Both amylose and cellulose have the same "monomer" structure (glucose), so what makes them look/form so differently?



Your structures don't clearly show the different configurations between α- and β- linked glycopyranosides. Cellulose is a non-branching (poly) β-glycopyranoside. Amylose (a component of starch) is a non-branching (poly) α-glycopyranoside. The enzymes in your mouth and digestive system are good at breaking apart the α-glycopyranoside linkage, but not the β-glycopyranoside (it doesn't fit into the enzyme).
Amylopectin (the other component of starch) is a branched (poly) α-glycopyranoside. It branches with a 1-6 linkage. Glycogen is another branched (poly) α-glycopyranoside.
Cellulose is not the only (poly) β-glycopyranoside; there is also β-glucans.

Cellulose and Amylopectin

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You can also add glyocogen to that list, a glycan found in us The difference is that cellulose has a linear structure, allowing parallel chains to lie in close proximity and for hydrogen bonding to predominate, strengthening the structure.. Starch, on the other hand, has side branches preventing as many hydrogen bonds from forming.

Note also that the site above shows very similar structures for starch and cellulose. The illustrations you've shown are using different methods to display the structure, emphasizing the cyclic nature in the first image, and the bond angles in the second. It would be interesting to see these molecules in atomic force microscopy, rather than in our conventional diagrams.

BTW, consider the different structure of carbon in diamond and graphite, where there is little bonding between layers of graphite, but diamond has three-dimensional strength.

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    $\begingroup$ Amylose is not really branched very much, although the other main component of starch, amylopectin, is. The difference in amylose and cellulose is the geometry of the 1,4 glycosidic linkage and the effect of on the resulting crystal structure of the two polymers. Inter-chain H bonding is greatly facilitated by the cellulose crystal structure, making it exceptionally resistant to hydrolysis, relatively insoluble, and a tougher material than amylose, all because it's 1,4 linkage is beta instead of alpha. $\endgroup$ – Curt F. Jun 19 '15 at 16:57
  • $\begingroup$ @CurtF., could you please explain (or attach a link explaining) the difference between alpha linkage and beta linkage? $\endgroup$ – Sparkler Jun 19 '15 at 17:02
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    $\begingroup$ There is some info here: chem.ucla.edu/harding/ec_tutorials/tutorial43.pdf $\endgroup$ – Curt F. Jun 19 '15 at 17:48

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