Bio required pracs

Question 1

Describe a method to investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition

Answer 1

1. Wash hands before and after
2. Sterilise work surfaces before and after
3. Use sterile petri dish and culture medium (agar gel) to ensure no microoganisms already present
4. Place antibiotic discs onto medium (with one paper disc with no antibiotic to act as a control)
5. (if using) inoculating loop to transfer culture to medium; sterilise it by holding it in bunsen burner until it glows red, thus killing the microorganisms on loop
6. Use loop/swab used to streak/spread microorganism across medium
7. The lid of the petri dish should be secured with adhesive tape, to prevent unwanted microorganisms entering and micro-organism leaving, but not fully as anaerobic conditions encourage growth of pathogens
8. Incubate to allow growth of pathogen, upside down (to prevent condensation falling) and at 25°C, lower than optimum conditions for pathogen growth
9. Leave for a few days, measure zone of inhibition and compare

Question 2

Describe a method to investigate the effect of pH on the breakdown of starch by amylase

Answer 2

• use range of 3 values/buffer solution
• keep concentration of amylase and iodine the same and use water bath to maintain constant temperature
• start timer
• do iodine test to make qualitative observations every 10 seconds by adding it to a food sample on spotting tile
• When colour does not change, stop the timer
• Calculate the rate of reaction
• repeat at different pHs and compare

Question 3

Describe a method to the investigate effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed

Answer 3

• Place a piece of pondweed in a large beaker of water.
• Shine a light at it using a lamp at a specific distance away.
• Record the number of bubbles of gas coming out of the pondweed in one minute (or other constant time).
• Repeat this with the lamp at different distances from the pondweed
• Make sure to leave time for the pondweed to adjust to the new light intensity and reach a steady rate of photosynthesis
• calculate rate of photosynthesis/oxygen production (number of bubbles / time) and compare
• CVs are type of pondweed used and the limiting factors

Question 4

Describe a method for an investigation into the effect of a factor on human reaction time.

Answer 4

• subject should drink decaffeinated drink and wait for 5-10 mins for internal conditions to adjust
• subject should sit with lower arm resting on table to keep arm steady during test
• investigator should hold ruler vertically above subject’s thumb and forefinger at same distance each time and let go without warning with no force
• subject should catch the ruler as soon as possible
• measure distance on ruler caught at
• repeat 5 rimes for repeatability
• calculate mean reaction time
• re-do experiment but with caffeinated (stimulant) drink (and allow 5-10 mins for adjustment to internal conditions
• make sure ruler is the same weight
• compare

Question 5

2 needed

Why might a computer system be better at calculating reaction time than a ruler?

Answer 5

1. no indication before hand whether colour will change
2. measurement of time more precise / resolution (of computer timer) is higher

Question 6

Describe a method to investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.

Answer 6

• Add lipase to a test tube and place in a water bath at 20^oC.
• In another test-tube add milk and 5 drops of Cresol red solution and sodium carbonate (to make it alkaline)
• Leave for 5 minutes to allow the solutions to reach the same temperature.
• Add the lipase to the milk and start the stopwatch immediately.
• Stir the mixture continuously and stop the stopwatch when the colour of the solution changes to yellow. Record time taken.
• Repeat this for different temperatures of water bath.