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I came across an article that says that lipids are more efficient energy storage molecules compared to starch because lipids have higher “hydrogen to oxygen ratio”.

I do not understand how “hydrogen to oxygen ratio” equates to more efficient energy storage.

I am guessing that since energy production have to do with reducing NAD+, so “hydrogen to oxygen ratio” is a measure of the ability to reduce NAD+. And since NADH is then later used in oxidative phosphorylation in mitochondria to produce energy currency ATP, “hydrogen to oxygen ratio” is a measure of the potential amount of ATP the molecule is able to produce. And thus it is a measure of energy storage efficiency.

Is this correct?

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Answer

In my opinion:

The original poster’s overall interpretation of the statement in the article (as reported) is most likely correct.

Problems with original statement

Although the statement may be clear in context, I would rephrase it because of two specific objections I have to it. These are:

  1. “Energy storage efficiency” in isolation is ambiguous. It is not clear whether it refers to storage per unit mass or per unit volume, both of which may be considerations for living organisms.
  2. “Hydrogen to oxygen ratio” is an imperfect heuristic for the reduction state of the carbon backbone.

I would also be specific in naming the lipids involved in ‘energy storage‘ — there are other lipids (e.g. steroids) that are not used as energy stores.

So my restatement (adapted from Berg et al. ‘Biochemistry’) would be:

Triacylglycerols are more concentrated stores of metabolic energy than polysaccharides such as starch or glycogen because they are more highly reduced. The energy yield per gram from the complete oxidation of triacylglycerol is over twice that from polysaccharides (38 kJ/g cf. 17 kJ/g).

Comments on restatement

  1. I have avoided the use of the word ‘efficiency’ for reasons of lack of precision, already mentioned. In general I deplore its use in biology, partly because it is seldom defined, and partly because it generally involves the implicit assumption that the most ‘efficient’ is the ‘best’. If this were so in the current case, potatoes would be bags of oil.
  2. There are other factors that contribute to the concentrated nature of triacylglycerols for storage of metabolic energy. These include dehydration and lack of branching (important for reducing the volume).
  3. It is important to talk in chemical terms — i.e. about oxidation and reduction. I reproduce below part of a diagram I used to use for teaching which summarizes the relationships between metabolic intermediates of different oxidation states. Metabolic oxidation/reduction
pairs
  4. The above diagram shows the limitations of the heuristic of “hydrogen to oxygen ratio”. The pair on the left (which might be found in saturated v. unsaturated fatty acids) both have the same H:O ratio (infinity) but are in different states of reduction.
  5. It may be better (at least in abstract) to consider carbon atoms as the basis on which to compare the oxidation states of different metabolites, rather than weight, which is influenced by the oxidation state itself.
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