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.
Pressure
Even though pressure is the most predictable variable in the ocean, increasing by 1
