Question

Metabolic Differences between Muscle and Liver in a "Fight-or-Flight" Situation When an animal confronts a "fightor-flight" situation, the release of epinephrine promotes glycogen breakdown in the liver and skeletal muscle. The end product of glycogen breakdown in the liver is glucose; the end product in skeletal muscle is pyruvate. a. What is the reason for the different products of glycogen breakdown in the two tissues? b. What is the advantage of these specific glycogen breakdown routes to an animal that must fight or flee?

Short answer

Liver releases glucose to the bloodstream for whole-body energy, while muscle uses pyruvate for immediate local energy.

Step-by-step solution

Understand Glycogen Breakdown in Liver

During a 'fight-or-flight' response, epinephrine signals the liver to break down glycogen into glucose 6-phosphate which is then converted into glucose by the enzyme glucose-6-phosphatase. The liver releases this glucose into the bloodstream to provide energy for various tissues, especially during stress or emergency situations.

Understand Glycogen Breakdown in Muscle

In skeletal muscle, glycogen breakdown also produces glucose 6-phosphate. However, muscle tissues lack the enzyme glucose-6-phosphatase, so glucose 6-phosphate is not converted into glucose. Instead, it enters glycolysis directly to form pyruvate, which is then converted to lactate or used in the Krebs cycle for ATP production, providing immediate energy for muscle contraction.

Compare the Metabolic Needs of Liver and Muscle

The liver functions as a glucose distributor for the whole body, hence it releases glucose into the bloodstream. Skeletal muscles are focused on rapid energy production and utilize glycogen stores directly for quick ATP generation needed during physical activities like fighting or fleeing.

Consider the Advantages of Metabolic Differences

The liver's conversion of glycogen to glucose supports energy supply to vital organs and other tissues through the bloodstream, which is essential during stress. Muscle tissue, on the other hand, directly uses pyruvate from glycogenolysis for rapid ATP generation, meeting the immediate energy demands for muscle contraction.

Key concepts

Fight-or-Flight Response

When faced with a stressful situation, the body initiates what is called the "fight-or-flight" response. This is an automatic response that prepares the body to either confront or flee from a threat. The hormone epinephrine, also known as adrenaline, plays a crucial role in this process. Its release triggers a series of reactions that prepare the body for quick action. The key effects include:

• Increased heart rate to pump more blood.
• Dilated airways for easier breathing.
• Elevated blood glucose levels to provide energy.

Epinephrine influences glycogen metabolism significantly, kickstarting glycogen breakdown to quickly supply energy necessary for survival in a threat scenario.

Epinephrine and Glycogen Breakdown

Epinephrine is essential for mobilizing energy stores during a "fight-or-flight" reaction. In the liver, epinephrine activates enzymes that break down glycogen into glucose 6-phosphate. This compound is then processed by glucose-6-phosphatase to release free glucose into the bloodstream, ensuring that energy is available globally across tissues. In muscle tissue, a slightly different process occurs. Although glycogen is also broken down into glucose 6-phosphate, muscles lack the enzyme glucose-6-phosphatase needed to convert it to free glucose. Therefore, glucose 6-phosphate in muscle cells is rerouted into glycolysis, quickly providing ATP— the energy currency of the cell. This immediate source of energy supports quick and sustained muscle contraction, crucial when rapid response is needed.

Liver vs. Muscle Metabolism

The liver and muscle tissues meet different metabolic needs during a stress response. The liver acts as a provider, releasing glucose to maintain blood sugar levels which fuel the brain and other critical tissues. This is essential because these organs cannot produce their own glucose efficiently. Muscle tissues, conversely, focus on themselves during a "fight-or-flight" moment. They utilize their glycogen reserves directly for swift ATP production through glycolysis. The immediate use of glycogen in muscles is advantageous, as it enables swift reactions of muscles without relying on external glucose sources. This capacity for rapid ATP production ensures that muscle contractions can happen rapidly and for longer durations when necessary.

Glucose-6-Phosphatase Function

Glucose-6-phosphatase is an enzyme found primarily in the liver. Its main function is converting glucose 6-phosphate into free glucose. This step is crucial for maintaining blood glucose levels, particularly during fasting or in stress scenarios. In the absence of glucose-6-phosphatase, as observed in muscle tissues, glucose 6-phosphate cannot be exported as glucose into the bloodstream. Instead, it undergoes glycolysis. By converting glucose 6-phosphate directly to energy within the muscle, the process circumvents the need for this enzyme and supports rapid energy use essential during periods of exertion, such as fighting or fleeing.