Practical Techniques

Question 1

Why does use of a colorimeter improve repeatability of results

Answer 1

It is quantitative and standardises the method

Question 2

What’s one way a student could ensure a sample is kept at the same temperature the whole way through the experiment

Answer 2

Measure the temperature at intervals and use appropriate corrective measure

Question 3

What’s the purpose of boiling the agar in the aseptic practical

Answer 3

So no contamination

Question 4

What’s the purpose of transferring the same volume of liquid culture onto each agar plate in the aseptic practical

Answer 4

So same number of bacteria transferred to allow comparison

Question 5

Describe how a student could make up a dilution series in bacteria practical

Answer 5

1. Add 1 part bacteria culture to 9 parts sterile liquid
2. Mix
3. Repeat using 9 parts fresh sterile liquid and 1 part of 10^-1 and 10^-2 dilutions to make 10^-3 dilutions

Question 6

Why does a student not use his preliminary dilution (aseptic practical)

Answer 6

• count is unlikely to be reliable
• because there’s too many cells

Question 7

Describe method for potato osmosis practical

Answer 7

1) weight first and then put each cut cube in a test tube of sucrose and dilute water solution that you have made up.

2) at room temperature

3) dry the cubes with paper towel to remove excess water before measuring

4) measure mass of cubes every 10 mins

5) calculate percentage change in mass

Question 8

In chromatography why is pencil used to mark and not ink

Answer 8

The ink and leaf pigments would mix

Question 9

Describe the method to separate leaf pigments in chromatography

Answer 9

• level of solvent below line
• stop before solvent reaches the top

Question 10

What is the purpose of incubating the root tip with HCl

Answer 10

• stops cell division from occurring
• hydrolyses the middle of the lamella so that the cells can be separated easily

Question 11

How do you carry out a serial dilution?

Answer 11

1) Take original solution of known concentration
2) fill additional test tubes with the same volume of distilled water
3) transfer the same volume of solution from one test tube to the next and mix

Question 12

Describe how to produce and use a calibration curve eg for concentration of protein in urine

Answer 12

• use water and your protein solution to produce a dilution series of known concentrations protein
• measure absorbance of each concentration using a colorimeter
• plot a graph of absorbance on Y axis against concentration on X axis and draw a curve
• use absorbance of samples to find protein conc from the curve

Question 13

Why must a eyepiece graticule be calibrated when switching to a higher magnification

Answer 13

The stage micrometer will appear larger and so each eyepiece division will be a different value

Question 14

Suggest why a stage micrometer and an eyepiece graticule are needed

Answer 14

• the size of the units of the eyepiece graticule change depending on the magnification being used
• a stage micrometer can be used to calibrate the graticule

Question 15

Describe how a student could use an eyepiece graticule to determine the mean diameter eg of stomata

Answer 15

• measure each stoma using eyepiece graticule
• calibrate eyepiece graticule against stage micrometer
• take a number of measurements to calculate a mean

Question 16

Give two precautions the students should take when setting up a potometer

Answer 16

• dry off leaves
• ensure there are no air bubbles present
• ensure it is airtight

Question 17

Why does a potometer not truly measure the rate of transpiration

Answer 17

• water used in photosynthesis
• apparatus is not sealed
• water produced in respiration

Question 18

How does temperature/acidity effect the permeability of the cell surface membrane

Answer 18

Permeability increases
- the proteins in the membrane denature as heat damages the ionic bonds in their tertiary structure
- so more pigment is released

Question 19

How does conc of ethanol effect the permeability of the cell surface membrane

Answer 19

Increases permeability
- the alcohol starts to dissolve the phospholipid bilayer in the cell surface membrane
- releasing pigment from the cell

Question 20

When would you use Log10 and why

Answer 20

• when you have a large range of data
• so you can read the data points off the graph accurately

Question 21

How would you work out the original value from the log value

Answer 21

10^log

Question 22

How could you reset a potometer

Answer 22

Open the tap

Question 23

What variable should be controlled if you wanted to do the potometer experiment on two different plants

Answer 23

Surface area of leaves - Number and size of leaves

Question 24

How would you work out rate of transpiration from a potometer

Answer 24

Volume of cylinder/time taken

Volume = pie x r^2 x distance bubble travelled along tube

Question 25

What things are required for a good scientific drawing

Answer 25

• no shading
• draw all parts to the same scale
• no overlapping lines/lines must be continuous
• must have a scale or magnification indicator
• diagram must have labels/a title

Question 26

Why is a mounted needle needed when using a microscope

Answer 26

To lower the cover slip and prevent air bubbles under the cover slip

Question 27

What are some safety precautions in the heart dissection practical

Answer 27

• wash hands with soap and water
• disinfect instruments/surfaces
• carry sharp instruments by pointing away from your body
• put gloves/paper towels in a separate bin to dispose

Question 28

How do you work out mitotic index

Answer 28

No. Of cells undergoing mitosis/total no. Of cells x 100

Question 29

Why would you stain the cells

Answer 29

To make chromosomes visible

Question 30

What’s the test for non-reducing sugars?

Answer 30

• heat/with acid
• cool and add an alkali to neutralise
• heat with Benedict’s solution
• turns red

Question 31

How do you calculate how many times bacteria divides in a certain amount of hours?

Answer 31

• work out the number of divisions
• 1 x 2^no. of divisions