Question

After an overnight fast, which of the following processes would be expected to occur at an elevated rate compared with the well-fed state? (A) Glycolysis (B) Glycogenolysis (C) Glycogenesis (D) Glycerol synthesis

Short answer

The process elevated during an overnight fast is glycogenolysis (B).

Step-by-step solution

Understand the Overnight Fast

An overnight fast refers to a period without food intake, typically lasting 8-12 hours.

Identify Body's Response to Fasting

During fasting, the body's priority is to maintain blood glucose levels. This is achieved by breaking down stored molecules.

Define Each Process

Glycolysis is the breakdown of glucose for energy. Glycogenolysis is the breakdown of glycogen into glucose. Glycogenesis is the formation of glycogen from glucose. Glycerol synthesis is the formation of glycerol, a component of fats.

Compare Processes to Fasting State

During a fast, the body needs to increase blood glucose levels. Therefore, it breaks down glycogen into glucose through glycogenolysis.

Determine Which Process is Elevated

Since glycogenolysis provides glucose by breaking down glycogen, it would be elevated during an overnight fast.

Key concepts

Glycogenolysis

When you haven't eaten for about 8-12 hours, your body needs to keep blood glucose levels stable. Glycogenolysis is one of the key processes that help maintain this balance. Glycogenolysis involves breaking down glycogen, a stored form of glucose in your liver and muscles, back into glucose. This glucose is then released into the bloodstream, providing energy to your body, especially your brain and red blood cells, during fasting. This process ensures that your body has a constant glucose supply, even when you're not eating.

Glycolysis

Glycolysis is a process where glucose is broken down to produce energy. It occurs in the cytoplasm of cells and doesn't need oxygen, making it an anaerobic process. Glycolysis breaks a glucose molecule (with 6 carbon atoms) into two molecules of pyruvate (each with 3 carbon atoms). This process produces a net gain of 2 ATP molecules, which are used as energy currency in cells.
While glycolysis provides energy during both fed and fasting states, it is not as elevated during fasting because the focus is on producing glucose, not breaking it down. However, during a well-fed state, glycolysis is more active as the body has a higher supply of glucose from the recent meal.

Glycogenesis

Glycogenesis is the process of converting glucose into glycogen for storage. This process mainly occurs in the liver and muscle cells and is an anabolic process (building up). Glycogenesis happens when there's excess glucose in the blood, such as after eating a meal. Your body stores this glucose as glycogen to be used later when glucose levels are low.
During an overnight fast, glycogenesis is not active. Instead, your body will rely on breaking down glycogen through glycogenolysis to maintain blood glucose levels. Understanding this balance helps explain why glycogenolysis, not glycogenesis, is elevated during fasting.

Blood Glucose Levels

One of the body's main priorities is to keep blood glucose levels steady, whether you are fasting or have just eaten. Blood glucose is crucial for energy, particularly for organs like the brain that rely heavily on glucose. During fasting, the body employs processes like glycogenolysis to maintain glucose levels. Additionally, gluconeogenesis (creating glucose from non-carbohydrate sources) also helps in prolonged fasting.
When you eat, your blood glucose levels rise, and insulin is released to help cells absorb this glucose and to store excess as glycogen. In contrast, during fasting, hormone levels shift to favor the release of stored glucose into the bloodstream. This dynamic regulation ensures that your body functions well under different dietary conditions.