Lock and key theory, denaturing, the three digestive enzymes, the four food tests, and the required practical on pH and amylase.
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1.What is an enzyme?
A biological catalyst. It is a protein that speeds up a reaction without being used up in it.
2.What is the active site of an enzyme?
The part of the enzyme that the substrate binds to, where the reaction happens.
3.Describe the lock and key theory.
The substrate has a shape complementary to the active site, so only that substrate fits. This is why each enzyme works on only one substrate.
4.What happens to an enzyme at high temperature?
The bonds holding it together break, the active site changes shape, and the substrate no longer fits. The enzyme is denatured.
5.Can a denatured enzyme recover if it is cooled down?
No. Denaturing is permanent.
6.What happens if the pH is too high or too low for an enzyme?
The active site changes shape and the enzyme denatures, so the reaction stops.
7.Why does the rate increase as temperature rises up to the optimum?
Molecules have more kinetic energy, so enzyme and substrate collide more often and with more energy.
8.What is the optimum temperature for most human enzymes?
About 37 degrees Celsius, which is body temperature.
9.Which enzyme breaks down starch, and where is it made?
Amylase, made in the salivary glands and the pancreas.
10.Which enzymes break down protein, and where are they made?
Proteases, made in the stomach, the pancreas and the small intestine.
11.Which enzyme breaks down lipids, and where is it made?
Lipase, made in the pancreas and the small intestine.
12.How do you test for starch, and what is a positive result?
Add iodine solution. It changes from orange-brown to blue-black if starch is present.
13.How do you test for sugars?
Add Benedict's solution and heat in a water bath. It changes from blue through green and yellow to brick red if a reducing sugar is present.
14.How do you test for protein?
Add Biuret solution. It changes from blue to purple if protein is present.
15.How do you test for lipids?
Add ethanol, then add water and shake. A cloudy white emulsion shows lipid is present.
16.Why is food ground up and mixed with water before testing?
To release the food molecules into solution so the reagent can reach and react with them.
17.Why is a water bath used instead of a Bunsen burner in the sugar test?
Ethanol and other substances used are flammable, and a water bath gives even, controlled heating.
18.What colour does Benedict's solution stay if no sugar is present?
Blue.
19.In the required practical on pH and amylase, what is the independent variable?
The pH, controlled using different buffer solutions.
20.What is the dependent variable in that practical?
The time taken for the starch to be completely broken down.
21.How do you know when all the starch has gone?
Take drops of the mixture onto a spotting tile with iodine. When the iodine stays orange-brown, no starch is left.
22.Name three control variables for the amylase practical.
Temperature, the volume and concentration of amylase, and the volume and concentration of starch solution.
23.Why is a buffer solution used?
It keeps the pH constant throughout the reaction, so pH is properly controlled.
24.Why is the reaction kept in a water bath at a fixed temperature?
So that temperature does not vary, and any change in rate can be attributed to pH alone.
25.How do you calculate rate of reaction from a time?
rate = 1000 divided by the time in seconds.
26.The starch disappears after 40 seconds. Calculate the rate.
1000 divided by 40 = 25 in arbitrary units.
27.What does denatured mean?
The enzyme has permanently lost the shape of its active site, so the substrate no longer fits and it cannot catalyse the reaction.
28.Why can an enzyme only act on one type of substrate?
Its active site has a specific shape, complementary to that substrate and nothing else.
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