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We have just been looking at the properties of intermediary metabolic enzymes and observing that changes were needed in enzyme structure to preserve functionality in the face of temperature change. Similarly, the proper environment within the cell with respect to substrates, cofactors, ionic concentrations, and all other properties must be maintained by the cell membrane as temperature changes. The cell membrane itself and its associated proteins govern what crosses it, how much, and the direction of net movement. Ultimately, the cell membrane is the biological barrier that allows the cell its limited autonomy. As a barrier, the membrane not only limits diffusion inward and outward but it also contains embedded transport proteins. The membrane’s effectiveness as a barrier and as a center for transport is highly dependent on temperature.

The cell membrane is critical to survival of an organism. If the membrane is breached, its highly regulated internal milieu will be compromised and the cell will die. If enough cells die, wherever they may be located, clearly the whole organism will be in trouble. More importantly, nerve, muscle, and sensory systems are totally dependent on ion transport for their functions. Propagation of signals down a nerve, contraction of muscle, and all sensory mechanisms require an intact, functioning, membrane transport system.

Figure 2.13 The relationship between the catalytic rate constant (K_cat) and adaptation temperature for A₄‐LDH orthologs from differently thermally‐adapted vertebrates.

Source: Hochachka and Somero (2002), figure 7.3 (p. 302). Reproduced with the permission of Oxford University Press.