Editing Metabolism (section)

===Digestion===
{{further|Digestion|Gastrointestinal tract}}
Macromolecules cannot be directly processed by cells. Macromolecules must be broken into smaller units before they can be used in cell metabolism. Different classes of enzymes are used to digest these polymers. These [[digestive enzyme]]s include [[protease]]s that digest proteins into amino acids, as well as [[glycoside hydrolase]]s that digest polysaccharides into simple sugars known as [[monosaccharides]].<ref>{{cite book| vauthors = Demirel Y |title=Energy : production, conversion, storage, conservation, and coupling|publisher=Springer|year=2016|isbn=978-3-319-29650-0|edition=Second|location=Lincoln|pages=431|oclc=945435943}}</ref>

Microbes simply secrete digestive enzymes into their surroundings,<ref>{{cite journal | vauthors = Häse CC, Finkelstein RA | title = Bacterial extracellular zinc-containing metalloproteases | journal = Microbiological Reviews | volume = 57 | issue = 4 | pages = 823–37 | date = December 1993 | pmid = 8302217 | pmc = 372940 | doi = 10.1128/MMBR.57.4.823-837.1993 }}</ref><ref>{{cite journal | vauthors = Gupta R, Gupta N, Rathi P | title = Bacterial lipases: an overview of production, purification and biochemical properties | journal = Applied Microbiology and Biotechnology | volume = 64 | issue = 6 | pages = 763–81 | date = June 2004 | pmid = 14966663 | doi = 10.1007/s00253-004-1568-8 | s2cid = 206934353 }}</ref> while animals only secrete these enzymes from specialized cells in their [[Gastrointestinal tract|guts]], including the [[stomach]] and [[pancreas]], and in [[salivary gland]]s.<ref>{{cite journal | vauthors = Hoyle T | title = The digestive system: linking theory and practice | journal = British Journal of Nursing | volume = 6 | issue = 22 | pages = 1285–91 | year = 1997 | pmid = 9470654 | doi = 10.12968/bjon.1997.6.22.1285 }}</ref> The amino acids or sugars released by these extracellular enzymes are then pumped into cells by [[active transport]] proteins.<ref>{{cite journal | vauthors = Souba WW, Pacitti AJ | title = How amino acids get into cells: mechanisms, models, menus, and mediators | journal = Journal of Parenteral and Enteral Nutrition | volume = 16 | issue = 6 | pages = 569–78 | year = 1992 | pmid = 1494216 | doi = 10.1177/0148607192016006569 }}</ref><ref>{{cite journal | vauthors = Barrett MP, Walmsley AR, Gould GW | title = Structure and function of facilitative sugar transporters | journal = Current Opinion in Cell Biology | volume = 11 | issue = 4 | pages = 496–502 | date = August 1999 | pmid = 10449337 | doi = 10.1016/S0955-0674(99)80072-6 }}</ref>
[[File:Catabolism schematic.svg|thumb|left|upright=1.35|A simplified outline of the catabolism of [[protein]]s, [[carbohydrate]]s and [[fat]]s<ref name="n009">{{cite journal | last=Sinthupoom | first=Nujarin | last2=Prachayasittikul | first2=Veda | last3=Prachayasittikul | first3=Supaluk | last4=Ruchirawat | first4=Somsak | last5=Prachayasittikul | first5=Virapong | title=Nicotinic acid and derivatives as multifunctional pharmacophores for medical applications | journal=European Food Research and Technology | volume=240 | issue=1 | date=2015 | issn=1438-2377 | doi=10.1007/s00217-014-2354-1 | pages=1–17 | url=http://link.springer.com/10.1007/s00217-014-2354-1 | access-date=2025-04-17}}</ref><ref name="k966">{{cite journal | last=Clark | first=Audra | last2=Imran | first2=Jonathan | last3=Madni | first3=Tarik | last4=Wolf | first4=Steven E. | title=Nutrition and metabolism in burn patients | journal=Burns & Trauma | volume=5 | date=2017-12-01 | issn=2321-3876 | pmid=28428966 | pmc=5393025 | doi=10.1186/s41038-017-0076-x | doi-access=free | page=}}</ref>]]