11.1 Challenges Associated with Energy Stress
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Evaluate the importance of energy balance in normal cellular function.
- Apply your understanding from Chapter 6 to describe which processes influence energy balance.
- Compare how levels of ATP and ROS (reactive oxygen species) differ under energy excess and energy limitation stress.
For cells to function properly, the amount of energy acquired by a cell must be approximately equal to the energy demands of that cell. This homeostatic state is referred to as energy balance, and is maintained through a variety of cellular processes (Figure 11.2). As discussed in Chapter 6.2, cells can acquire energy through the uptake of organic molecules (heterotrophs, e.g., animal cells) or by harnessing other energy sources (like light) to synthesize their own organic molecules (autotrophs, e.g., plant cells). Energy transformed from the oxidation of these molecules powers synthesis of ATP. This ATP can then be used to support processes that require an energy source, such as anabolic reactions, growth, and reproduction. Cells also lose energy to the environment, e.g., in the form of heat. Cells have various mechanisms to maintain energy balance (Figure 11.2), but when these mechanisms fail the cell can experience energy stress.
Factors that Influence Energy Balance
Several factors influence the amount of energy available to a cell, as well as a cell’s energy demand – the amount of energy a cell needs to use. The source of organic molecules (for heterotrophs) or light energy to make organic molecules (for most autotrophs) is important for determining energy available to a cell. For example, a plant cannot grow well in darkness because its energy source (sunlight) is limited. However, even if a cell has access to appropriate organic molecules (e.g., sugar), that energy only becomes available to the cell via metabolic pathways like cellular respiration that can be used to synthesize ATP. This ability to metabolize organic molecules and produce ATP is regulated by abundance and activity of various enzymes. Oxygen availability is also usually important for these reactions because it acts as the final electron acceptor during aerobic cellular respiration, the process that is responsible for the bulk of ATP production in eukaryotic cells. The energy demands of a cell depend on its need to conduct processes that require energy. For example, cells that have high activity levels (e.g., highly motile cells) have high energy demands that must be met to maintain energy balance. This cellular activity also depends on functional proteins, including enzymes.
Energy Stress
Energy stress occurs when a cell is unable to maintain energy balance for prolonged periods, and can occur in two forms: energy limitation or energy excess. Energy limitation can occur when metabolic demand surpasses energy availability, or there is a lack of organic molecules being b