EnzymesEdexcel A-Level Biology A: Subtopic test

1. 1
   Catalase is an enzyme found inside liver cells. It breaks down hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. Hydrogen peroxide breaks down very slowly if no enzyme is present.

   (a)

   What effect does catalase have on the breakdown of hydrogen peroxide?

   [1 mark]

   • AIt is used up as the reaction proceeds
   • BIt increases the total volume of oxygen produced
   • CIt lowers the activation energy of the reaction
   • DIt supplies energy to the hydrogen peroxide molecules

   (b)

   Catalase does not break down starch. Which statement best explains this?

   [1 mark]

   • AStarch is too small to bind to an active site
   • BThe shape of the active site is not complementary to the shape of starch
   • CCatalase has no active site
   • DEnzymes only act on lipids and proteins

   (c)

   Describe how catalase lowers the activation energy of the breakdown of hydrogen peroxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Trypsin is a digestive enzyme made by cells of the pancreas and released into the small intestine, where it hydrolyses proteins in food. Pancreatic cells also contain many enzymes that catalyse reactions of respiration, which provides the cell with ATP.

   (a)

   How is trypsin best described?

   [1 mark]

   • AAn extracellular enzyme, made inside cells but working outside them
   • BAn intracellular enzyme that works in the cytoplasm of pancreatic cells
   • CA substrate that is hydrolysed by respiratory enzymes
   • DA non-protein catalyst that is unaffected by shape

   (b)

   Which of these is an intracellular enzyme?

   [1 mark]

   • ATrypsin in the lumen of the small intestine
   • BAmylase in saliva in the mouth
   • CLipase in the duodenum
   • DAn enzyme in the cytoplasm of a pancreatic cell that catalyses a reaction of respiration

   (c)

   Suggest why enzymes that digest food, such as trypsin, are extracellular.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates the effect of enzyme concentration on the rate at which catalase breaks down hydrogen peroxide. She adds 1.0 cm³ of catalase solution to 10 cm³ of hydrogen peroxide solution, which is in excess, in a buffer at 25 °C, and collects the oxygen produced in a gas syringe. With a catalase solution of concentration X she collects 9 cm³ of oxygen in the first 30 seconds. With a catalase solution of concentration 2X she collects 18 cm³ of oxygen in the first 30 seconds.

   (a)

   Calculate the mean rate of oxygen production over the first 30 seconds for each concentration of catalase, and state what the results show about the effect of enzyme concentration on rate.

   [3 marks]

   (b)

   Explain why doubling the concentration of catalase doubled the rate of reaction. Explain also why the rate would level off if she kept the catalase concentration constant and increased the concentration of hydrogen peroxide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician investigates the effect of starch concentration on the initial rate of reaction of amylase, an enzyme that hydrolyses starch to maltose. She has a stock solution of 2.0% starch, amylase solution, a buffer solution, iodine in potassium iodide solution, a colorimeter, water baths, standard glassware and stopwatches. Her initial rates increased steadily as the starch concentration rose from 0.2% to 1.0%, and then stayed almost constant at 1.2%, 1.6% and 2.0%.

   (a)

   Describe how the technician could carry out the investigation and use her results to find the initial rate for each starch concentration.

   [6 marks]

   (b)

   Explain the shape of the technician's results. Predict, with an explanation, what would happen to the rate at 2.0% starch if the concentration of amylase were doubled.

   [6 marks]

   Total for question 4: 12 marks

End of questions