Required Practicals Paper 2

Question 1

How to investigate the effect of light intensity on the height of seedlings

Answer 1

• Place cotton wool in 3 petri dishes and soak them with equal volumes of water
• Place 10 mustard seeds in each dish
• Leave the dishes in a warm place and allow the seeds to germinate and water them everyday
• Make sure that all the dishes have the same number of seedlings
• Use a piece of thread and ruler to measure the intital height of the seedling
• Place one dish in full sunlight, another in partial light and the last in darkness
• Measure the height of each seedling each day for a week and record the results in a table
• The seedlings in the light grow towards the Sun due to phototropism
• The seedlings in the dark are the longest as they grow rapidly to reach the light and their leaves are small and yellow
• Draw and label biologial diagrams to show the effects

Question 2

variables for the light intensity seedling investigation

Answer 2

• Independent v. - light intensity
• Dependent - height of the seedlings
• Control - volume of water in the dishes, number of seedlings, type of seedlings, temperature

Question 3

Describe how to measure a persons reaction time using a ruler

Answer 3

• Person 1 sits on a stool and puts the forearm of their dominant arm across the table with their hand overhanging the edge
• Person 2 holds the ruler vertically with the 0cm mark being just above person 1’s thumb and first finger
• Person 2 then tells them to prepare and drops it at a random time
• Person 1 has to catch the ruer with their thumb and first finger as quickly as possible
• Person 2 then records the measurement on the ruler that is level with the top of person 1’s thumb
• Repeat the test to calculate a mean
• Convert the measurement to a reaction time using a conversion table
• Repeat with Person 2 now catching the ruler
• Independent variables you could investigate

Question 4

Different independent variables you could test with the reaction time experiment

Answer 4

• Effect of practise - ONE person catches the ruler a large number of time to see if the reaction time changes
• Dominant vs non-dominant hand - carry out tests with both of the same person
• Chemicals e.g. caffeine - do the test then drink a cup of coffee and then do the test again 30 mins later

Question 5

How to carry out random sampling with a quadrat

Answer 5

• Place 2 tape measures 90 degrees to each other over a habitat
• Draw a grid of the sample area with each square being a 0.5 x 0.5 area and number each square to give it coordinates
• Use a random number generator to select 10 squares
• Place the 0.5 x 0.5 quadrat in one of the selected squares and record the number of organisms or the percentage of the quadrat covered by the organism
• Repeat this for all other squares and calculate a mean
• calculate the mean number per m² then multiply by the total area

Question 6

How to invesitgate the distribution of a species across a habitat

Answer 6

• Place a transect (tape measure) across the habitat
• Place a quadrat at regular intervals on the same side of the transect
• Count the number of organisms in each quadrat and see how it changes
• The distrubution can then be correlated with an environmental factor

Question 7

How to investigate the effect of temperature on the decay of fresh milk by measuring pH change

Answer 7

• Label a test tube lipase and put 5cm³ of the lipase solution into it using a pipette
• Label a test tube milk, add 5 drops of cresol red indicator and add 5cm³ of milk and 7cm³ of sodium carbonate solution and the test tube should be purple (alkaline conditions)
• Place a thermometer into this test tube
• Place both test tubes into a water bath at the chosen temp and wait for them to equilibrate
• Use a to pipette to transfer 1cm³ lipase into the milk test tube, stir and start a timer
• The lipase will break down the milk which releases fatty acids which lowers the pH
• In acidic conditions, cresol red will change from purple to yellow so stop the timer once it turns yellow
• Repeat the experiment at different temperatures e.g. 0°C 20°C 40°C 60°C 80°C with CLEAN TEST TUBES
• Share data with other groups and calculate a mean

NB: This is not actaully decay but it models it pretty well

Question 8

Variables for the decay experiment

Answer 8

• Independent - temperature
• Dependant - time taken
• Volumes of milk, cresol red, sodium carbonate and lipase solution

Question 9

Describe a decay graph

Answer 9

• At low temp, the rate of decay is low because the enzymes controlling decay work slowly at low temps
• The rate of decay then increases up to the optimum
• In temperatures higher than the optimum, the enzymes controlling decay denature at high temperatures