Question

Rate of ATP Breakdown in Insect Flight Muscle ATP production in the flight muscle of the fly Lucilia sericata results almost exclusively from oxidative phosphorylation. During flight, maintaining an ATP concentration of \(7.0 \mu \mathrm{mol} / \mathrm{g}\) of flight muscle requires \(187 \mathrm{~mL}\) of \(\mathrm{O}_{2} / \mathrm{h} \bullet \mathrm{g}\) of body weight. Assuming that flight muscle makes up \(20 \%\) of the fly's weight, calculate the rate at which the flight-muscle ATP pool turns over. How long would the reservoir of ATP last in the absence of oxidative phosphorylation? Assume that the glycerol 3-phosphate shuttle transfers the reducing equivalents and that \(\mathrm{O}_{2}\) is at \(25{ }^{\circ} \mathrm{C}\) and \(101.3 \mathrm{kPa}(1 \mathrm{~atm})\).

Short answer

The ATP turnover rate is approximately 1093 times per hour, and the ATP reservoir lasts 3.29 seconds without oxidative phosphorylation.

Step-by-step solution

Determine the Oxygen Utilization Rate for Flight Muscle

Given that flight muscle constitutes 20% of the fly's body weight, we first need to calculate the oxygen utilization rate specific to the flight muscle.The oxygen utilization rate is given as 187 mL of O₂/h/g body weight. Since flight muscle makes up 20% of the fly's weight, the oxygen consumption specific to the muscle is: \[187 \text{ mL O}_2/\text{h/g body weight} \times 0.20 = 37.4 \text{ mL O}_2/\text{h/g flight muscle}\]

Calculate ATP Production Rate from Oxygen Utilization

ATP is produced through oxidative phosphorylation, with approximately 1 mol of ATP generated per 0.5 mol of O₂ consumed (dependent on P/O ratio, typically ~2.5 ATP per 0.5 O₂ with mitochondrial electron transport).First, convert mL O₂ to mol O₂ using the ideal gas law, assuming standard conditions of temperature and pressure (STP: 25°C, 101.3 kPa):- 1 mole of O₂ occupies 24.45 L at STP (since 24.45 L/mol is used at STP for gases): \[37.4 \text{ mL O}_2/\text{h} = 0.0374 \text{ L O}_2/\text{h} = \frac{0.0374}{24.45} \text{ mol O}_2/\text{h} \approx 0.00153 \text{ mol O}_2/\text{h}\]Assuming 2.5 mol of ATP is produced per 0.5 mol of O₂: \[0.00153 \text{ mol O}_2/\text{h} \times 5 \approx 0.00765 \text{ mol ATP/\text{h}}\]

Convert ATP Production Rate to Micromoles

Continuing from the ATP production rate (in moles per hour), we convert it to micromoles for easier interpretation related to concentration: \[ 0.00765 \text{ mol ATP/\text{h}} = 7650 \mu\text{mol ATP/\text{h}} \]

Determine Turnover Rate of ATP Pool

The concentration of ATP in the muscle is given as 7.0 \(\mu\text{mol/g}\) of muscle. The turnover rate is how many times this pool is replenished in an hour given its rate of consumption: \[\text{Turnover Rate} = \frac{7650 \mu\text{mol ATP}\, \text{ per hour}}{7.0 \mu\text{mol/g}} \approx 1093 \text{ turnovers per hour}\]

Calculate Duration of ATP Reservoir Without Oxidative Phosphorylation

Determine how long the ATP reservoir lasts without replenishment:- The muscle has a reservoir of 7.0 \(\mu\text{mol/g}\). Rate of consumption is 7650 \(\mu\text{mol/h}\). \[\text{Duration} = \frac{7.0 \mu\text{mol/g}}{7650 \mu\text{mol/h}} \approx 0.000915 \text{ hours} = 3.29 \text{ seconds}\]

Key concepts

Oxidative Phosphorylation

Oxidative phosphorylation is a crucial process in the energy metabolism of cells, particularly in flight muscles of insects like the Lucilia sericata, the subject of our exercise. When flies engage in intense activities such as flight, they require substantial energy, which is primarily derived from ATP. This molecule is produced in large amounts through oxidative phosphorylation, occurring in the mitochondria. Here, electrons are transferred through the electron transport chain, driving the formation of a proton gradient across the inner mitochondrial membrane. This gradient powers ATP synthase, an enzyme that synthesizes ATP from ADP (adenosine diphosphate) and inorganic phosphate.

• In the case of the flight muscle of Lucilia sericata, we are specifically interested in the ATP produced via oxidative phosphorylation, due to its efficiency and capacity to deliver immediate energy.
• The P/O ratio (the number of ATP molecules synthesized per oxygen atom reduced) plays a significant role. Typically, around 2.5 ATP molecules are synthesized per 0.5 oxygen molecules consumed, reflecting the efficiency of this system.
• Since oxidative phosphorylation is directly associated with oxygen consumption, the fly's flight muscles, with high metabolic rates, necessitate a steady oxygen supply to continuously produce ATP and support sustained flight.

ATP Production Rate

Understanding the ATP production rate in insect flight muscles, such as those in Lucilia sericata, illuminates how these muscles handle intense demands during flight. During the process of oxidative phosphorylation, ATP is generated at a rate proportional to the oxygen consumed. In our specific example, we begin with the knowledge that per hour, the flight muscle consumes 0.00153 mol of oxygen.
Conversion to ATP occurs through the stoichiometry that involves ATP being formed through the reduction of oxygen. Mathematically, it can be expressed as:

• Obtaining approximately 0.00765 mol of ATP per hour based on the O₂ consumption.
• This process requires efficient coupling in the electron transport chain and optimal functioning of ATP synthase.

After calculating the ATP generation in moles, converting to micromoles helps us understand more intuitively the scale and speed of ATP turnover, particularly relevant when concentrations in tissues are usually discussed in such small units. This conversion gives us 7650 \( ano mol \) ATP per hour.
These calculations reveal not only the capacity of flight muscles to produce ATP but also imply their incredible efficiency in terms of sustaining high-energy activities like flight without exhaustive depletion of ATP stores.

Oxygen Utilization in Muscle

Oxygen utilization in insect flight muscles is an essential component for ATP synthesis, supporting energetic activities like flight in Lucilia sericata. Given that oxygen use directly influences ATP production, understanding this concept is vital.

• Flight muscle physiology is adapted for high oxygen uptake, enabling rapid ATP turnover and sustained aeration during flight.
• The rate at which oxygen is utilized offers insight into the muscle's metabolic demand. In our scenario, specific oxygen utilization for the flight muscle at 37.4 mL O₂/h/g highlights the precise and intense metabolic needs during flight.
• Knowing the percentage of body weight composed of flight muscle (20% in this case) helps tailor oxygen calculations exclusively to the muscle, ensuring accuracy.

During flight, these muscles continuously consume oxygen because it is the terminal electron acceptor in the electron transport chain. Without this constant oxygen supply, the ATP production from oxidative phosphorylation would halt, leading to rapid depletion of the muscle's ATP reservoir. This relationship underlines the critical importance of oxygen in maintaining the high ATP turnover needed for flight muscle functions.