Biochemistry II

Lab 3

Carbohydrate Characterization

Biochemistry II

Lab Section: B2

Date of Experiment: February 22nd, 2017

Question 1: Glucose is a monosaccharide, more precisely, a reducing aldohexose. Fructose is a monosaccharide, a reducing ketohexose, but its reducing power comes from isomerisation to aldose. Sucrose is a non-reducing disaccharide composed by glucose and fructose. Xylose is a monosaccharide, a reducing aldopentose.

Question 2: Benedict’s reagent is composed by CuSO4 and other alkaline species. When reducing sugars or other reducing substance contact it under heating, its aldehyde or ketone group becomes enediol and reduce Cu2+ to Cu2O. This compound is insoluble and has a brick-red color which means positive. Glucose, fructose and xylose show positive and sucrose is negative. Bial’s reagent is composed of HCl, orcinol and FeCl3. Under acidic condition, it dehydrates pentose to form fufural compounds; it can react with orcinol to generate a colored compound. In this laboratory, only xylose shows a positive reaction, because it’s the only pentose in all 4 sugars samples. Seliwanoff’s reagent consists of resorcinol and HCl. Ketose dehydrates under acidic condition and reacts with resorcinol to produce a red colored compound. Fructose and sucrose gives a positive result because they have ketose contain. Molisch’s reagent consists of phenol, often alpha-naphthol, dissolve in ethanol. All carbohydrates, nucleic acids and glycoproteins should give a positive reaction. By adding H2SO4, carbohydrates hydrolysis into monosaccharides and then dehydrated to react with phenol to form a purple dye. In cause of hydrolysis reaction time, disaccharide reacts slower than monosaccharide. Therefore, sucrose gives a positive test after 2 minutes, and glucose, fructose and xylose gives a positive test immediately.

Question 3: Sample A was monosaccharide, a reducing aldopentose identify as xylose. Sample B is a monosaccharide, a reducing aldohexose identify as glucose. Sample C is disaccharide, a non-reducing sugar contain ketose and without pentose, identify as sucrose.

Question 4: Pregnant cows have a lower free glucose level in their blood because they need to feed itself and the fetus, so the use of glucose is very high. In the peak lactation period, the lactose and even the milk fat come from the cow’s glucose. That’s why its blood glucose level is the lowest value in 3 samples. Heifers use its glucose only for itself, so its blood glucose level is the highest in all 3 samples.

References

Koshiyama, I. (1966). Carbohydrate component in 7S protein of soybean casein fraction. Agricultural and Biological Chemistry, 30(7), 646-650.

Haynes, W. M. (2016). CRC handbook of chemistry and physics, (97th ed.).

Hunt, I. R. (2009).Reducing sugars, University of Calgary.

Martin, D. (2017). Biochemistry II. Lab 3: Carbohydrate Characterization.

Svennerholm, L. (1957). Quantative estimation of sialic acids: II. A colorimetric resorcinol-hydrochloric acid method. Biochimica et biophysica acta, 24, 604-611.

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