Starch and Cellulose

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The sketch shows a fragment of cellulose. It is also a very long polymer of glucose. There are roughly 4,000 to 8,000 glucose molecules strung together.
The glucose units are sketched as a stair step to get the beta OH of one near the #4 OH of another. The molecule never has branches. The only difference from linear starch is beta-1-4 links instead of alpha-1-4 links, but it makes a world of difference in properties. Cellulose is very strong. Its links are broken by cellulase enzymes that are far less common in nature than are the amylase enzymes that split starch. Cellulose confers strength and structure to plants, and it would be bad if there were a plethora of organisms in nature that could break down cellulose. Cellulose is found as a composite mixture with hemicellulose and lignin.

Two glucose molecules reacted with a beta-1-4 link make the disaccharide cellobiose. It is an important intermediate in the enzymatic hydrolysis of cellulose.

The sketch of linear starch has many glucoses strung together with alpha-1-4 linkages. It would take dozens of sheets of paper to show a typical starch molecule at the scale of this sketch. Starch can be mostly linear, mostly branched, or a mixture depending on its source. A fragment of branched starch is shown in another sketch. There can be branches on the branches and several branches on any stem. The proportions of linear and branched and the degree of branching cause starch from different sources to have different properties.

  • A few concluding remarks
  • DONE
  • Back to boat and chair structures
  • Back to ring structures of sugars
  • Back to start