Problem 9 - Campbell Biology Concepts & Connections

Question

A strain of mice remains healthy as long as they are fed regularly. After they eat, their blood glucose level rises slightly and then declines to a homeostatic level. However, when these mice fast or exercise, their blood glucose drops dangerously. Which hypothesis best explains their problem? (Explain your choice.) a. The mice have insulin-dependent diabetes. b. The mice lack insulin receptors on their cells. c. The mice lack glucagon receptors on their cells. d. The mice cannot synthesize glycogen from glucose.

Step-by-Step Solution

Answer

Hypothesis (c): The mice lack glucagon receptors on their cells.

Step 1: Understand the Context

Identify the main issue: The mice experience dangerous drops in blood glucose when fasting or exercising.

Step 2: Examine Each Hypothesis

Go through each choice and see if it fits the context of the problem.

Step 3: Evaluate Hypothesis (a)

Hypothesis (a) suggests the mice have insulin-dependent diabetes. If true, one would expect consistently high blood glucose levels, not dangerously low levels when fasting or exercising. So, this hypothesis does not fit.

Step 4: Evaluate Hypothesis (b)

Hypothesis (b) suggests the mice lack insulin receptors. This lack would lead to high blood glucose levels rather than dangerously low levels during fasting or exercise.

Step 5: Evaluate Hypothesis (c)

Hypothesis (c) suggests the mice lack glucagon receptors. Without glucagon receptors, the liver could not release glucose into the blood during fasting or exercise, leading to dangerously low blood glucose levels.

Step 6: Evaluate Hypothesis (d)

Hypothesis (d) suggests the mice cannot synthesize glycogen from glucose. This would result in an inability to store glucose, leading to low blood glucose levels even after eating, potentially causing issues during fasting and exercise.

Step 7: Choose the Best Explanation

After evaluating all hypotheses, hypothesis (c) is the most consistent with the symptoms observed in the mice, because without glucagon receptors, they cannot adequately raise their blood glucose levels during periods of fasting or exercise.

Key Concepts

Homeostasis

Homeostasis is the body's way of maintaining a stable internal environment, despite external changes. It keeps conditions like temperature, pH, and glucose levels within a narrow, healthy range. This balance is vital for the body to function correctly. In terms of blood glucose regulation, homeostasis involves maintaining a balanced glucose level in the bloodstream. When you eat, glucose levels rise, and the body responds by storing or using this glucose. During fasting or exercise, the body releases glucose to ensure it remains within the optimal range.

Insulin

Insulin is a hormone produced by the pancreas that plays a key role in blood glucose regulation. When you eat and your blood glucose levels rise, insulin is released into the bloodstream. It helps cells absorb glucose to be used for energy or stored for future use. Insulin also promotes the synthesis of glycogen from glucose, primarily in the liver and muscles. Without enough insulin or if cells cannot respond to insulin, blood glucose levels can become dangerously high, as seen in conditions like diabetes.

Glucagon

Glucagon is another hormone produced by the pancreas. It works in opposition to insulin. When blood glucose levels drop, such as during fasting or exercise, glucagon is released. It signals the liver to break down glycogen into glucose, which is then released into the bloodstream. This process ensures that the body has a consistent supply of glucose for energy, even when you are not eating. Without proper glucagon function or its receptors, maintaining adequate blood glucose levels can be challenging during times of fasting or physical activity.

Glycogen Synthesis

Glycogen synthesis is the process of forming glycogen from glucose. This is facilitated by insulin and occurs mainly in the liver and muscles. After eating, when glucose levels are high, insulin promotes the storage of glucose in the form of glycogen. During times when glucose is needed, like fasting or exercising, glycogen can be broken down back into glucose. This process is essential for maintaining stable blood glucose levels. Problems with glycogen synthesis can lead to issues like low blood glucose levels, as the body cannot store or release glucose effectively.