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As 03 Enzymes
 

As 03 Enzymes

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    As 03 Enzymes As 03 Enzymes Presentation Transcript

    • A S Biology Exam Doctor
      • ENZYMES
      Question 2 Question 3 Question 1
    • A S Biology Exam Doctor
      • ENZYMES
      QUESTION 1
    • The equation shows the effect of an enzyme on carbohydrates in the buccal cavity. (b) Explain: (i) the role of the chloride ions in this reaction. (a) Identify the enzyme involved in this reaction. (ii) why this reaction does not continue in the stomach. (c) Maltose digestion is completed elsewhere in the gut. Name the enzyme involved, the digestive juice which contains the enzyme and the end product. Enzyme: Digestive juice: [2] [3] [3] Product:
    • Amylase. The equation shows the effect of an enzyme on carbohydrates in the buccal cavity. (b) Explain: (i) the role of the chloride ions in this reaction. (a) Identify the enzyme involved in this reaction. (ii) why this reaction does not continue in the stomach. (c) Maltose digestion is completed elsewhere in the gut. Name the enzyme involved, the digestive juice which contains the enzyme and the end product. Enzyme: Digestive juice: Product: [2] [3] [3] [3] They act as a cofactor which binds to the enzyme allowing it to work. pH 7.0 is the optimum pH at which this enzyme works best. The stomach pH is very acid (pH 1.5 to 2.5) which is too low for the enzyme to work. Maltase. Pancreatic juice. Glucose
    • MARK SCHEME (a) salivary; amylase; 2 (b) act as a cofactor/allosteric effector; binds to the enzyme altering its (molecular) shape; this enables the active sites to bind to the substrate/become operative; 3 (c) pH 7.0 (in the buccal cavity) is the enzyme optimum at which it works best; the stomach pH is too low/too acidic for the enzyme to work; this low pH/pH 1.5 to 2.5 denatures the enzyme; 3 (d) (i) maltase; 1 (ii) intestinal juice/succus entericus; 1 (iii) glucose; 1 TOTAL 11 The equation shows the effect of an enzyme on carbohydrates in the buccal cavity. (a) Identify the enzyme involved in this reaction. (b) Explain: (i) the role of the chloride ions in this reaction. (ii) why this reaction does not continue in the stomach. (c) Maltose digestion is completed elsewhere in the gut. Name the enzyme involved, the digestive juice which contains the enzyme and the end product.
    •  Total 6 out of 11 Amylase. The equation shows the effect of an enzyme on carbohydrates in the buccal cavity. (b) Explain: (i) the role of the chloride ions in this reaction. (a) Identify the enzyme involved in this reaction. (ii) why this reaction does not continue in the stomach. (c) Maltose digestion is completed elsewhere in the gut. Name the enzyme involved, the digestive juice which contains the enzyme and the end product. Enzyme: Digestive juice: Product: [2] [3] [3] [3] They act as a cofactor which binds to the enzyme allowing it to work. pH 7.0 is the optimum pH at which this enzyme works best. The stomach pH is very acid (pH 1.5 to 2.5) which is too low for the enzyme to work . Maltase. Pancreatic juice. Glucose There were two marks available. You should have mentioned ‘salivary’ to distinguish the enzyme from ‘pancreatic amylase’. Three marks were available – you should also have referred to the cofactor changing the shape of the enzyme & enabling the active site to become operative. Because three marks were available you should have made a third point.  MP 1   MP 2  MP 1  
    • A S Biology Exam Doctor
      • ENZYMES
      QUESTION 2
    • (b) Explain the shape of the graph between: (i) a and b. The graph shows the effect of substrate concentration on the rate of an enzyme controlled reaction. (ii) c and d. [3] [3] (a) Describe the overall shape of the graph. [3]
    • (b) Explain the shape of the graph between: (i) a and b. The graph shows the effect of substrate concentration on the rate of an enzyme controlled reaction. (ii) c and d. [3] [3] (a) Describe the overall shape of the graph. [3] As the substrate concentration increases so does the reaction rate. This is because more active sites become occupied. Eventually, when all active sites are occupied any further increase in substrate concentration does not increase the rate any further. More active sites are available than there are substrate molecules, so as more substrate molecules are added more active sites can be occupied thus increasing the rate. There are more substrate molecules present than there are active sites. Thus the enzyme concentration (number of active sites) is a limiting factor preventing any further rate increase.
    • MARK SCHEME (a) as substrate concentration increases the rate of reaction increases; up to substrate concentrations at which the rate of reaction slows; at a high substrate concentration any further increase in substrate concentration fails to increase the rate of reaction; 3 (b) (i) substrate concentration is the limiting factor; some active sites on the enzyme remain free/are not occupied by substrate; thus, as more substrate is available it can occupy the vacant active sites so increasing the reaction/turnover rate; 3 (ii) all the available active sites are occupied by substrate molecules; number of substrate molecules exceed the number of active sites; thus enzyme concentration is the limiting factor; 3 TOTAL 9 (a) Describe the overall shape of the graph. (b) Explain the shape of the graph between: (i) a and b. (ii) c and d.
    • There are more substrate molecules present than there are active sites. Thus the enzyme concentration (number of active sites) is a limiting factor preventing any further rate increase. More active sites are available than there are substrate molecules, so as more substrate molecules are added more active sites can be occupied thus increasing the rate. As the substrate concentration increases so does the reaction rate. This is because more active sites become occupied. Eventually, when all active sites are occupied any further increase in substrate concentration does not increase the rate any further. (b) Explain the shape of the graph between: (i) a and b. Total 5 out of 9 The question states ‘Describe…’. Thus any explanatory statements about ‘active sites’ are irrelevant. This is a very common error – always take note of the verbs ‘describe’ and ‘explain’. No reference was made to region b – c on the graph (mark scheme point 2). Examiners are not allowed to transfer mark scheme points between sections – in this case (a) and (b). A reference to ‘substrate concentration being the limiting factor’ was also needed. A reference to ‘all the active sites are occupied by substrate molecules’ was also required. The graph shows the effect of substrate concentration on the rate of an enzyme controlled reaction. (ii) c and d. [3] [3] (a) Describe the overall shape of the graph. [3]  MP 2  MP 1  MP 3  MP 3  MP 2
    • A S Biology Exam Doctor
      • ENZYMES
      QUESTION 3
    • The table below refers to the enzymes amylase and lactic dehydrogenase. If the feature is correct, place a C in the appropriate box and if the feature is incorrect, place an I in the appropriate box. [6]
    • [6] I I C C I C I I I C I C The table below refers to the enzymes amylase and lactic dehydrogenase. If the feature is correct, place a C in the appropriate box and if the feature is incorrect, place an I in the appropriate box.
    • The table below refers to the enzymes amylase and lactic dehydrogenase. If the feature is correct, place a C in the appropriate box and if the feature is incorrect, place an I in the appropriate box. TOTAL 6 I I C C I I I C I I C I MARK SCHEME
    •  Total 2 out of 6 The table below refers to the enzymes amylase and lactic dehydrogenase. If the feature is correct, place a C in the appropriate box and if the feature is incorrect, place an I in the appropriate box. [6] I I C C I C I I I C I C Lactic acid is the substrate of lactic dehydrogenase, not lactose. Lactic dehydrogenase is an oxido-reductase enzyme If required, any proteins can be genetically engineered – enzymes are proteins Lactobacilli are used to make yoghurt and although lactic dehydrogenase is involved, so are many other enzymes secreted by the Lactobacilli.      