Eduqas Biology for A Level Book 1: 2nd Edition

Study point If two solutions with different concentrations have the same treatment in the Benedict’s test, the more concentrated solution will have a greater colour change. The test does not actually measure the concentration, but indicates which solution is more concentrated. It can, therefore be described as a semi- quantitative test. Study point If a cell absorbs too much water, its solutes would not be at the appropriate concentration for the cell’s reactions. If too much water were absorbed into an animal cell, it would burst. Study point Make sure you can distinguish between these three terms: Qualitative – tells you if a molecule is present. Semi-quantitative – tells you the relative concentrations of solutions, but no actual values. Quantitative – gives a numerical value for the concentration. Key terms Biosensor: A device that combines a biomolecule, such as an enzyme, with a transducer, to produce an electrical signal which measures the concentration of a chemical. Polymer: A large molecule comprising repeated units, monomers, bonded together. Monomer: Single repeating unit of a polymer. Link The effects of osmosis are described on p61. Exam tip When describing the Benedict’s test, do not forget to say: • The sample is mixed with Benedict’s reagent and strongly heated. • The colour change is from blue to green / orange / red. The Benedict’s test Some disaccharides, such as sucrose, are non-reducing sugars and give a negative result, i.e. the solution remains blue. Sucrose can only be detected if it is first broken down to its constituent monosaccharides, for example by heating with hydrochloric acid. Benedict’s reagent needs alkaline conditions to work, so alkali is added. Benedict’s reagent is then added and heated as before. If the solution now turns red then a non-reducing sugar was initially present. Another way of detecting sucrose would be to use sucrase, an enzyme that hydrolyses sucrose into glucose and fructose. The Benedict’s test will then give a positive result. However, enzymes are specific. Sucrase will only hydrolyse sucrose, so other non- reducing sugars still give a negative result. Giving an actual value to the concentration of sugar present is much more useful. This is described as a quantitative measurement, and using a biosensor , an accurate measurement may be obtained. This is important in monitoring medical conditions such as diabetes, where an accurate measurement of the concentration of blood glucose is required. Polysaccharides Polysaccharides are large, complex polymers . They are formed from very large numbers of monosaccharide units, which are their monomers , linked by glycosidic bonds. Glucose is the main source of energy in cells and it must be stored in an appropriate form. It is soluble in water and so it would increase the concentration of the cell contents, and consequently draw water in by osmosis. This problem is avoided by converting the glucose into starch in plant cells or glycogen in animal cells. Starch and glycogen are storage products. They are polysaccharides. They are more suitable than glucose for storage because: ▪ They are insoluble so they have no osmotic effect. ▪ They cannot diffuse out of the cell. ▪ They are compact molecules and can be stored in a small space. ▪ They carry a lot of energy in their C–H and C–C bonds. Starch Starch is the main store of glucose for plants. Starch grains are found in high concentrations in seeds and storage organs such as potato tubers. Starch is made of α -glucose molecules bonded together in two different ways, forming the two polymers, amylose and amylopectin. ▪ Amylose is a linear, unbranched molecule with α -1,4-glycosidic bonds forming between the first carbon atom (C1) on one glucose monomer and the fourth carbon atom (C4) on the adjacent one. This is repeated, forming a chain, which coils into a helix. Fig 03 Test results negative very low low medium high Colour depends on concentration of reducing sugar present in sample. Equal volume of Benedict’s reagent added to glucose sample dissolved in water. Heated in water bath. If reducing sugar present solution changes colour. heat Eduqas Year 1/AS Biology 18