AI Required Practicals

Question 1

How do you calculate mitotic index?

Answer 1

Mitotic index = (number of cells with visible chromosomes) / (total number of cells in sample)

Question 2

Where do plant cells undergo mitosis at?

Answer 2

At shoot and root tips in areas known as meristems. Cells in these areas are totipotent.

Question 3

What are calibration curves?

Answer 3

Calibration curves are graphs used to determine an unknown concentration of a sample by comparing the unknown to a set of standard samples with known concentrations

Question 4

How is calibration curve used to find the concentration of plant tissue?

Answer 4

• Plot a calibration curve of percentage change in mass against concentration.
• Find the x-intercept where the plant tissue is isotonic to the sucrose solution

Question 5

Why are potato discs/tubes/etc left in solution for 20 minutes?

Answer 5

To allow time for osmosis until the plant tissue reaches equilibrium with its surrounding solution.

Question 6

Outline a procedure of investigating osmosis using potato tissue.

Answer 6

1) Make a simple dilution of 1M sucrose to produce 5 concentrations. Add 5cm3 to 5 different test tubes
2) Cut a potato into equal sized chips and weigh each before
3) Place a chip in each test tube and leave for 20 minutes
4) Take out and dab the excess water, and weight them again
5) Calculate the percentage change in mass

Question 7

Why is the percentage change used rather than the actual change in mass when investigating osmosis?

Answer 7

Potato chips may not all have the same starting mass, meaning percentage allows for fair comparison

Question 8

Why are potato chips dabbed dry after removing from the sucrose solution when investigating osmosis?

Answer 8

To remove any excess water clinging to the surface that will raise the mass measured

Question 9

What are the controlled variables of the osmosis practical?

Answer 9

• Volume of sucrose solution
• Size of potato chips
• Length of time left in solution

Question 10

How is a control set up in a practical investigating enzyme activity?

Answer 10

Replace the enzyme solution with distilled water or boiled enzyme solution

Question 11

Outline the practical procedure used to measure the effect of temperature on enzyme activity, using trypsin and milk.

Answer 11

• Immerse equal volumes of trypsin and milk, stored in different test tubes, in a water-bath for 5 minutesfor the temperature to equilibrate
• Mix together and immediately start timing, recording the time taken for the milk to be completely hydrolysed (become colourless)
• Test 5 different temperatures with 3 repeats each

Question 12

Outline the procedure to prepare a root tip slide.

Answer 12

1) Warm 1M HCl to 60° using a water bath
2) cut a root tip using a scalpel and add to the HCl, leave for 5 minutes
3) Remove from HCl and wash with distilled water
4) Cut the tip of the root sample and place on a slide
5) Add a few drops of stain to make chromosomes visible.

Question 13

State 6 aseptic techniques

Answer 13

• Wipe down surfaces with antibacterial cleaner, before and after experiment
• Use a Bunsen burner so that convection currents draw microbes away from the culture
• Flame the wire hoop before using it to transfer bacteria
• Flame the neck of any bottles before using them to prevent any bacteria entering the vessel
• Keep all vessels containing bacteria open for the minimum amount of time
• Close windows and doors to limit air currents

Question 14

Why is bacteria incubated at 25°C?

Answer 14

To prevent the growth of pathogenic bacteria, which occurs at highe temperatures

Question 15

How can you compare the effectiveness of different antibiotics applied to the same bacteria?

Answer 15

Measure the diameter and calculate the area of the zone of inhibition (clear zone) on the agar

Question 16

What does the zone of inhibition indicate?

Answer 16

• It indicates the bacteria killed by the antibiotic. The larger the zone the more effective the antibiotic.
• If an antibiotic has very little or no zone of inhibition, the bacteria is likely resistant to the antibiotic.

Question 17

Why should the lid not be completely taped to the Petri dish?

Answer 17

To allow oxygen to enter the Petri dish, preventing the growth of harmful anaerobic bacteria.

Question 18

What is the function of methylene blue when investigating respiration in single-celled organisms?

Answer 18

• It is a redox dye and acts as an alternate electron acceptor of the electrons transferred during ATP synthesis.
• It turns from blue to colourless

Question 19

Outline the procedure to investigate the effect of temperature on the rate of respiration of yeast.

Answer 19

1) Set up a water bath at 35°c
2) Add equal volumes of the yeast and glucose solution to three test tubes. Place test tubes in the water bath and leave them to equilibrate for 10 minutes
3) Add 2cm3 of methylene blue to the test tubes and start the timer. Shake for 10 seconds and place the test tube back in the water bath, and record the time taken for methylene blue to turn colourless for each tube
4) repeat the experiment using temperatures of 40, 50, 60 and 70°c

Question 20

How can you calculate the rate of respiration at each temperature?

Answer 20

Rate = 1 / time taken for decolorisation of methylene blue