11.3 Strategies & Mechanisms of Energy Stress Tolerance
11.3 Strategies & Mechanisms of Energy Stress 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 energy limitation and energy excess are effective at restoring energy balance.
- Give examples of two mechanisms that can be used to increase available energy under energy limitation.
- Give examples of two mechanisms that can be used to decrease available energy under energy excess.
In response to energy stress, cells have evolved a range of strategies to restore homeostasis and maintain optimal energy level. The major strategies that can be used to combat energy limitation stress are increasing available energy and decreasing energy demand. For example, to increase available energy, cells may increase uptake of organic molecules, or intensify the activity of catabolic processes that generate ATP (Figure 11.6). To decrease energy demand, cells can downregulate processes that require lots of ATP, similar to how cells can tolerate hypoxia (Chapter 9.3). Conversely, the major strategies to tolerate energy excess include decreasing available energy and increasing energy demand. To decrease available energy, cells can store organic molecules in larger macromolecules rather than breaking those molecules down for ATP synthesis. To increase energy depend, cells can ramp up the activity of anabolic processes that consume ATP (Figure 11.6). AMPK plays a central role in in coordinating the multiple mechanisms employed by cells to carry out these strategies.
Mechanisms Under the Influence of AMPK
In times of energy limitation, activated AMPK can promote the acquisition of organic molecules such as glucose, and initiate catabolic processes that produce ATP such as glycolysis and fatty acid oxidation (Figure 11.6). In addition, AMPK can inhibit anabolic reactions that consume ATP such as protein synthesis, glucose synthesis (gluconeogenesis), and fatty acid synthesis (Figure 11.6). Under energy excess, AMPK is inactive. The catabolic processes normally stimulated by AMPK will no longer be stimulated. In addition, the anabolic processes normally inhibited by AMPK will be more likely to occur. Below, some of the many mechanisms used by active AMPK to increase energy availability in cells are described.
Carbohydrate & Lipid Metabolism
One way AMPK activation increases energy availability in the cell is by promoting the breakdown of carbohydrates and lipids, while also inhibiting their synthesis into storage molecules. In animal cells, the carbohydrate glucose can be stored as glycogen (decreasing energy availability) or broken down via cellular respiration to make ATP (increasing energy availability). Active AMPK inhibits an enzyme (glycogen synthase) that is important for glycogen synthesis, increasing glucose availability for glycolysis – the first part of cellular respiration (Figure 11.6). Fatty acids, a component of most lipids, can also be broken