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Lactase Enzyme Lab


How does heat affect the efficacy of lactase?


Lactose is found in milk and is a disaccharide composed of glucose and galactose (both six-sided sugars). Sucrose, the ordinary table sugar, is also a disaccharide composed of fructose and glucose. Glucose is a six-sided sugar and fructose is a five-sided sugar.

Lactase is an enzyme that breaks lactose down into galactose and glucose. Lactase can be purchased in pill form by people who are lactose intolerant. These people lack the enzyme, lactase, and cannot break down the sugar lactose into its comp1nt parts.

Although lactose is similar to sucrose, lactase will break down only lactose because of the shape of the sugar.


A prediction using the "If __ then ___, because ____" format is provided by the student.



  1. Get and wear your safety goggles.
  2. Label the test tubes 1, 2, 3, 4, and 5.
  3. Label one beaker "enzyme solution" and the other beaker "sucrose solution".
  4. In the beaker labeled "enzyme solution", carefully open the capsules and pour the contents into the beaker. Add 8 mL of water. Stir.
  5. In the beaker labeled "sucrose solution", mix 5g sugar and add 8 mL of water.
  6. In test tube #1 add 2 mL of milk and 1 mL of enzyme solution.
  7. In test tube #2 add 2 mL of milk and 1 mL of water.
  8. In test tube #3 add 2 mL of sucrose solution and 1 mL of enzyme solution.
  9. Place the beaker with the remaining enzyme solution on the hot plate. Turn on the hot plate to about medium heat.
  10. In test tube #4 add 2 mL of milk and 1 mL of HOT enzyme solution.
  11. In test tube #5 add 2 mL of the sucrose solution and 1 mL of water.
  12. Add 1 ml of Benedict's Solution to each test tube.
  13. Place the test tubes in the hot water bath. Observe for color changes.
  14. If there is glucose present mark a "+". If glucose was absent mark a "-" in the table.
  15. Clean up all materials.


Test tube Solution Glucose results
1 Milk & enzyme solution  
2 Milk & water  
3 Sucrose & enzyme solution  
4 Milk & denatured enzyme solution  
5 Sucrose & water  
  1. Diagram and describe the lactose and lactase reaction.
  2. Why did the enzyme react to lactose and not to sucrose?
  3. What happened when the enzyme was boiled?
  4. Another way to affect the enzyme is by lower the pH of the solution. Would lower the pH of the enzyme solution affect this enzyme? Why or why not? (Think about where lactose is broken down in your body.)
  5. Write a paragraph describing substrate specificity in enzymes.


Provided by student.


Mr. Sodek's Science Site. El Camino High School.