Question

Muscle fibers that have a large number of mitochondria, contain large amounts of myoglobin, and exhibit low rates of ATPase activity are called _____ fibers. a. slow-glycolytic b. fast-glycolytic c. intermediate d. fast-oxidative e. slow-oxidative

Short answer

The muscle fibers described in the problem are slow-oxidative fibers (Option e).

Step-by-step solution

Consider the characteristics mentioned

First, we consider the three characteristics mentioned in the question. The muscle fibers in question have a large number of mitochondria, contain large amounts of myoglobin, and show low rates of ATPase activity.

Compare with given options

We then compare these characteristics with the known characteristics of the five fiber types listed. In this comparison, we should consider the role of mitochondria, myoglobin and ATPase activity in muscle fibers. Mitochondria are involved in energy production, myoglobin stores oxygen in muscle cells, and ATPase activity is related to the rate of energy utilization.

Identify the correct muscle fiber type

Upon comparing the characteristics, it is apparent that the fibers referred to in the question are slow-oxidative fibers. These fibers have a high mitochondrial and myoglobin content and a slow ATPase activity, which matches with the provided characteristics.

Key concepts

Mitochondria in Muscle Fibers

Muscle fibers are specialized cells that require energy to contract and thereby engage in physical movement. The mitochondria, often referred to as the 'powerhouse' of the cell, play a vital role in energy production. They produce adenosine triphosphate (ATP), the cell's energy currency, through a process called oxidative phosphorylation.

In muscle cells, the number of mitochondria corresponds with the type of activity they are optimized for. Fibers with a high number of mitochondria, such as slow-oxidative fibers, are designed for endurance and sustained activity. The abundance of mitochondria in these muscle fibers ensures a steady supply of ATP through aerobic metabolism, which is efficient and generates ATP using oxygen. This is why they are prevalent in muscles that maintain posture and those engaged in activities like long-distance running.

Myoglobin Function

Myoglobin is a protein found in muscle fibers that is structurally similar to hemoglobin—the oxygen-carrying protein in the blood. Myoglobin's primary function is to store oxygen within muscle tissue. This oxygen reserve is particularly important during intense exercise when the oxygen supply from hemoglobin might not meet the muscle's demand.

Muscle fibers that contain large amounts of myoglobin, such as slow-oxidative fibers, appear reddish due to this oxygen-storing protein. The stored oxygen is readily available when the energy demand increases, making these fibers especially suited for endurance activities. Not coincidentally, such fibers possess characteristics that enable them to resist fatigue, allowing athletes like marathon runners to sustain effort over prolonged periods.

ATPase Activity

ATPase is an enzyme that catalyzes the hydrolysis of ATP, releasing energy that the muscle can use for contraction. ATPase activity is an indicator of how fast a muscle fiber can use ATP. A low rate of ATPase activity suggests that the fiber uses ATP more slowly. Therefore, it contracts more slowly and is suited for endurance, as it conserves its energy resources for prolonged periods.

Muscle fibers with low ATPase activity, such as slow-oxidative fibers, contrast with fast-glycolytic fibers, which have high ATPase activity and are thus able to contract quickly and with greater force. However, these fast-twitch fibers fatigue rapidly and are best suited for short bursts of high-intensity activities, like sprinting. The inherent efficiency of slow-oxidative fibers in using ATP make them ideal for activities that do not require rapid, high-powered muscle contractions.