IMAT 2011 Q47 [Oxygen Food Burning]

A food item was burned in pure oxygen and released 830 kJ of energy. An identical food item of the same mass was found to produce 8 ATP’s in respiration.

Assuming it takes 31 kJ to produce one ATP molecule, estimate the efficiency of respiration.

A. 10%

B. 25%

C. 30%

D. 45%

E. 50%

Let’s analyze the question;

“A food item was burned in pure oxygen and released 830 kJ of energy.” - What does it mean?

In a perfect, pure burning, (100% yield) a food item is producing 830 kJ of energy. (Remember, kJ represents kilo Joule and amount of energy).

The same food item of the same mass (meaning the same as the 830 kJ 100% yield one) is producing 8 ATP molecules (energy molecules) during respiration (the process and making of energy in the mitochondria). We are given that the same mass of food produces 8 ATP molecules, 31 kJ is taken (from the food we eat) to create one ATP molecule, how many kJ have we produced from 830 kJ of food?

Eight molecules time 31 is 248 kJ. We consume 830, but only 248 kJ of the calories consumed were actually converted to the making of 8 molecules of ATP! If you want to calculate the yield, all we need to do is to divide the actual yield by the theoretical perfect yield.

248/830 time 100% = 29.9% = 30% EFFICIENCY = actual yield.

So, in this case, only 30% of the energy we consumed from the food item went to the creation of ATP!