How can stomach acid be strong enough to dissolve thin razor blades, but stuff like corn and tomato skin can pass through seemingly completely untouched?
In: Biology
Your stomach acid is composed of HCl which is good at dissolving metals and peptide bonds (thus dissolving proteins). It’s not so great at dissolving sugars and fats. Your intestine releases two enzymes – amylase and lipase – which dissolve starches (long-chain sugars in alpha-glucose form, a 3D structure that is compact but has bonds that are easily attacked by these enzymes) and fats, respectively.
Notably, amylase can *only* effectively break down alpha-structured sugars. Cellulose, on the other hand, is composed of sugars in a beta orientation leading to long, tightly bound structural chains (or “fibres”). Your body cannot digest these sugars and so they are simply excreted as waste.
Aside from serving a structural/protective role for the plants they originate in, the body *does* still use cellulose. Whenever you hear that you need more “fibre” in your diet, that is what is being referred to. While not having any nutritional significance, these fibres stimulate mucous secretion in the intestine. This mucous later allows the diffusion of nutrients from the excrement into the body, as well as lubricates the intestine so that waste can move more easily.
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