eli5 : Why can’t we (humans) eat wood? It contains glucose, and termites can eat it for example! (Even if raw wood would be pretty hard to chew, we could mix it down into some kind of paste or powder, so idk)
In: Biology
It doesn’t contain glucose as such. Cellulose is a polymer of sugar-type molecules bonded together. They are not released by our digestive system but some animals can break it down.
Wood consists mostly of celulose. Our digestive tract cannot digest cellulose. The calorific value of anything else in wood makes the material a very inefficient fuel, from a human perspective. Only certain animals (like termites and beetles) can digest cellulose and metabolize it into useful energy.
Termites have microbes in their digestive tracts that break down the cellulose into sugars that they can digest. I suppose if you could get those to live in your gut you could probably eat it too.
Cows and other ruminants have multiple stomachs that let the cellulose in grass ferment into something digestible, which is a similar situation.
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Cellulose is a not-uncommon food additive. It’s why Taco Bell meat wasn’t 100% meat! It’s a good gelling agent, and it can emulsify fat somewhat, or at least absorb it.
But we can’t really digest it, it’s just more of a bulking agent.
Wood doesn’t contain glucose itself but rather cellulose, which is basically a long chain of glucose molecules. If you want to break the cellulose apart you need a cellulase enzyme and our body can’t produce that. Cows, for example, can produce it though and that’s why they can survive off of grass alone.
>it contains glucose
Pay attention to the ose part of that. That particular one is a monosaccharide, mono meaning singular(chain). There are also disaccharides meaning two chains. Lactose is one of those(galactose + glucose).
Celulose is a polysaccharide and poly means many. So our digestive system has nothing to handle that many chains. Fleetwood mac wrote a song about it.
The takeway is that certain plants that bear ‘fruit’ or ‘veg’ have also enclosed their seeds with it. Its a clever way for them to break up some of those chains for us, leading to far less necessary digestive prowess, and a free amount of fertilizer for their seeds on the way out.
Wood pulp is used in foods to keep stuff separated like cheese. https://www.cornucopia.org/2017/11/brief-history-wood-pulp-food/
Wood is made os cellulose which is a bunch of glucose molecules hooked together. Wood is almost exactly the same as starch but the glucose are hooked in equatorial positions and the starch is hooked in axial positions. Think of equatorial as side to side and axial as up and down. The side to side is more energetically favorable and so requires to much energy for our stomachs to break down. Cows and horses can eat cellulose (grass) because they have a symbiotic relationship with the bacteria in their guts that break apart the linkages and then they digest the remaining glucose.
This may not sound like a true eli5 but it is the correct answer and it was the most satisfy to me when I had this same question.
To be a little more specific about it, think of a body of an animal like you would a budget. Every body has a set amount of energy resources that it can use for certain functions. Think D&D skill points or a fallout/skyrim character generator.
Animals that can eat wood and cellulose plant fibers, like cattle and other herbivores have spec’ed a majority of their points into their gut, they spend most of their time chewing and digesting food. Their gastrointestinal tract is massive compared to their body size. Cows have a 4 chambered stomach, and a long intestine to squeeze literally every last bit of nutrient out of grass(grass is very low calorie) This leaves fewer points to put into other energy dependent things like a large brain.
Humans have one of the largest brain to body ratio’s in the animal kingdom. We’re totally playing life as an INT main playthrough. The trade off to that is our gut tract is a pretty simple tube that is pretty bad at digesting complex starches like cellulose so most of it just passes straight through (corn). Luckily we’ve figured out how to cook our food to get more nutrients out of it, and have figured out how to find and grow the most nutrient rich plants on the planet.
TLDR If we had a gut system to eat grass and wood, we wouldn’t have the metabolic energy left to have as large a brain as we do.
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