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From my (limited) understanding, animals get energy from food by breaking chemical bonds between molecules.

There's a lot of water here [citation needed], so it seems like natural selection would favor any organism that could live off it. Why, in the history of life on earth, hasn't an organism appeared that eats water?

Same applies to dirt, rocks -- (although maybe lichen eat rocks? I'm fuzzy on lichen.)

Or air?

Why are humans and other animals like us only equipped to eat certain types of molecules, when others are so much more plentiful?

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    $\begingroup$ Because breaking H2O into H2 and O2 is an endothermic reaction: that is, it takes energy to break the bonds. Food has molecules that can be broken down, or combined with other molecules such as oxygen, in exothermic (that is, energy producing) reactions. $\endgroup$ – jamesqf Apr 25 '16 at 18:38
  • $\begingroup$ Food has two purposes: providing energy and providing raw material for growth. Water can only be broken down into hydrogen and oxygen which requires energy input, and water isn't by itself a raw material for anything other than ice. It is however important to life on Earth as a solvent (non-polar and electrolytic), and component of some structural chemicals (hydrates). $\endgroup$ – Anthony X Aug 27 '17 at 0:52
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You don't get energy from breaking chemical bonds, you only get energy from making chemical bonds, while breaking chemical bonds requires the input of energy. However, in practice chemical bonds are always broken as others are formed, and the net number of bonds is generally constant. Otherwise you would end up with free radicals, which are highly reactive and looking to form a chemical bond with the next thing it touches.

For example, the burning of glucose in respiration involves the breaking of C-C, C-H, C-O and O-H bonds in glucose and O=O bonds in oxygen, with the simultaneous formation of C=O bonds in CO2 and O-H bonds in water. But the C=O bonds in CO2 and O-H bonds in water contain much less energy than the bonds we break within in glucose. So the key is that the bonds formed contain less energy than the bonds broken, and energy is released overall.

And actually it's a bit more complicated than that, because you also have to consider entropy, which is how disordered the system is. Generally the more molecules in a system, the more entropy, so breaking a few big molecules down into lots of small molecules tends to release energy by increasing entropy.

So to answer your question, "food" is what we can draw useful energy from in the process of turning it into something else as we make and break chemical bonds. We can't do this with water, or indeed with rocks, because their chemical bonds would require more energy to break than the energy we'd get back out by forming alternative bonds with anything else.

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    $\begingroup$ Oh, that's really clear! So to attempt to parrot it back to you: animals that eat food are turning high-energy bonds into low-energy bonds, and using the energy that results. The low-energy bonds that get created in this process are in things like water and CO2, which get turned into high-energy bonds again by plants (they get the extra energy from sunlight). $\endgroup$ – Eli Rose Apr 27 '16 at 18:56
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    $\begingroup$ So if something is "food" to an animal, it's probably because it has a lot of high-energy bonds. $\endgroup$ – Eli Rose Apr 27 '16 at 18:56
  • $\begingroup$ Yes, that pretty much sums it up. Ultimately, food stores energy in high energy bonds, and we extract the energy by breaking them and replacing them with low energy bonds in CO2 and water. In fact most organic compounds contain high energy bonds, which is why they burn in air to give out energy (the heat and light of a flame), although we haven't evolved the enzymes to use them all as food. $\endgroup$ – Kelvin Apr 27 '16 at 21:41
  • $\begingroup$ So we're talking about bonds that have a lot of potential energy. Are these always bonds that are easy to break, or do the two categories just have some overlap but are unrelated? $\endgroup$ – Eli Rose Apr 28 '16 at 2:24
  • $\begingroup$ The energy of a bond and whether it is easy to break are not directly linked as it depends on the energy of the transition state, but generally the more energy stored in the bond, the easier it is to break and the less energy required to break it via the transition state. However most enzymes work by reducing the energy of the transition state so that bonds break and form more easily, and thus they speed up the reaction. $\endgroup$ – Kelvin Apr 28 '16 at 6:49
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Plants use both water and air as food using photosynthesis where water is split and recombined with carbon dioxide from air to make glucose.

Overall, the chemical reaction of photosynthesis is as follows:

Light energy + plant enzymes 6CO2 + 12H2O ------------------------------------------------> C6H12O6 + 6O2 + 6H2O

...which means that it takes

six molecules of carbon dioxide plus
12 molecules of water

in the presence of light and the proper enzymes in the cell, to make

one molecule of glucose
6 molecules of oxygen
6 molecules of water

Energy is released when glucose is burnt

about 16 enzymatic rxns C6H12O6 + 6O2 + 6H2O ------------------------------------> 6CO2 + 12H2O + ENERGY

...which means that

one molecule of glucose in the presence of
six molecules of oxygen and
six molecules of water

can be "burned" to release stored energy as well as the "waste" products of

6 molecules of carbon dioxide and
12 molecules of water

http://www.bio.miami.edu/dana/dox/photosynthesis.html

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  • $\begingroup$ "Plants use both water and air as food using photosynthesis ... to make glucose." - No, glucose itself is the "food" that is burned with oxygen in respiration, while CO2 and water are used by plants to make the glucose food by photosynthesis with the sun's energy. Water and air could never be described as "food". Food is what you get energy from by oxidation (burning it with oxygen). $\endgroup$ – Kelvin Apr 27 '16 at 8:40
  • $\begingroup$ The Cambridge dictionary disagrees with you. $\endgroup$ – Graham Chiu Apr 27 '16 at 9:51
  • $\begingroup$ I'm just saying it is more accurate to say that plants make their own food by photosynthesis, and that food is glucose, not "air and water", because there is no energy to extract from air and water: you have to put energy in to get the glucose (food), from which you can then get energy out. $\endgroup$ – Kelvin Apr 27 '16 at 10:10
  • $\begingroup$ There's a quote from Richard Feynman floating around in which he says that trees "eat air" to get sustenance. Over 95% of the trees mass is obtained from atmospheric Co2 and water. $\endgroup$ – Graham Chiu Apr 27 '16 at 11:03
  • $\begingroup$ One could also say that houses "eat bricks" to get sustenance, but that doesn't make it food. Food is a source of both energy and building blocks. Glucose provides both, but air and water provide no energy. $\endgroup$ – Kelvin Apr 27 '16 at 11:06

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