1. **Energy Source**: Carbohydrates are the primary source of energy for most organisms. Simple sugars like glucose are quickly metabolized to release energy. In humans, glucose is a key energy source, especially for the brain and muscles during physical activity.
2. **Energy Storage**: Carbohydrates are stored for future energy needs. In plants, energy is stored as starch, while animals store carbohydrates as glycogen in the liver and muscles.
3. **Structural Function**: Carbohydrates are essential for building and maintaining the structure of cells and tissues. For example, cellulose, a polysaccharide, is a major component of plant cell walls, providing rigidity and strength. Chitin, another carbohydrate, is found in the exoskeletons of arthropods and the cell walls of fungi.
4. **Cell Recognition and Signaling**: Carbohydrates are involved in cell-cell recognition and signaling processes. Glycoproteins and glycolipids on the surface of cells play a crucial role in immune responses and in the communication between cells.
5. **Building Blocks for Other Molecules**: Carbohydrates are precursors for the synthesis of other essential molecules, including nucleic acids (DNA and RNA), which contain sugar moieties as part of their structure, and certain amino acids and lipids.
6. **Regulation of Biological Processes**: Carbohydrates are involved in the regulation of various biological processes. For instance, they can influence protein folding, stability, and activity, and are involved in the regulation of metabolic pathways.
7. **Protection**: Some carbohydrates serve protective functions. For instance, mucus, which contains glycoproteins, protects the linings of the respiratory, digestive, and reproductive systems.
8. **Dietary Fiber**: In the human diet, certain carbohydrates, such as dietary fiber, are important for digestive health. They help regulate the body’s use of sugars, helping to keep hunger and blood sugar in check.
These diverse functions highlight the importance of carbohydrates in both the structure and function of living organisms.
In a situation where there is not enough glucose to meet the body’s needs, glucose is synthesized from amino acids. Because there is no storage molecule of amino acids, this process requires the destruction of proteins, primarily from muscle tissue. The presence of adequate glucose basically spares the breakdown of proteins from being used to make glucose needed by the body.
As blood-glucose levels rise, the use of lipids as an energy source is inhibited. Thus, glucose additionally has a “fat-sparing” effect. This is because an increase in blood glucose stimulates release of the hormone insulin, which tells cells to use glucose (instead of lipids) to make energy.
Adequate glucose levels in the blood also prevent the development of ketosis. Ketosis is a metabolic condition resulting from an elevation of ketone bodies in the blood. Ketone bodies are an alternative energy source that cells can use when glucose supply is insufficient, such as during fasting. Ketone bodies are acidic and high elevations in the blood can cause it to become too acidic. This is rare in healthy adults, but can occur in alcoholics, people who are malnourished, and in individuals who have Type 1 diabetes.