7.3 Strategies and Mechanisms of Stress Tolerance

There is so much diversity in biological systems, and this diversity extends to stress tolerance. We have seen that stress tolerance can differ among species and over time, so it is useful to have a framework in which we can organize our understanding of stress tolerance. One framework is that of strategies and mechanisms, which we will explore in this chapter section. This framework helps us better understand the similarities and differences in stress tolerance among organisms.

Stress Tolerance Strategies

In the context of stress tolerance, a strategy is an overall “approach” to dealing with the stress, without getting into the details of how that approach is carried out. For example, let’s consider the serious stress of being a human that lives in a house that doesn’t contain enough chocolate for you and all your housemates. What could someone do to tolerate this stressful situation? One strategy would be to increase the amount of chocolate in the house so that everyone’s chocolate needs are met. An alternative strategy would be to decrease everyone’s chocolate consumption, so that it doesn’t matter if there is a low chocolate supply. A third option would be to not share the chocolate with your housemates and keep it all to yourself (so you can eat the amount of chocolate you need to eat). All of these are general approaches, with our any detail about how this would happen. (Where could you buy more chocolate? How would you cope with decreased chocolate consumption? How would you hide the chocolate from your housemates?) In the context of a biological stress (e.g., low oxygen availability, high temperatures), organisms do not necessarily make a conscious “choice” about their stress tolerance strategies, but we can still categorize their stress tolerance into strategies. We will see examples of various stress tolerance strategies in the chapters that follow.

Stress Tolerance Mechanisms

So what about the details? In the context of stress tolerance, mechanisms refer to the biological processes an organism uses to carry out its strategy. If we come back to our example of the chocolate problem, let’s talk about some potential mechanisms for the strategy of increasing the amount of chocolate in the house. You could, for example, go to a store and buy more chocolate using mechanisms like walking or riding your bike (to go to the store) and paying with cash or card (to buy the chocolate). An alternative mechanism for the same strategy would be to make some chocolate at home, using the appropriate ingredients and a recipe. A third mechanism for the strategy (which the authors do not condone) would be to steal some chocolate from a neighbouring house. All of these mechanism are different, but could support the same strategy (increase the amount of chocolate in the house). In the context of a biological stresses (e.g., low oxygen, high temperature), there are usually multiple possible mechanisms that can support a stress tolerance strategy. One organism might use multiple mechanisms, and those mechanisms may be similar or different to other organisms. We will see examples of various stress tolerance mechanism in the chapters that follow.

Relationship between Strategies and Mechanisms

As you can probably infer from the text and figure above, strategies and mechanisms are interrelated. Generally, the strategy is the broad approach to being stress-tolerant, while the mechanisms describe how that strategy is carried out. Mechanisms can be understood at different levels of biological organization. For example, if you are going to get more chocolate, we can think broadly about how you might get to the store; walking and biking (cycling) are alternative mechanisms that could be involved in this strategy, and are mechanisms at the whole organism level (How does the organism move?). We can also think about the physiological mechanisms involved, for example the different muscles that must contract to facilitate walking or biking, and how your respiratory and circulatory systems support those muscle contractions. These physiological processes are also all mechanisms supporting the strategy of getting more chocolate. And we can think about cellular mechanisms, for example how the proteins in your muscle cells are interacting to support muscle contraction, or how oxygen is getting into your muscle cells to support the metabolism that underlies muscle contraction. These are additional mechanisms that support the strategy of getting more chocolate. There are often many mechanisms that support a single strategy, and in the following chapters we will focus largely on the cellular mechanisms!