The Reason for Gluconeogenesis
Our bodies are capable of harvesting energy in the form of adenosine triphosphate (ATP) from various macromolecules, including carbohydrates, lipids, and proteins. Cellular respiration, or the extraction of ATP from glucose (a simple carbohydrate), is the main pathway by which the body obtains ATP to power cellular processes. Units of proteins and lipids are also capable of entering respiration after conversion to pathway intermediates. For example, in beta (β) oxidation, fatty acids are converted to acetyl CoA, which is a reactant in the Krebs cycle of respiration. Deaminated amino acids can be converted to respiration intermediates as well.
Because of this interconnectivity between carbohydrate, lipid, and protein metabolism, interconversions are possible. One example of an interconversion is gluconeogenesis, or the synthesis of glucose from non-carbohydrate sources (such as lipids or proteins). This process occurs with high frequency when dietary intake of glucose is insufficient for metabolic demands (such as in ketosis, described here), therefore helping maintain adequate blood glucose levels.
The process of gluconeogenesis.
One may ask why it is ever necessary to synthesize glucose from other molecules, when it is possible to derive ATP from converting molecules to respiration intermediates, and subsequently conducting respiration in a single direction. However, one must consider blood glucose levels. If one is not consuming enough carbohydrates, blood glucose levels will be too low to support metabolism. Gluconeogenesis addresses this problem by synthesizing glucose from other dietary inputs so that blood glucose levels remain constant.
The reason that glucose is needed (instead of just respiration intermediates) concerns timely energy supply to all organs. Cellular respiration must occur at different rates in different organs, and the chemically unstable intermediates and products of cellular respiration cannot possibly be stored long enough for use when needed. Glucose, however, is chemically stable, and therefore a prime candidate for transport in the blood to organs that require energy. These organs can then conduct cellular respiration and fuel their activities.
If you would like to learn more about how fats and proteins also can be metabolized for energy, check out this article from Khan Academy: https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/connections-between-cellular-respiration-and-other-pathways
