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The properties of enzymes
1. Enzymes are protein catalysts - they remain unchanged after the reaction.
2. Enzymes are sensitive to (a) pH, (b) substrate concentration, (c) enzyme concentration, and (d) temperature.
3. Enzymes are specific - each enzyme is limited to acting as a catalyst for only one type of reaction.
4. Some enzymes require the presence of other chemicals, called co-factors.
Active site: The point on an enzyme of temporary attachment to the substrate. It is specific.
Substrate: The substance that attaches to the enzyme at the active site. It is converted to the product.
Denatured enzyme: The enzyme has lost its ability to function as a biological catalyst. Its active site has changed shape preventing the substrate binding to it. This can occur because of high temperature or change in pH.
Co-enzyme: A non-protein part of some enzymes. It is essential for the enzyme to function.
How enzymes work - 'The lock and key theory'
Enzymes work as catalysts by combining with a substance known as the substrate, to form the enzyme substrate complex. This then breaks down releasing the enzyme and the product.
Factors affecting enzyme activity:
1. pH
Enzyme activity is high only within a narrow range of pH. Outside this range the activity of this enzyme falls quite rapidly. This is because the enzyme loses its shape i.e. it is denatured.
2. Enzyme concentration
The rate of enzyme activity increases as the enzyme concentration increases (provided that the substrate concentration is not limiting). (Fig. 1)
3. Substrate concentration
The rate of enzyme activity increases as the substrate concentration increases. When all the active sites are occupied, increasing the substrate concentration will no longer increase the rate - substrate saturated. (Fig. 2)
4. Temperature
The rate of chemical reactions increase as temperature increases. Human enzymes work best at 37 degees Celcius while most plant enzymes work best at about 30 degees Celcius. These temperatures are called the optimum temperatures. Above these temperatures the enzymes start to denature.
(Fig. 3)
Five Enzyme Experiments
For the five experiments: The substrate is starch. The enzyme is amylase. The product is maltose.
The amylase breaks down the starch and converts it into maltose.
To test for starch add Iodine
Starch present (i.e. it has not been broken down) - blue-black
Starch absent (i.e. it has been broken down by the amylase) - remains yellow-brown
To test for maltose heat with Benedicts solution
Maltose present (i.e. the amylase has broken down the starch into maltose) - brick red.
Maltose absent (i.e. the amylase has not broken down the starch) - remains blue.
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Experiment 1 To investigate the effect of amylase on starch Note: Learn this experiment well, as the next four are very similar. |
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Experiment 2 To investigate the effect of pH on the rate of enzyme activity |
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Experiment 3 To investigate the effect of temperature on the rate of enzyme activity |
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Experiment 4 To investigate the effect of enzyme concentration on the rate of enzyme activity |
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Experiment 5 To investigate the effect of substrate concentration on the rate of enzyme activity |
Leaving Certificate Higher Level Exam questions
1986 Q. 12; 1988 Q.9; 1989 Q. 10 (b); 1992 Q.15 (a); 1994 Q.10; 1995 Q.4; 1998 Q.2
