Digestion of sugars

In more complicated molecules each end of these molecules can join up with further sugar units. When many hundreds of units of carbohydrates are linked together, the compounds are called polysaccharides . Starch and cellulose are such compounds. 

The large molecules of the polysaccharides (starches and cellulose) present in foods are too big for us to digest directly. Our bodies hydrolyse (i.e. action of water in the presence of an enzyme) the starches and break them up into smaller molecules, e.g. glucose. Our systems cannot hydrolyse the complicated long chains of sugar units in cellulose, but those of grazing animals can. 

The small glucose molecules are then small enough to be absorbed by the villi on the surface of the small intestine and carried to the liver. The glucose can enter the blood system to be circulated and be readily available to the cells for energy release, for example at muscle endings, as required. 

One gram of carbohydrate gives 17 kJ of energy in the process of respiration. The carbon dioxide is removed from muscle endings by the blood and is transported into the lungs for exhalation. 

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The reaction can be performed chemically adding simple acids to the sugar solution drives the reaction toward equilibrium. Our stomach acids perform it as the first step in our digestion of sugar. It can also be done biologically, using an enzyme catalyst, invertase. That is the first of many steps summarized by the overall reaction Eq. 16.A. 

Table 1. Apparent digestibility (%) of sugar (A) andfodder (C) beet pulp fractions in 50Wi. 

The a-glucosidase inhibitors, acarbose (Precose) and miglitol (Glyset), lower blood sugar by delaying die digestion of carbohydrates and absorption of carbohydrates in the intestine. 

Figure 5.31 LC-electrospray-MS-MS spectrum of the column eluate at around 22 min in the analysis of the peptide mixture from the tryptic digest of glycoprotein TIME-EA4 from silkworm diapause eggs. Reprinted from Bioorg. Med. Chem., 10, Kurahashi, T., Miyazaki, A., Murakami, Y., Suwan, S., Franz, T., Isobe, M., Tani, M. and Kai, H., Determination of a sugar chain and its linkage site on a glycoprotein TIME-EA4 from silkworm diapause eggs by means of LC-ESI-Q-TOF-MS and MS/MS , 1703-1710, Copyright (2002), with permission from Elsevier Science.

Sugar composition (mol%) of MHR population A and fractions I-IV, obtained after Sephacryl S200 size-exclusion chromatography of the digest of MHR population A with RGase [39]. 

Protoplasmic structures are not very stable. Consequently, if protective agents were not employed, destruction of the walls was accompanied by a rapid lysis of the protoplasts, followed by the liberation of most of the cell protein and nucleic acid in soluble form. This could be prevented by employment of the enzyme in a 0.2 M solution of sucrose or cane sugar. After digestion of the cell walls, the living protoplasts rounded up into spheres. 

Suppose we start with a starch-rich meal, say one containing a lot of pasta or bread. The digestion of starches begins in the mouth. Saliva contains an enzyme, salivary amylase (aka ptyalin), which catalyzes the conversion of starch to simple sugars such as glucose. This process is completed in the small intestine under the influence of other enzymes in the amylase class. This completes the first phase of carbohydrate catabolism the conversion of complex, polymeric carbohydrates (e.g., starches) to their simple monomeric units, the sugars. 

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