Wouldn't it be simpler and more efficient if life ran on hydrocarbons?

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Oxygen is needed for either to put out energy.

cellular respiration
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy

gasoline combustion
2 C6H14 + 19 O2 = 12 CO2 + 14 H2O

(I hope I balanced the equations correctly)

Seems like they work pretty much the same.

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Which carbohydrates are water soluble?

Which hydrocarbons?

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I wouldn't think so. We run out of oil, worldwide, all known reserves, and future predicted reserves, even using technology we don't yet possess, sometime mid-century.

Carbohydrates, proteins, etc., will be around for billions of years yet!

Which hydrocarbons do you have in mind and where would life get them?

Simple hydrocarbons like methane and ethanol are produced by many organisms as byproducts or waste products.
Methanogen bacteria were probably the most common form of life at one time on our world (I think) so there were plenty of producers of these compounds; but as methane and ethanol are volatile perhaps there were simply no good ways for a life form to develop which could collect and use them.
A hydrocarbon metabolism would need free oxygen as well (I think) so could not develop in the environment of the Early Earth.
Carbohydrates have the advantage of being nonvolatile, so can be stored and digested more easily than a trace gas or volatile liquid.

Long chain hydrocarbons are quite difficult to synthesis apparently - it takes a long time to make gasoline under extreme conditions- so there wouldn't be many sources of petrol around to feed our hypothetical fuel bugs.

Which incidentally aren't so very hypothetical after all; a number of different organisms are able to use diesel fuel for energy, as you may know; see here
http://www.fueldoctors.com/fuel.htm

but they rely on humans to collect the stuff for them.

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And of course those methanol and ethanol are the result of organisms, which means the organisms would not arise unless they were already here making their own hydrocarbons. Catch 22.

No. Carbohydrates are plentiful; hydrocarbons rare.

Short answer -- plants produce carbohydrates, so that's what animals use.

Corollary question -- why don't plants produce hydrocarbons instead?

I think the answer is, plant photosynthesis (well, back then it was cyanobacteria photosynthesis) developed in the absence of free oxygen. Carbohydrates break down into water and CO2 with net release of energy without oxygen -- it's called anaerobic respiration. Hence carbohydrates were useful to plants as energy storage, and to animal which developed to eat plants. Hydrocarbons are at the bottom of energy well -- they do not exothermicaly break down into anything, so they were of no use to the plants.

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Carbohydrates don't provide as much energy per unit weight when oxidised as hydrocarbons. Plants store carbohydrate as energy store, as weight is not critical to a plant. It can be polymerised for storage as starch grains and quickly broken down into soluble sugars when required.

Animals store fat, which is almost a hydrocarbon. It's three hydrocarbon chains linked to a small alcohol unit. I don't think it will be much different to a hydrocarbon in terms of energy density. It's a lot more wasteful than storing carbs though, as it takes 5 times as much energy to synthesise the fat from carbohydrate as the animal gets back from oxidising the fat. Presumbly the weight and space saving, coupled with the insulation and padding properties of fat, makes the trade-off worthwhile.

If we are going to include fats and vegetable oils in this discussion there is some interesting reading here-
http://www.whatislife.com/reader2/M...fattyacids.html

Biological oils and lipids are widely encountered in metabolism; but the heavier alkanes, alkenes, and propanol-type compounds are less frequently used in biological systems.
Some turpentine and resin producing trees can probably produce spirit which could be used as motor fuel...

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A matter of solubility? Life as we know it is an aqueous affair. You can dissolve a lot more sugar in your body than you can hexane.

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Life runs on what's available, hydrocarbons aren't as easily available as carbohydrates.
I'm assuming you're talking animals when you say life.

As a whole, life runs on either solar energy or chemical energy from volcanic vents. Everything else is derivative.

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And the solar energy outweighs the chemical energy from volcanic vents by about a factor of a billion to one!

The future of our energy solution is NUCLEAR, folks. Modern-technology fission followed closely by fusion.

There simply IS no other solution that provides enough energy to replace natural gas and oil!

Natural gas runs out around 2017. That's just 12 years away.

Oil runs out around 2045. We've a little more time, but that's still just half a lifetime away.

After that, there simply IS no more!

Please stop dooming our civiliation to ruin and join ranks with the only solution that will work in the forseeable future? Particularly if we wind up running on either battery power or fuel cell (O & H) - that power has to come from somewhere, and that somewhere is nuclear!

[quote=genebujold]

Natural gas runs out around 2017. That's just 12 years away.

[quote]

May I ask where you got this information?

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I would also be interested in learning where you got this information.

As far as I understand it, global reserves of petroleum are sufficient to last us through 200+ more years, even with extrapolations from increased usage (we use more every year, extend the trend, you still get 200 years of oil).

I think the common misperception about oil abundance stems from the economics of extraction. Oil wells get shut down all the time when they stop being productive. This does not mean the oil field the well is tapping has been drained dry. On the contrary, I think most oil wells get shut off when the field drops to 60% remaining. The cost of oil determines how much is extracted. Extracting oil requires the input of various factors (energy, typically electricity to run pumps, liquid mud to pump down to get the oil up, etc), all of which cost money. If you can sell your oil for more, it becomes economically feasible to extract more from previously "empty" fields.

However, I agree with you that nuclear energy generation is the way to go. Not because of any lack of oil, but because nuclear is so much cleaner - especially if anybody ever cracks the fusion problem.

Didn't somebody say that Saturn's moons (some of them) might be covered in simple hydrocarbons? I forget where I saw the story, I think it was from one of the Cassini flybys or possibly the Huygens mission. Apologies to people who know about those missions, I'm probably mangling the mission details and misspelling the names.

If that were the case, we might expect life to evolve on a cold, hydrocarbon-rich world that does use the chemical energy stored in hydrocarbon molecules. If you can break down carbohydrates and get energy without ever involving free oxygen, can you do the same with butane? I think I saw a discussion in NatureNews about xenobiologists (love that word) looking into Formamide as a solvent rather than water. Formamide disolves nicely in water, but I think it's also good as disolving some nonpolar stuff like small hydrocarbons. Any physical chemists reading this?

OK, this may take me a while. Bear with me, folks, and I'll try to provide some answers.

First off, ethanol and methanol are closer to being carbohydrates than hydrocarbons, because they contain oxygen.

There are bacteria that can consume methane (it is slightly soluble in water), but they have to start out by using energy to convert it to methanol before they can extract any energy from the reactions. So to make methanol from methane in a controlled fashion requires energy (the C-H bond is very stable), and it is only by oxidising methanol that the bugs can obtain energy from the chemical reactions. If methanol is available, they will tend to favour that.

There are fungi that can metabolise long-chain hydrocarbons such as are found in crude oil. However, I do not know very much about these organisms. I believe they metabolise hydrocarbons in order to reduce their toxicity and not primarily to obtain energy. This ability to reduce the toxicity of toxic hydrocarbons gives these fungi an adaptive advantage in certain environments.

The point made in earlier posts about hydrocarbons being poorly soluble in water is a valid one, but it is not the most important consideration. There are two other points that I think are more important:

1. Since carbon and hydrogen have very similar electronegativity, a hydrocarbon molecule is very uniform, or non-polar. It does not develop areas of small positive or negative charges because the electrons that make the chemical bonds between atoms (or, more accurately, the electrons that occupy the highest occupied molecular orbital, or HOMO) are evenly "spread" over the molecular orbitals in the molecule. A consequence of this is that the enzymes that catalyse metabolism would have nothing with which to "grab hold" of the hydrocarbon molecule. Enzymes are biological catalysts; they are protein molecules, and as such have to carry out their catalytic function using chemical groups that exist on the 22 or so amino acids in proteins (there are 20 a.a.s from which proteins are made, but sometimes some are modified afterwards). The enzyme must align the substrate molecule in the correct orientation for its catalysis to be effective. To do this, it needs the substrate molecule to have areas that are chemically different. The simplest way to do this chemically is by having heteroatoms such as oxygen in the substrate molecule. Because oxygen has a high electronegativity, the electrons in bonds adjacent to O atoms spend more time near the oxygen than near, say, the C or H atoms. Therefore, the O atom develops a slight negative charge, making the molecule polar. This difference is exploited by an enzyme molecule to control the orientation of the substrate molecule and hence facilitate catalysis. Simply put, it is easier for an enzyme to catalyse a reaction with a carbohydrate molecule than with a hydrocarbon molecule.

2. Metabolism is often compared to burning, but there are important differences in the chemistry. The breakdown of carbohyrate into CO2 and water is done in a gradual and controlled fashion. If it were to happen all at once, the energy would be lost (i.e. not captured in a useful form) and might actually boil the cell. Some very sophisticated systems have evolved to capture the energy of the oxidation of carbohydrate and "store" it in a molecule that can be moved around. This molecule is ATP (adenosine triphosphate). The breakdown (hydrolysis) of the phosphodiester bond between the second and third phosphate groups of ATP has a high Gibbs free energy associated with it. This can be coupled to the various processes for which living things require energy, and hence used to do work (e.g. to move other molecules around, to make DNA, to make structural molecules and so on).

There's more. We do metabolise molecules that are nearly hydrocarbons. The fats mentioned by kzb in post #11 are triacyl glycerols. Glycerol is simply three carbons, each with an alcohol group (OH) attached (the rest of the bonds are occupied by hydrogen atoms). The triacyl part will be three fatty acids. A fatty acid is a long-chain hydrocarbon (typically about 18 - 20 carbons, often saturated with hydrogen atoms, but sometimes having one or more points of unsaturation, meaning carbon-carbon double bonds) with a carboxylic acid group at one end. This acid group allows enzymes to get a hold on the molecule, and can also take part in useful, reversible chemical reactions. The acid groups can form ester bonds with glycerol (these are bonds that can be made and broken with only small changes in energy required so they are quite convenient for a system that requires them to be made reversibly) to form the triacyl glycerol molecule. Per unit weight, these provide more energy when broken down than carbohydrates (because oxygen is available from the air - carbohydrates are denser than fats because they contain oxygen, which has a higher molecular weight than carbon or hydrogen). Animals store surplus food as triacyl glycerol; plants store their food as carbohydrate because the do not need to carry it anywhere. Consequently, plants need to express fewer enzymes for their energy metabolism (not counting photosynthesis). Animals do store a small amount of carbohydrate in the form of glycogen. This is why breakfast is the most important meal of the day: first thing in the morning, your glycogen reserves are low, so your blood-sugar level will be low.

However (because nothing is ever that simple), not all of our tissues can metabolise fatty acids as a source of enegy. In particular, the brain requires glucose. It cannot obtain energy from any other source (which is why diabetics need to regulate their blood sugar level through diet or regular doses of drugs). One of my physiology professors once told us that the brain requires one Mars bar's-worth of glucose every day. So that works quite well for me... Anyway, because the brain requires glucose, it is easier for us to obtain that as carbohydrate in our diet (starch is a polymer of glucose) than to synthesize it from fats and oxygen.

One caveat here: I'm a little hazy in my recollection of lipid metabolism; it may be that we cannot synthesize glucose from fats, but I cannot recall any reason why this should be so. I do know that we can make glucose from other carbohydrates or from amino acids (with the excess nitrogen from amino acids being incorporated into urea and than passed from the body in urine).

So, although that was a bit long-winded, I think I've covered all the basics.

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Carbohydrates are soluble in water, and therefore easier for our bodies to transport.