipids vary greatly in character with the changes in habitat temperature typical of different zoogeographic regions, but considerable acclimation to temperature change by membrane lipids can also occur within a period of days to weeks. Such short‐term change can be considered part of the overall acclimation process that allows a species to adjust its upper and lower lethal limits (see Figure 2.2a, the tolerance polygon).

Table 2.2 Chemical formulas and melting points for a selection of saturated and unsaturated fatty acids.

Carbon atoms	Common name	Empirical formula	Chemical structure	Melting point (°C)
Saturated fatty acids
3	Propionic acid	C3H6O2	CH3CH2COOH	−22
12	Lauric acid	C12H24O2	CH3(CH2)10COOH	44
14	Myristic acid	C14H25O2	CH3(CH2)12COOH	54
16	Palmitic acid	C16H32O2	CH3(CH2)14COOH	63
18	Stearic acid	C18H36O2	CH3(CH2)16COOH	70
20	Arachidic acid	C20H40O2	CH3(CH2)18COOH	75
Unsaturated fatty acids
16	Palmitoleic acid	C16H30O2	CH3(CH2)5CH=CH(CH2)7COOH	−0.5
18	Oleic acid	C18H34O2	CH3(CH2)7CH=CH(CH2)7COOH	13
18	Elaidic acid	C18H34O2	CH3(CH2)7CH=CH(CH2)7COOH	13
18	Linoleic acid	C18H32O2	CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH	−5
18	Linolenic acid	C18H30O2	CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH	−10
20	Arachidonic acid	C20H32O2	CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH	−50

Figure 2.15 The relationship between adaptation temperature and percentage of unsaturated acyl chains in synaptosomal phospholipids of differently adapted vertebrates. Each symbol represents a different species. Open symbols denote phosphatidylethanolamine; filled symbols denote phosphatidylcholine.

Source: Hochachka and Somero (2002), figure 7.27 (p. 372). Reproduced with the permission of Oxford University Press.

Figure 2.16 Temperature acclimation and phospholipid class. Time course of change in the ratio of phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) in gill cell membranes of rainbow trout acclimating to the indicated temperatures. *indicates a statistically significant difference (P<0.05) compared to the day zero mean.

Source: Hazel and Carpenter (1985), figure 4 (p. 599). Reproduced with the permission of Springer.