Rise of the ATP Storage Man
We’ve all probably heard (or tried not to hear) in biology class that ATP is the energetic molecule that sustains our life and is constantly replenished.
The question we are addressing here is: Why do we need to constantly restore it? Why not just store it in appreciable amounts like we do fat and glycogen?
Here, you'll find the calculations that cover a large reason why we store so little ATP.
Computing “multipliers” for fat was done by multiplying the number of ATP produced by one fatty acid by the difference in size(palmitate is half the size so in this case 2 is multiplied by the number of ATP palmitate produces). This concept is used to compute the "multipliers" for each stored substrate:
Total ATP from a 16-C fatty acid (palmitate)= 106 ATP and palmitate's molecular weight= 256g/mole; One molecule of palmitate is half the size of an ATP molecule and produces 106 times the ATP. 2x106= 212
Computing multiplier for stored carbohydrate:
Total ATP per glucose from glycogen= 23; glucose= 180 g/mol; Multipliers 2.5x23= 57.5
Computing multiplier for stored phosphocreatine:
Total ATP per phosphocreatine= 1; Molecular weight= 211g/mol; Multiplier= 2.5
For Reference: ATP = 507 g/mol ATP
Now using a 155 pound man as an example we will use these multipliers and estimates of how much of each substrate this man would store to compute what he would weigh if he only stored ATP for his energy needs:
155 lb. man
20% fat= 31 lb. of fat; 31x212= 6572 lbs. needed to be stored as ATP
600g glycogen (1.32 lbs.) = 1.32x57.5= 76 lbs. needed to be stored as ATP
120g phosphocreatine (0.26 lbs.) = .26x2.5= .65 lbs. needed to be stored as ATP
6649 extra pounds + All non-fat tissues (123) = 6773
Assuming all other organs/substrates would stay the same size (they would not) a 155-pound man would need to be ~6773 pounds or 44 times the size in order to store energy as ATP.
There are many reasons why we store so little ATP and that substrates other than ATP are needed as "energy harbors". One of which is size; it would be very inefficient to be sallying around at near 7000 pounds of essentially non-functional (other than providing energy) weight! The necessity and unique capabilities/functions of creatine (size not a big factor as seen above) is covered in previous blog post!