Science Makes Sense-Week43:Organic Chemistry-Starches, sugars, cellulose

As children, we were fascinated by travel and far-off destinations. We would implore our parents to take a trip around the world and the answer would invariably be:” We cannot afford it, money does not grow on trees!”  If I had studied science then, I could have smartly retorted,” Oh but it does! Money is made from paper and paper comes from trees!”

Starch and cellulose molecules are polysaccharides,which literally means ‘many sugars’. (Ref.1). Cellulose is a long chain of linked sugar molecules that gives wood its strength and is the main component of plant cell walls and the starting point for textiles and paper.(Ref.2)  Cellulose is a poly saccharide polymer with many glucose mono saccharide units (Ref.3) and so is starch.   What is the difference?  Now each glucose unit is joined by acetal linkages.   These acetal linkages are actually an oxygen atom, connecting each glucose unit, left after a reaction between an alcohol group,-OH,with an aldehyde ,CH=O group where the water molecule is removed. (Ref.1)  In cellulose the acetal linkages are beta linkages (acetal group in the upper position) while in starch these are alpha linkages (acetal group in the lower position).  This interesting variation in acetal linkages between starch and cellulose  results in major difference in digestibility in humans.  Humans cannot digest cellulose since they lack the enzymes needed to break the beta linkages, while they can easily  digest starch.  Animals such as cows,sheep, goats and termites  have symbiotic bacteria in their intestinal tract that breakdown cellulose.(Ref.3)

The structure of cellulose and starch vary because of the different acetal linkages as well.  The angles in the beta linkages makes the polysaccharide cellulose a linear chain.  Meanwhile the angles of the alpha acetal linkages in starch form a spiral like a coiled spring.(Ref.3)

These polysaccharides are abundant in nature and their main function is energy storage and are the components of cell walls. They are called mono-polymers because they yield only one type of monosaccharide,namely glucose, after hydrolysis. Hetero-polymers also exist in nature (namely those polysaccharides which yield many kinds of mono-saccharides upon hydrolysis) and include gum, pectin.(Ref.4)  

Polysaccharides are not sweet-tasting like mono or disaccharides. They are non- reducing carbohydrates and do not undergo mutarotation.(Ref.4)  What is mutarotation?  A monosaccharide like D- glucose has two stereo isomers that can co-exist because of change in specific rotation of the chiral compound (a compound that does not superimpose on its mirror image).  The two isomers are called alpha D-glucose and beta D- glucose respectively.  Both are cyclic compounds with one acetal group, 5 carbons, 5 hydroxyl groups (-OH).   In the alpha form, one of the -OH is in the lower position whereas in the beta form it is in the upper position.(Ref.5)

Uses of sugars, cellulose and starch:  these mono/ di and polysaccharides can also be broadly classified as carbohydrates, sources of energy.  Green plants manufacture sugars and most of it is used for plant metabolism and very little accumulates.  However many vegetables and trees are sources of commercial sugar.(SeeNuggets)  Cellulose is a complex carbohydrate and cotton, pure cellulose, is the starting point for artificial fibers.  When wood, also cellulose, is treated with concentrated acids and alkalis, the bond between wood fibers and the lignin ( which holds them together tightly) is broken.  These can be reorganized to form paper.  Treated with more chemicals this can lead to the production of artificial fibers and cellulose plastics.  If you further break it down to the individual elements of carbon, C, hydrogen,H, and oxygen,O, (which is the fundamental elements in cellulose) these elements  can be recombined to form wood sugar, yeast and alcohol.  These are the raw materials for many industrial products.(Ref.6)

From time immemorial, we humans have depended on starch, cellulose and sugar for our energy intake, today we have broadened its usage considerably.

Activities for Middle School Teachers:

Construct molecules with acetal or hydroxyl linkages.  What changes in structures are observed with alpha or beta linkages? Where do you see stereo isomerism? Why or why not? (Remember the lack of rotation in the poly saccharides like cellulose and starch)

Students will look at the timeline for cotton production in the U.S. and other countries.  What was the impact of slavery on cotton productions?  How long did the production of cotton depend on African American labor even after slavery was dismantled?  As students study the structure of cellulose, let them study the history of  cotton production here in the US.

Look at potato, a common starch source for the western world. How were the Irish affected by the potato blight?  How did it change the demographics in the U.S.?

Nuggets of Information:

Cotton is the purest form of cellulose.  In the laboratory, ashless filter paper is, for all practical purposes,a source of pure cellulose.(Ref.2) 

Sucrose or table sugar is the most familiar disaccharide made by linking fructose and glucose.(Ref.2)

The length of the cellulose chain varies greatly: a few 100 sugar units in wood pulp to 6000 units  for cotton! (Ref.3)

Linking just two sugars produces a disaccharide called cellobiose, whereas cellulose is a polysaccharide produced by linking additional sugars in the same way.(Ref.3)

Storage sugars are found in roots of plants like beets, carrots and in stems of plants like sugar cane, sorghum and in flowers such as palm sugar and sugar maple.(Ref.6)

Maize/Indian corn is the source of 80% of the starch made in the U.S., while Europe is the principal producer of potato starch.(Ref.6)



5. ( look for sugars,starch and cellulose)



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