Carbon dioxide, CO2, or O=C=O, is a gas at the temperature and pressure of our environment.
Water, H2O, or H-O-H, is a liquid.
Both compounds are relatively stable in chemical terms, and abundant in our biosphere and in the kosmos.
Carbon dioxide is soluble in water, and in solution the two compounds may combine to form carbonic acid:
CO2 + H2O <---> H2CO3
This reaction is incomplete: at equilibrium carbon dioxide, water, and carbonic acid exist together in significant proportions in the solution. Of course, removal of one component will pull the reaction in that direction. So, for example, adding calcium hydroxide leads to the precipitation of solid calcium carbonate, eventually trapping all the carbon dioxide. This process cleared huge quantities of carbon dioxide from the atmosphere of the young earth.
The formation of carbonic acid is an important process in geochemistry and physiology, but a second and much more difficult reaction of carbon dioxide and water is perhaps the most important chemical reaction in our biosphere.
The vital reaction is the combination of carbon dioxide and water to produce formaldehyde and oxygen:
CO2 + H2O ---> H2C=O + O2
Formaldehyde is the simplest carbohydrate - compounds which have molecular formulas which reduce to multiples of hydrated carbon, (C.H2O)n.
Under normal conditions this reaction does not proceed at all. The reason is that the products - formaldehyde and oxygen - together contain much more energy than the starting materials. Energy from radiation - photon energy - can drive the reaction. Radiation from stars can cause the production of formaldehyde in surrounding gas clouds. In our biosphere photosynthesis traps solar radiation energy to fix carbon dioxide to produce carbohydrates. Photosynthesis does not produce formaldehyde, which is very toxic to cells, but glucose and other carbohydrates, which are polymers of formaldehyde. Let me explain.
Two molecules of formaldehyde can combine to form glycol aldehyde:
H2C=O + H2C=O ---> H2COH.HC=O
Glycol aldehyde still has the reactive HC=O aldehyde group, so it can react again with formaldehyde to produce glyceraldehyde:
H2COH.HC=O + H2C=O ---> H2COH.HCOH.HC=O
Glyceraldehyde also keeps an aldehyde group, so it too can react further with formaldehyde. In theory the process can go on and on, producing huge molecules. In biology it stops usually at six - glucose, galactose, fructose, and other sugars.
The photosynthetic reaction may be written thus:
(C.H2O)5 + CO2 + H2O ---> (C.H2O)6 + O2
The stages of the reaction are complicated, but overall 1 formaldehyde unit is added to ribulose, a 5-fold formaldehyde polymer, producing the 6-fold polymer glucose, and molecular oxygen. The reaction is driven by radiation energy from the sun.
Glucose is a remarkable molecule. It will form a 6-membered ring structure, by the interaction of the aldehyde and the hydroxyl group on carbon 5 in the chain. The resulting ring has 5 carbon and 1 oxygen atoms. Excellent diagrams can be found here.
Glucose is an important biological fuel, releasing energy as it is broken down in stages, eventually releasing the carbon dioxide and water from which it was made. Glucose is also a starting material in the biosynthesis of many other compounds, and it can polymerize into chains of glucose molecules, notably cellulose, the dominant biological product on earth.
Glucose molecules may combine together in one of two ways. The first is the 1-4-beta linkage. Cellulose is a high molecular weight polymer of glucose units linked in this way. The resultant chain molecules have abundant hydroxyl groups, which are in a configuration favouring intermolecular linkages, excluding water. In this way the insoluble long fibres of cellulose are made and are woven to form the cell walls of plants.
Wood is cellulose impregnated with lignins: complex organic molecules which waterproof, stabilise and protect the cellulose.
Chitin, the tough outer shell of insects, is a similar polymer of a simple derivative of glucose - N-acetyl-2-glucosamine.
The second linkage, 1-4-alpha, produces polymers which tend to form branching coils rather than fibres, more soluble in water, and used as biological energy stores: starch in plants, and glycogen in the muscle and liver of animals.
Starches and glycogen are easily broken down to release glucose: the necessary enzymes are common in the biosphere.
Cellulose is much more difficult to digest, and few organisms have enzymes to do this. Those which do are mostly bacteria or fungi, and animals able to digest cellulose usually rely on bacteria in the gut to start the process for them: wood-boring insect larvae, for example. Cattle and other ruminants have complex stomachs in which bacterial enzymes ferment cellulose from plant structures reduced by grinding teeth, often given a second grinding by 'chewing the cud'.
In our biosphere the rarity of cellulases is remarkable, given the energy potentially available from the digestion of cellulose. In consequence woods are biologically stable, and forests are possible.
But here's another spooky fact. At 19% the concentration of oxygen in the atmosphere is near the maximum if cellulose is to be stable. At 25%, for example, fire might well be so frequent and so extensive that forests could not survive. At 33% cellulose would degrade rapidly on exposure to air, and fire would spread explosively.
Life and the planet must interact to maintain the environment most favourable for life, in ways which we do not understand.
Next time you look at a tree, remember that most of what you see is polymerized formaldehyde, the virtual first product of that remarkable reaction of carbon dioxide and water, energetically unfavourable but vital; which biology achieves with aplomb, and which chemists strive to emulate.
And next time you see wood burning, remember you are seeing the release of energy formed by nuclear fusion reactions in the solar core, many years ago.