Lesson #2: Our Story

Directions:

Before the video, write down everything that you know about a marathon.

During the video, write down questions you are wondering. 

After the video, write down what you have learned about our story. 

The Chemistry of the Marathon

Running a marathon requires a massive amount of energy. To keep moving for 26.2 miles, the muscle cells in a runner's legs must constantly produce ATP (adenosine triphosphate), the molecule that powers cellular work. This process relies heavily on the interaction between body systems. As the runner begins the race, the nervous system signals the respiratory system to breathe faster and the circulatory system to pump harder. This coordination ensures that oxygen and glucose are delivered rapidly to the mitochondria in the muscle cells.

For the majority of the race, the runner’s body performs aerobic cellular respiration. In this efficient process, enzymes in the mitochondria break down glucose in the presence of oxygen. This reaction produces a large amount of ATP, along with water and carbon dioxide as waste products. As long as the runner maintains a steady pace, their heart and lungs can supply enough oxygen to keep this process going, maintaining a state of dynamic equilibrium (homeostasis).

However, if the runner sprints or pushes too hard, the demand for energy may exceed the supply of oxygen. When the circulatory system cannot deliver oxygen fast enough to meet the muscles' needs, the cells switch to anaerobic respiration (fermentation). While this allows the runner to keep moving briefly, it produces very little ATP compared to the aerobic method. Additionally, it results in the production of lactic acid. As lactic acid builds up in the muscle tissue, it changes the pH of the cells, causing the enzymes to function less efficiently.

This chemical change leads to muscle fatigue and the "burning" sensation runners often feel near the end of a race. Even after the runner crosses the finish line and stops moving, their breathing rate remains high for several minutes. This is the body’s attempt to restore homeostasis. The heavy breathing brings in extra oxygen to break down the accumulated lactic acid and helps clear the excess carbon dioxide from the blood.

Multiple Choice Questions

1. Based on the passage, which statement best describes the interaction between the respiratory and circulatory systems during the marathon? 

A. The circulatory system breaks down glucose, while the respiratory system produces ATP. 

B. The respiratory system takes in oxygen, which the circulatory system transports to muscle cells for respiration. 

C. The circulatory system signals the lungs to stop breathing so that anaerobic respiration can begin. 

D. The respiratory system filters lactic acid out of the blood, while the circulatory system cools the body down.

2. In Paragraph 3, the passage describes a switch to anaerobic respiration. What is the direct cause of this shift? 

A. The muscle cells run out of glucose and begin burning protein. 

B. The runner’s body temperature drops too low for enzymes to work. 

C. The circulatory system cannot deliver oxygen fast enough to match the demand for ATP production. 

D. The mitochondria stop working because they have produced too much carbon dioxide.

3. Why does the runner’s breathing rate remain high even after the race has finished (Paragraph 4)? 

A. The body is trying to eliminate the excess glucose stored in the lungs. 

B. The nervous system has stopped sending signals to the diaphragm. 

C. The body is using a negative feedback mechanism to restore proper pH and oxygen levels. 

D. The digestive system requires oxygen to begin breaking down the runner's next meal.