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I have Read that we cannot digest cellulose because we do not have enzymes to digest Beta glycosidic linkages in Cellulose

Then how is it that we have an enzyme called Lactase to digest the Beta glycosidic linkages in lactose?

Also If we have evolutionarily developed Lactase ,Why didnt we develop "Cellulase " too ?

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  • $\begingroup$ Humans have β-galactosidase (lactase) which allows them to break down the β-glycosidic linkages in lactose. This enzyme is inactive against cellulose. We also haven't evolved a cellulase, probably because we didn't need to. It's quite simple. Are you asking any further questions beyond these? $\endgroup$ – S Pr Oct 30 '19 at 14:43
  • $\begingroup$ Please quote or provide a reference to what you have "read" when asking questions on this site. It is quite likely that you are misquoting or misunderstanding, and by checking the original we can put you right. Enzymes recognize their substrates — in this case not just a chemical bond, but the two sugars that it joins. These are different for cellulose (glucose only) and lactose (glucose, galactose). Also, please restrict yourself to one question at a time. The second question about why few animals have cellulases is quite separate. (It should be obvious why we evolved lactase.) $\endgroup$ – David Oct 30 '19 at 16:01
  • $\begingroup$ @SPr — I'm not sure it's quite so obvious why very few animals have not evolved cellulases. Ruminants and most insects that do "need to" use symbiotic bacteria. I think this might be an interesting question in itself. Is it something to do with the waxy coat on leaves, perhaps? I don't know. $\endgroup$ – David Oct 30 '19 at 16:06
  • $\begingroup$ Few animals have evolved cellulases, but they are a very tiny minority and they tend to be aquatic invertebrates or small insects with little gut tracts like termites; always different to large mammals. I must admit I don't understand "why didn't we develop X" questions though. I think the onus is on the person to reason that it would have been beneficial in the first place, at least to a more competitive degree than the absence of a trait. Consider free glucose in the anaerobic gut, in the presence of a diverse microbiome... Consider a lifelong lack or reduction of fiber... No thanks. $\endgroup$ – S Pr Oct 30 '19 at 16:25
  • $\begingroup$ Although tangential to the OP question, the presence of lactase in adult humans is 'quite a story' and is considered by some to be an example of 'natural selection in action' of the type that @SPr alludes to. Few adults have the ability to digest lactose (after weaning the activity is lost), but (maybe) descendants of human populations 'that have traditionally practiced cattle domestication' retain the ability to do so into adulthood (ref. See also here) $\endgroup$ – user1136 Oct 31 '19 at 2:14

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