9.1 Challenges Associated with Oxygen Limitation
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Evaluate the importance of aerobic processes for eukaryotic cells.
- Compare how normoxia, hypoxia, and anoxia affect cellular metabolism and other processes.
- Give examples of environments in which hypoxia or anoxia can occur.
- Explain why processes that produce toxic products (e.g., lactic acid fermentation, ethanol fermentation) are still useful under hypoxia
To produce large amounts of ATP, most eukaryotic cells require oxygen. By using aerobic respiration (which requires oxygen), cells can produce 30-38 ATP molecules from the energy stored in a single glucose molecule (Chapter 6). Starting with glycolysis and ending with oxidative phosphorylation, this metabolic pathway efficiently supplies ATP that is required to power many cellular processes. When a cell has limited or no access to oxygen, normal cell functioning is impaired. In this section, you will learn about how a cell functions under optimal oxygen levels and what happens when oxygen supply does not support the demands of the cell.
Understanding Hypoxia
Hypoxia occurs when a cell’s oxygen demand exceeds its oxygen supply. A cell may experience hypoxia due to increased demand for oxygen (e.g., increased metabolic activity) or a decreased supply of oxygen available (e.g., at high altitudes, Figure 9.2). Under extreme conditions, cells can experience the total absence of oxygen, or anoxia. It is important to note that not all cells experience hypoxia at the same level of oxygen. Some organisms (and therefore their cells) have adapted their physiology to survive at lower oxygen levels. For instance, the naked mole rat, who primarily reside underground are generally unbothered by the low oxygen content. However, other organisms, such as humans are not accustomed to such oxygen levels, and therefore do not inhabit those environments.
Cells are likely to experience hypoxia in terrestrial environments such as high-altitude areas or in aquatic environments with high abundance of algal species. High altitude hypoxia is caused by a decrease in the partial pressure of oxygen, making it more difficult for oxygen to diffuse into cells or be transported by hemoglobin. In aquatic environments, the overgrowth of algal blooms consumes available oxygen, leaving other cells and organisms in hypoxic stress. Cells that regularly experience hypoxia from their environment have certain strategies or have adapted in a way to withstand the effects (more in Chapter 9.3).
Human Health Example – Anemia
Anemia is a condition where one’s hemoglobin levels or red blood cell count is below average. Red blood cells are responsible for transporting oxygen to tissues of the body and hemoglobin is the protein which oxygen molecules bind to. With decreased hemoglobin or red blood cell levels, an anemic person’s cells would experience hypoxia as their cells are not receiving enough oxygen to m