Carbohydrate forms

Storage polysaccharides are an important carbohydrate form in plants and animals. It seems likely that organisms store carbohydrates in the form of polysaccharides rather than as monosaccharides to lower the osmotic pressure of the sugar reserves. Because osmotic pressures depend only on numbers of molecules, the osmotic pressure is greatly reduced by formation of a few polysaccharide molecules out of thousands (or even millions) of monosaccharide units. 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

Carbohydrates occur in every living organism. The sugar and starch in food and the cellulose in wood, paper, arid cotton are nearly pure carbohydrates. Modified carbohydrates form part of the coating around living cells, other carbohydrates are part of the nucleic acids that carry our genetic information, and still others are used as medicines. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Disaccharide (Section 25.8) A carbohydrate formed by linking two simple sugars through an acetal bond. 

If allowance is made for the fact that much of the carbohydrate formed from protein, which is oxidized in the non-diabetic, may escape metabolism by the diabetic and be excreted in the urine, the protein R. Q. for the diabetic should be 0.632 instead of the normal value of 0.802.134 The R. Q. for fat is also altered because of the failure of a complete combustion of this foodstuff, due to the attendant ketonuria. 

Sweetness free, or at least virtually free, from calories establishes the need for intense sweeteners while suitability for diabetics, absence of carcinogenicity with functionality similar to sucrose and related carbohydrates form the basis for application of bulk sweeteners. 

Oxygen released in photosynthesis is taken up in respiration. Carbohydrates formed in photosynthesis are consumed in respiration. Sunlight drives the synthesis of ATP. 

Carbohydrates Ultimately, carbohydrates cire the product of photosynthesis, the process in plants that combines carbon dioxide, water, and energy with chlorophyll and other biomolecules to produce Ccirbohy-drates and release oxygen gas. The major carbohydrate formed during photosynthesis is glucose. Plants and animals sometimes combine simple carbohydrates such as glucose into more complicated carbohydrates such as stcirch, glycogen, and cellulose. 

If the carbohydrate formed is cellulose, then the reaction in effect is the reverse of the burning of wood, and obviously requires considerable energy input. 

Carbohydrates form the major structural components of the cell walls. The most common form is cellulose which makes up over 30 per cent of the dry weight of wood. Other structural forms are hemicellulose (a mixed polymer of hexose and pentose sugars), pectins and chitin. Apart from contributing to the structure, some polymers also act as energy storage materials in living systems. Glycogen and starch form the major carbohydrate stores of animals and plants, respectively. Carbohydrate structure, like that of nucleic acids and proteins, is complex, and various levels of structure can be identified. 

Dipole moment (/i) (Section 1.9) The product of the amount of charge separation in a molecule times the distance of the charge separation. Disaccharide (Section 25.6) A carbohydrate formed from two monosaccharide units tliat are connected by a glycosidic bond. 

Monosaccharide (Section 25.1) A carbohydrate formed from one sugar unit. 

Polysaccharide (Section 25.7) A polymeric carbohydrate formed from monosaccharides that are linked by glycosidic bonds. 

Acetyl-CoA is produced from fatty acids, proteins, and carbohydrates and is a central and major compound in intermediary metabolism. The mechanism of its formation from the degradation of fatty acids and proteins is discussed in Chaps. 13 and 15, respectively here, the means whereby carbohydrates form this most important molecule will be presented. The glycolytic pathway can yield pyruvate from all degradable sugars, and this can be converted to acetyl-CoA. Pyruvate enters the mitochondrial matrix and is the substrate for the multienzyme complex pyruvate dehydrogenase. 

In Chapter 4 (Section 4.1D), we indicated that the radiation input of the sun to the earth s atmosphere averages 1366 W m-2 (the solar constant ). Some of the radiant energy is used to form ATP and NADPH in chloroplasts. In turn, these energy currencies lead to the reductive fixation of CO2 into a carbohydrate in photosynthesis (see Fig. 5-1). In the same photosynthetic cells, in other plant cells, and in animal cells, the carbohydrates formed during photosynthesis can serve as the energy source for mitochondrial respiration, which leads to the generation of ATP by oxidative phosphorylation. 

Carbohydrates form another class of biologically important molecules. They serve as a food source for most organisms and as a structural material for plants. Because many carbohydrates have the empirical formula CH2O, it was originally believed that these substances were hydrates of carbon, thus accounting for the name. 

Lactose, a carbohydrate formed from two simple sugars, glucose and galactose, is the principal sugar in dairy products. Many individuals, mainly of Asian and African descent, lack adequate amounts of the enzyme necessary to digest and absorb lactose. This condition, lactose intolerance, is associated with abdominal cramping and recurrent diarrhea, and is precipitated by the ingestion of milk and dairy products. In Chapter 27, we learn about the structure, synthesis, and properties of carbohydrates like lactose. 

Carbohydrates form a class of compounds of carbon with hydrogen and oxygen. The name comes from the chemical formulas of these compounds, which can be written C (H20) suggesting a hydrate of carbon. Simple sugars, or monosaccharides, are carbohydrates with the chemical formula C H2 0 . Sugars with three, four, five, and six carbon atoms are called trioses, tetroses, pentoses, and hexoses, respectively. 

Galactose oxidase (EC 1.1.3.9, GAO) from the fungus Fusarium sp. was used as a starting gene for directed evolution. The GAO has a molecular weight of 68 kDa, and is a monomeric glycoprotein with 1.7% carbohydrate, formed by 639 amino acid residues. It catalyzes the following reaction ... 

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