9.3 Strategies and Mechanisms of Hypoxia and Anoxia Tolerance
9.3 Strategies and Mechanisms of Hypoxia and Anoxia Tolerance
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Explain how the main strategies organisms and their cells can use to combat hypoxia are effective at minimizing the negative effects of low oxygen.
- Give examples of two mechanisms that can be used to decrease oxygen demand, and organisms that use those mechanisms.
- Give examples of two mechanisms that can be used to increase oxygen uptake, and the organisms that use those mechanisms.
Hypoxia and anoxia create many challenges that force cells to adjust their physiology. Once the cell has detected the oxygen limitation, there are multiple possible responses. If we consider hypoxia as a problem of oxygen demand exceeding oxygen supply, there are two main strategies an organism can use: decreasing their demand for oxygen, or increasing cellular access to oxygen (increasing supply). Organisms may employ both of these strategies, depending on the species. This chapter section will explore mechanisms that can support each of these strategies.
Mechanisms to Decrease Oxygen Demand
There are a variety of mechanisms that cells can put into place to reduce their demand for oxygen. Two examples include 1) downregulating cellular process that consume ATP, and 2) increasing ATP synthesis in the absence of oxygen via anaerobic pathways. Both of these mechanisms decreases the organism’s reliance on ATP produced via aerobic respiration.
Decreasing Cellular Activity that Consumes ATP
A myriad of cellular processes requires ATP. However, under hypoxia, it is difficult for the cell to synthesis enough ATP to keep all these processes going. For a cell to survive, certain cellular processes, such as the maintenance of the ion balance across cell membrane with Na+/K+ ATPase, cannot fully shut down. Nevertheless, there are a variety of cellular processes that require ATP (e.g., protein synthesis) which can be downregulated to conserve ATP within the cell. Therefore, to decrease its demand for oxygen a cell can downregulate certain processes that consume ATP. Since cellular activity is decreased, the need for oxygen to generate ATP is reduced, resulting in a net decrease in oxygen demand.
One species that clearly uses this strategy is the anoxia-tolerant painted turtle (Chrysemys picta; Figure 9.5). [1]A study conducted by Hochachka et al. concluded that during exposure to anoxic conditions, multiple pathways including protein synthesis, gluconeogenesis (synthesis of glucose), and protein breakdown were downregulated in the hepatocytes (liver cells) of this species. The downregulation of these pathways resulted in 94 % suppression of the total ATP demand. This response reduces the demand for oxygen in the cells and significantly contributes turtle’s ability to tolerate anoxia.
Anaerobic ATP Pathways
Even though anaerobic metabolism only produces 2 ATP per glucose (compared to 30-32 ATP per glucose with aerobic metabolism), the c