Yeast practical - respiration and effect of temperature

Question 1

Why is a water bath in a conical flask used instead of a Bunsen burner ?

Answer 1

• By using a kettle to fill initial water-bath the risk of burning using the Bunsen burner is removed.

Question 2

Aims of respiration practical

Answer 2

To show that carbon dioxide is produced when yeast respires anaerobically by using either bicarbonate indicator or limewater and recognising the colour change associated with carbon dioxide being released

Question 3

Why is the water firstly boiled?(2)

Answer 3

• To remove any dissolved oxygen in the water to produce anaerobic conditions
• to sterilise it by killing microorganisms

Question 4

Method of respiration practical (5)

Answer 4

• Boil some water to remove dissolved oxygen
• Dissolve a small amount of sugar in the boiled water, and allow it to cool
• When it is cool, add yeast and stir with glass rod
• Sent up apparatus and add the liquid paraffin by tricking it gently down the side of the tube, using a Pipette
• Leave apparatus in a warm place (23-30 degrees ideal)

Question 5

Changes made to the apparatus to prove that it is the yeast that is producing carbon monoxide ?

Answer 5

Repeat the experiment without yeast

Question 6

Why must sugar solution be cooled before adding the yeast ?

Answer 6

• Stops the yeast from being killed since high temperature could denature enzymes in the yeast, stopping biological reactions

Question 7

Purpose of the layer of liquid paraffin / oil?

Answer 7

Prevents oxygen from entering the solution

Question 8

End of experiment (3)

Answer 8

• The solution is slightly warmer since anaerobic respiration is exothermic (not as much as Aerobic)
• Ethanol is produced in the Sugar Solution
• CO2 turns limewater milky

Question 13

How to measure effect of temperature

Answer 13

Count the number of bubbles produced in a period of time

Question 14

Independent variable

Answer 14

• Temperature
• Presence of oxygen
• Different sugars

Question 15

Independent variable definition

Answer 15

The variable that is changed or controlled to test the effects on the dependant variable

Question 16

Dependant variable

Answer 16

The one being measured and tested. As the independent variable is changed, the effect on the dependant variable is observed

Question 17

Dependant variable in this practical

Answer 17

• Respiration of yeast

Question 22

Name 3 safety risk assessments

Answer 22

• The water-bath will contain water at 70oC initially, so there is a risk of scalding from the water.
  • Yeast will not be handled directly pupils, minimising allergy risk.
• Clean up any spillages immediately

Question 23

Conclusion

Answer 23

As the temperature increased to 40oC, the rate of respiration of the yeast increased. Above 50oC the rate of respiration then decreases.

Question 24

Hypothesis of temperature

Answer 24

• The higher the temperature, the more carbon dioxide will be released by yeast, therefore forming a greater amount of bubbles.
• Once the temperature gets above a certain point the rate of respiration will decrease.

Question 27

The factors affecting respiration in yeast are: (4)

Answer 27

• pH
• nutrient availability
• build up of waste materials
- temperature