So, I'm designing a species of sapient extraterrestrials for a series of short stories, and I've gotten to the point of needing to find a significant biochemical difference between life on their world and life on Earth.

One idea I had was to replace glucose with ethanol as the primary molecule for energy storage. This would mean that plants would directly produce ethanol from sunlight, CO2, and water and directly metabolise it; Similarly, ethanol would be the molecule that animal life would prefer to use over anything else and would attempt to break down more complex molecules into. Just to clarify, when I speak of using ethanol for energy, I am not talking about fermenting it into acetic acid, I am talking about aerobically "burning" ethanol for energy. It's fine if glucose has to be used as an intermediate molecule in constructing other things such as cellulose, but everything should be broken down to ethanol when being used for energy.

With that out of the way, what biochemical mechanisms and metabolic pathways would be needed to accomplish this, what is their relative efficiency, and is there anything interesting or unusual about them that I should know about?

There are some things you should bear in mind about local conditions and life:

  1. The local atmosphere has around 22,000ppm CO2, so any mechanisms that do not work in acidic environments are irrelevant.
  2. Life on the planet we are discussing uses amino acids not found in any Earth life, so anything that requires that is fine.

So, just to recap: I want to replace glucose with ethanol as the primary energy storage molecule. Ethanol should be directly, aerobically metabolised, although glucose can be used as an intermediary in constructing structural and genetic polymers. It's fine to use amino acids not found in Earth life so long as it can resist acidic conditions. How do I do this, what is the relative efficiency, and is there anything else about the means of doing this I should know?

  • $\begingroup$ I don't have biochemistry at hand to put an answer, but I wonder.. wouldn't this require some physiological changes as well ? Animals now keep their energy stock in fats. These are solid substances, carried around quite easily. Ethanol is either fluidic, or dissolved. For storing ethanol in large volumes in an animal or plants, you'd need some kind of fluid container inside. $\endgroup$
    – Goodies
    Jan 23, 2022 at 12:01
  • $\begingroup$ Anaerobic respiration. Plants produce alchohol when respiring without oxygen. Or fungi do? Anyway... lack of oxygen is probably the key. $\endgroup$
    – Richard
    Jan 25, 2022 at 1:28

2 Answers 2


We already get up to something similar without even trying (doesn't even need to include a visit to a pub or bar).

Our natural digestive processes produce a couple of grams of ethanol just from the normal diet of an average person (whoever that might be).

  • The first part takes an input of a little energy (47.2 kJ/mol):

C2H6O(ethanol) + NAD+ → C2H4O(acetaldehyde) + NADH + H+

  • The next gives off reasonable energy (262.3 kJ/mol), it also produces acetic acid, which fits with the acid-tolerant environment requirements:

C2H4O(acetaldehyde) + NAD+ + H2O → C2H4O2(acetic acid) + NADH + H+

  • A little energy is developed in the next step (46.8 kJ/mol):

C2H4O2(acetic acid) + CoA + ATP → Acetyl-CoA + AMP + PPi

  • The final stage (the Krebs Cycle/Citric-Acid Cycle) in this breakdown gives the most energy (1110.5 kJ/mol), it's also the only one with a nice diagram to illustrate (acetyl-CoA joins the reaction top left in a lovely salmon colour):

Krebbs cycle.

Free license 2022 Wikipedia.

In animals, this last part occurs on the delicate membranes of the mitochondria present in virtually every cell in the body. The total energy produced (maximum theoretical) after deductions for step one is: 1325 kJ/mol (This doesn't take into account the reduction of NAD in step two, that's separate and would depend on a number of factors)

So you're fitted up with a complete aerobic metabolic pathway for ethanol in an acid environment, supplying a whopping 1325 kJ/mol.


To get ethanol the easiest chemical path would be to first make glucose and then oxidize it to get ethanol.

$C_6H_12OH (aq)\rightarrow 2C_2H_5OH (l) + 2CO_2 (g)$

Apart from the obvious waste in energy, plants would have a lot of troubles using ethanol: while glucose can be polymerized to form cellulose, lignin, starch and so on, ethanol cannot. Basically, by using ethanol you are cutting down on the basic brick for plant construction.

  • $\begingroup$ I appreciate the answer, but this takes away a lot of the effect I was going for... $\endgroup$ Jan 23, 2022 at 7:19

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