Our brain cells exclusively rely on glucose for ATP production

Our brain cells exclusively rely on glucose for ATP production. What are the mechanisms by which the glucose becomes available to these cells during a fasting period?

A Use of fat molecules B Glycolysis C Breakdown of glycogen D Synthesis of glycogen

The correct answer and explanation is:

The correct answer is C: Breakdown of glycogen.

During a fasting period, the body must ensure that glucose is still available for vital organs like the brain. While the brain does primarily rely on glucose for ATP production, it can also use ketone bodies in prolonged fasting states. However, the immediate source of glucose during fasting is the breakdown of stored glycogen.

Explanation:

1. Glycogen Breakdown (Glycogenolysis): The body stores glucose in the form of glycogen in the liver and muscles. When fasting, the liver breaks down glycogen into glucose through a process called glycogenolysis. This glucose is then released into the bloodstream to be used by tissues, especially the brain, which cannot store glucose and relies on a constant supply.
2. Gluconeogenesis: After glycogen stores are depleted (usually after 24-48 hours of fasting), the body starts synthesizing glucose from non-carbohydrate sources like amino acids (from muscle protein) and lactate. This process occurs in the liver and is known as gluconeogenesis. This helps maintain blood glucose levels when glycogen stores are no longer sufficient.
3. Use of Fat Molecules (Lipolysis): While fat molecules can be broken down into fatty acids and glycerol (a process called lipolysis), they do not directly provide glucose. Fatty acids are primarily used for energy by many tissues in the body. However, the brain can also use ketone bodies (derived from fatty acids) after several days of fasting when glucose is in short supply.
4. Glycolysis: Glycolysis is the breakdown of glucose into pyruvate to generate ATP. This process happens once glucose is available, either from glycogen breakdown or gluconeogenesis. It does not produce glucose; it consumes it to generate energy.
5. Synthesis of Glycogen: Glycogen synthesis occurs when glucose is abundant, such as after eating a meal. This process is the opposite of glycogenolysis and is not involved during fasting periods.

Thus, the breakdown of glycogen is the primary mechanism by which glucose becomes available to brain cells during fasting.