Viable alternatives to lignin and cellulose for cell walls and wood?

Question

I want to design alien plants in which wood and cell walls are not made of the lignin/cellulose mixture wood and plant cell walls on Earth are made from, but I don't actually know what materials would be suitable. On it's own, I admit, this isn't a very helpful thing to say, as what counts as "suitable" could be almost infinitely variable, so I have a list of criteria for my definition of suitability:

1. Its strongest form must have a ratio of compressive strength to weight at least 75% that of average Earth wood

2. It should be sufficiently rigid that tall trees made of it would remain upright

3. It should be composed entirely of carbon, hydrogen, oxygen, and nitrogen, with no other elements as part of its core chemical structure

4. It should not be so reactive that it would be significantly more unstable than flammable wood in an oxygen-rich atmosphere

5. It should be possible to biodegrade but not so easy to digest that it cannot form fossil fuels; If it cannot biodegrade at all, it is not viable for use by living things, but I would also like the planet these plants to inhabit to experience an industrial revolution at some point, which requires at least coal, so it must be plausible for it to take long enough for biodegradation of it to evolve that something like what happened in the Earth Carboniferous era could take place whereby it could trap enough carbon before biodegradation evolved that large amounts of carbon-rich material derived from it could be fossilized

With all that out of the way, what materials meet the above criteria and, therefore, could comprise the wood and plant cell walls of an alien world, and what interesting uniquenesses do these have?

Answer 1

The space of possible structural chemicals, as AlexP observed, is Quite Big.

Cellulose is a polysaccharide, for example. Chitin is another, and is used for the cell walls of fungi. You don't have any cell walls, but various bits of you are held together or held up by polypeptides like collagen or keratin. Various things can synthesize silk, another protein based material.

And terrestrial life's building blocks don't even end there... lignin is a different kind of polymer again (a polyphenol). For a non-polymer example, corals use calcium carbonate, though that's perhaps a little less convenient above the water's surface and not quite so suitable for a fuel, but other minerals could substitute. Styrene, the monomer from which polystyrene is formed, can be extracted from the resin of a particular species of tree. Latex rubber is a natural product, and although is isn't used as a natural structural material it clearly could be.

And that's barely scratching the surface of structural compounds. There's plenty of carbon in all those things (apart from maybe conventional corals), and so plenty of scope for forming fossil fuels given enough time and suitable conditions.

The point is you can handwave in pretty much anything you like, given your relaxed and plausible requirements. You're not asking for supernatural properties, so you can just declare your material to exist and you can't really be "wrong". There's no need to go into any detail... just make up a name. It'll be fine. This isn't like inventing a new element, or natural superconductor, or nuclear fuel or whatever. Don't overthink it, and use your limited and valuable brainwidth on other parts of your story and setting.

It should be possible to biodegrade but not so easy to digest that it cannot form fossil fuels

Lignin was obviously undigestible for quite some time, hence coal, but under the right conditions all sorts of organic matter can turn into fossil fuels. The precursors to oil and gas would have been very much digestible under normal circumstances, but dead stuff sinking to the bottom of an anoxic abyssal plain isn't going to be broken down in the same way as the same material sitting around in a damp, temperate, oxygenated location. Wood free plant matter can form peat, and peat can turn into coal too; its about the environment of decay as much as the materials involved.

Remember also the existence of nylonase! Just because something might seem unbiodegradeable now, does not mean it is intrinsically immune to all forms of digestion.

an industrial revolution at some point, which requires at least coal

It isn't a fundamental requirement, merely a very convenient thing to use. Oil and gas are more awkward to work with, but they can do all the necessary things.

That said, there's nothing to stop you using your authorial fiat to say that your xenolignoid polymers can undergo coalification (which is a real word, apparently).

Answer 2

Chitin or Keratin

From Wikipedia:

Chitin $(C_8H_{13}O_5N)$ is a primary component of cell walls in fungi, the exoskeletons of arthropods such as crustaceans and insects, and the radulae, cephalopod beaks and gladii of molluscs. It is also synthesised by at least some fish and lissamphibians.[1] The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers. It is functionally comparable to the protein keratin.

Chitin is structurally similar to cellulose and suitable for uses ranging from cell walls to shells to exoskeletons, and presumably everything in between. It would work for tree bark too.

Answer 3

Polycarbonate.

https://en.wikipedia.org/wiki/Polycarbonate#Construction_materials

lexan sheets

There are many types of plastic. Above is polycarbonate; the stuff is strong, light and durable. There are other kinds of plastic. Probably you are sitting on some. You are! And that button on your pants is plastic. You can leave it buttoned for now.

Plastic I am reminded sometimes has nonorganic interlopers but polycarbonate is just C H and O. There is no reason biology could not make it. It would be easier than making diamonds which was my initial idea. I like the idea of Gaudiesque organic plastic structures comprising the plants of your world. In a world of plastic plants there would of course eventually evolve things that could eat the carbon in the plastic. Our world is not quite there yet, and so plastic accumulates the way cellulose did before fungi evolved the ability to eat it.

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Skeptical Mark. Luckily I have a love of learning!

Its strongest form must have a ratio of compressive strength to weight at least 75% that of average Earth wood

Polycarbonate properties Compressive Strength 121- 159 MPa 17.5496 - 23.061 ksi 1 ksi = 1000 psi. 23.061 ksi = 23061 psi.

Southern red oak wood Compressive Strength 3.79 MPa 550 psi Green, perpendicular to grain 6.00 MPa 870 psi 12% moisture content, perpendicular to grain 20.9 MPa 3030 psi Green, parallel to grain 42.0 MPa 6090 psi 12% moisture content, parallel to grain

Oak = 0.52g/cc compressive strength / weight = 11711

Polycarbonate = 1.22g/cc compressive strength / weight = 18902

Polycarbonate has a little less than 400% of the compressive strength of the strongest oak. Compressive strength / weight exceeds oak.

It should be sufficiently rigid that tall trees made of it would remain upright

I think this has more to do with the 3dimensional structure of the trunk than the stiffness of the material. But for your viewing pleasure here is a polycarbonate dome measuring about 3m.

https://vikingdome.com/en-be/products/auradome-60-standard-geodesic-dome

It should be composed entirely of carbon, hydrogen, oxygen, and nitrogen, with no other elements as part of its core chemical structure

Yes but no nitrogen. You can add your own when you are done with it, if no-one is watching. This is supposed to be a tree after all.

It should not be so reactive that it would be significantly more unstable than flammable wood in an oxygen-rich atmosphere Yes.

It should be possible to biodegrade but not so easy to digest that it cannot form fossil fuels

Yes. They are just carbon carbon bonds. No funky halogens to make the molecule indigestible. In an oxygen atmosphere they would provide good energy.