7.1 What is Stress?

For any cell to function properly, it needs to control its internal environment so that important biological processes can occur. For example, the cytosol of most cells is around pH 7 (neutral) because an acidic (pH < 7) or basic (pH > 7) environment would inhibit many biochemical reactions. Cells therefore put substantial energy reserves into maintaining homeostasis: a steady (relatively unchanging) state of physical and chemical internal conditions. If the external environment changes (e.g., pH increases), cells can respond to ensure the internal environment remains the same (e.g., cytosol with pH 7). Homeostasis can occur at the cellular level, or at the level of a whole multicellular organism (e.g., temperature of a human body). Not every parameter is tightly controlled in every organism. For example, most mammals (including humans) and birds maintain their body at a constant warm temperature, but most other animals, plants, fungi, and unicellular organisms do not. However, every organism needs homeostasis in some form so their cells function properly.

Definitions of Stress

A stressor is anything (e.g., a change in environmental conditions) that causes stress to an organism. ^[1]There are many ways to define stress itself. In some animals, stress can be defined physiologically – there are certain hormones that are released into the bloodstream during stress (e.g., cortisol), so stress can be defined as a state in which there are high circulating levels of stress hormones. This is a less useful definition for single cells, which do not have a circulatory system. Stress can also be defined in terms of an organism’s fitness. If a stressor reduces the ability of an organism to perform normal activities required for survival or reproduction, we can say that the organism is stressed. However, fitness is often difficult to measure for a single stressor, especially because organisms may experience many stressful events over their reproductive lifetime. The broadest definition of stress is a deviation from homeostasis (Figure 7.2) caused by a change in environment. A stress response is then something that the organism does to return to the homeostatic state (Figure 7.2).

Let’s consider an example of stress using the “deviation from homeostasis” definition. Most cells use lots of ATP (energy) to tightly control the concentration of sodium ions in the cytosol of cells. There are some normal biological processes (e.g., neuron function in animals) that require changes in sodium ion concentrations inside the cell – this change in sodium concentration would not be considered a stress. However, if the concentration of sodium ions inside a fish’s cells suddenly increased because that fish moved from freshwater into salt water (change in environment), that deviation from homeostasis (change in sodium ion concentrations) would be considered stressful. The salt water itself would be considered a stressor. The fish would need to expend extra energy on ion transport to re-establish homeostatic levels of sodium concentrations in their cells.

Severity of Stress

It is not always possible for organisms to return to a homeostatic state following stress. If a stressor is sufficiently severe, there may be permanent damage in the organism as a result, possibly leading to death. Several factors can impact the severity of a stressor, including intensity, duration, and frequency of that stressor (Figure 7.3). Intensity refers to the magnitude of the stressor – for example a large change in temperature, or pH, or ion concentration would be a more intense (and more stressful) experience. Duration simply refers to how long an organism is experiencing the stressor; longer durations (exposure times) are usually more stressful. Frequency refers to how often an organism experiences a stressor; high frequency stressors (multiple exposures to the same stressor over time) are often more stressful than a stressor that only occurs once or a few times.

Let’s consider these terms (intensity, duration, frequency) use the example of environmental temperature. Low temperatures can be stressful for lots of organisms, especially organisms known as ectotherms, who can only get heat from their environment. When environmental temperature starts to drop, an ectotherm’s temperature also starts to drop, which interferes with most biological processes (causing stress!). The lower the temperature gets, the more intense the stress gets, especially if fluids inside or around cells begin to freeze. In addition, longer duration exposures to cold (at any temperature) are usually more stressful than short exposures to cold Finally, if the temperature fluctuates quite a bit (it gets cold, then warm, then cold, then warm, etc.), this can also increase the stress associated with cold because of the high frequency of cold exposures.

Multiple Stressors

Much of the research on stress and stressors is studied in the context of a single stressor (e.g., temperature, pH, ion concentration). However, in natural environments, organisms often experience more than one stressor at the same time (Figure 7.4). For example, at high altitudes (e.g., at the top of a mountain; Figure 7.4A), an organism will likely experience low temperature stress (because it’s cold) as well as stress associated with low oxygen availability (because the air is “thinner”). During a hot summer day (Figure 7.4B), organisms may experience both high temperature stress as well as dehydration (water loss) due to those high temperatures. In ocean environments (Figure 7.4C), warmer temperatures are also associated with acidification (lower pH) and low oxygen availability (because oxygen is less soluble in warm water). These stressors may occur at the same time, or sometimes one happens after another. Exposure to more than one stressor can greatly exacerbate (worsen) the stress an organism experiences. The chapters later in this unit each focus on a separate stress. However, it is important to remember that stressors are rarely experienced in isolation in the real world.