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Carbohydrates: from one sugar to a store of thousands questions
Monosaccharides including alpha and beta glucose, ribose and deoxyribose. The glycosidic bond formed by condensation, and the disaccharides maltose, sucrose and lactose. Starch as amylose and amylopectin, glycogen, and cellulose. The Benedict's and iodine tests, including a quantitative method.
8 original questions · 25 marks · the carbohydrates: from one sugar to a store of thousands notes · Biological molecules, water and inorganic ions
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Describe the structure of a molecule of starch.
Mark scheme
- B1 a polysaccharide made of many α-glucose monomers joined by glycosidic bonds
- B1 it is a mixture of two polysaccharides, amylose and amylopectin
- B1 amylose has 1,4 glycosidic bonds only and coils into a helix
- B1 amylopectin has 1,4 glycosidic bonds along the chain and 1,6 glycosidic bonds at the branch points
Cellulose and amylose are both unbranched polysaccharides of glucose. Explain how the structure of cellulose makes it suitable for its role in a plant cell wall.
Mark scheme
- B1 cellulose is made of β-glucose, so every other glucose molecule is rotated through 180°
- B1 this makes the chains straight rather than coiled, so they lie parallel to one another
- B1 hydrogen bonds form between hydroxyl groups on neighbouring chains, holding them together as microfibrils
- B1 the many hydrogen bonds give high tensile strength, so the wall resists the pressure of a cell taking in water and the cell does not burst
A colorimeter is available. Describe how you would find the concentration of reducing sugar in a fruit juice.
Mark scheme
- B1 make up a series of glucose solutions of known concentration
- B1 add the same volume of Benedict's solution to each, in excess, and heat all the tubes in a water bath at the same temperature for the same time
- B1 filter off the precipitate and measure the absorbance of each filtrate in the colorimeter, then plot absorbance against known concentration as a calibration curve
- B1 treat the fruit juice in exactly the same way and read its concentration from the curve, diluting the sample and repeating if its reading falls outside the range of the standards
One molecule of glycogen was built from 2,400 molecules of α-glucose. Taking the relative molecular mass of water as 18, calculate the total relative mass of the water released while that molecule was assembled.
Compare the structure of glycogen with the structure of amylose.
A student adds Benedict's solution to a solution of a carbohydrate and heats it in a water bath. The mixture stays blue. Suggest why this result does not show that the solution contains no carbohydrate, and suggest what the student should do next.
State what is meant by a monosaccharide, and state the molecular formula of glucose.
Name the bond formed when two monosaccharides are joined together, and name the two monosaccharides that make up sucrose.
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