Which of the following components of a tossed salad will pass through the human digestive tract and be digested the least?

* cellulose (in the lettuce)
* starch (in the croutons)
* protein (in the bacon bits)
* sugar (in the dressing)
* oil (in the dressing)

* cellulose (in the lettuce)

Explanation:

Carbohydrates molecules that supply energy and provide support consist of mainly sugars or starches in long chains and rings to form monosaccharide monomers. Inclusive of monosaccharides, disaccharides and polysaccharides (which describes the type of bonding and the degree of complexity of the polymers), their basic makeup comprises C, H, O -with many polar OH groups.

Enzymes are proteins which catalyze chemical reactions like the breakdown of large polysaccharides. Some enzymes act on polysaccharides bonds at random points along the chain by splitting the 1-4 glycosidic bonds to produce monomers such as maltose, dextrin and glucose for use in ATP synthesis via respiration, ethyl alcohol fermentation and lactic aicid fermentation.

However, cellulose is a structural polysaccharide, made up of special β,  1-4 glycosidic bonds, which give structural polysacharides their rigid, strong structure. However, the human body does not contain the correct enzymes cellulases to split β,  1-4 glycosidic bonds, so they pass through the intestinal tract unchanged as dietary fiber - cellilases are naturally found in protozoans,  bacteria and fungi.  Animals such as pigs, horses and cows digest cellulose with the help of bacteria in their intestines, while invertebrate earthworms, and snails readily digest the compound on their own.

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