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O-Level Biological Molecules
What the O-Level syllabus expects for Biological Molecules, and how to practise it.
What the syllabus expects
- Name the chemical elements found in carbohydrates, in fats and in proteins.
- Give the chief functions that carbohydrates, fats and proteins serve in living organisms: carbohydrates supplying energy right away, fats providing insulation and long-term energy stores, and proteins supporting the growth and repair of cells.
- Set out and perform the tests for starch (iodine in potassium iodide solution), reducing sugars (Benedict's solution), protein (biuret solution) and fats (ethanol).
- Note that large molecules get assembled out of smaller units: glucose builds cellulose, glycogen and starch; amino acids build polypeptides and proteins; while glycerol together with fatty acids builds lipids such as fats.
- Account for how enzymes work, referring to the active site, the enzyme-substrate complex, the lowering of activation energy and enzyme specificity, drawing on the 'lock and key' hypothesis.
- Investigate and account for how temperature and pH influence the speed of enzyme-catalysed reactions.
How it's examined
Questions on this topic most often ask you to explain, find, outline, suggest. About 21% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (5 marks)
9 OR (a) Outline the characteristic properties of enzymes.
Show the worked answer
Characteristic properties of enzymes: - They are biological catalysts: they speed up the rate of chemical reactions without themselves being used up/changed, so they can be reused. - They are proteins with a specific three-dimensional shape, including an active site. - They are specific: each enzyme acts only on one substrate (or type of reaction) whose shape is complementary to the active site. - They are affected by temperature: activity increases with temperature up to an optimum, then falls as the enzyme is denatured at high temperatures. - They are affected by pH: each has an optimum pH; extremes of pH denature the enzyme. - They are required in small amounts because they are not used up.
Example 2 (4 marks)
Table 2.1 lists four biological molecules. Table 2.1: Starch, Glucose, Glycogen, Haemoglobin Sort each of these into the categories in Table 2.2; a single molecule may belong to more than one category. Table 2.2 Categories: monomers, polymers, monosaccharides, polysaccharides
Show the worked answer
Glucose is a single sugar unit, so it is both a monomer and a monosaccharide. Starch and glycogen are large molecules made of many glucose units, so they are polymers and polysaccharides. Haemoglobin is a protein made of many amino acids, so it is a polymer but not a carbohydrate (not a mono- or polysaccharide).
Example 3 (2 marks)
Explain why pectinase works best at the pH you gave in (i).
Show the worked answer
Every enzyme has an optimum pH. At this pH the active site of pectinase has its correct three-dimensional shape, complementary to the pectin substrate, so the maximum number of enzyme-substrate complexes form and the reaction rate is highest. At higher or lower pH, the changed H+ concentration disrupts the bonds holding the active site's shape, so the active site changes shape, the substrate no longer fits, and activity falls.
More worked questions on this topic
- The Atkins diet recommends eating more protein. Propose and explain drawbacks that this diet mi (2 marks)
- Using the data in Table 24.1 (enzyme activity at 0-50 degC), plot rate of enzyme activity again (4 marks)
More O-Level Pure Biology topics
Cell Structure and Organisation · Movement of Substances · Nutrition in Humans · Transport in Humans · Respiration in Humans · Excretion in Humans · all of O-Level Pure Biology