Practical Techniques Flashcards
Why does use of a colorimeter improve repeatability of results
It is quantitative and standardises the method
What’s one way a student could ensure a sample is kept at the same temperature the whole way through the experiment
Measure the temperature at intervals and use appropriate corrective measure
What’s the purpose of boiling the agar in the aseptic practical
So no contamination
What’s the purpose of transferring the same volume of liquid culture onto each agar plate in the aseptic practical
So same number of bacteria transferred to allow comparison
Describe how a student could make up a dilution series in bacteria practical
- Add 1 part bacteria culture to 9 parts sterile liquid
- Mix
- Repeat using 9 parts fresh sterile liquid and 1 part of 10^-1 and 10^-2 dilutions to make 10^-3 dilutions
Why does a student not use his preliminary dilution (aseptic practical)
- count is unlikely to be reliable
- because there’s too many cells
Describe method for potato osmosis practical
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
In chromatography why is pencil used to mark and not ink
The ink and leaf pigments would mix
Describe the method to separate leaf pigments in chromatography
- level of solvent below line
- stop before solvent reaches the top
What is the purpose of incubating the root tip with HCl
- stops cell division from occurring
- hydrolyses the middle of the lamella so that the cells can be separated easily
How do you carry out a serial dilution?
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
Describe how to produce and use a calibration curve eg for concentration of protein in urine
- 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
Why must a eyepiece graticule be calibrated when switching to a higher magnification
The stage micrometer will appear larger and so each eyepiece division will be a different value
Suggest why a stage micrometer and an eyepiece graticule are needed
- 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
Describe how a student could use an eyepiece graticule to determine the mean diameter eg of stomata
- measure each stoma using eyepiece graticule
- calibrate eyepiece graticule against stage micrometer
- take a number of measurements to calculate a mean
Give two precautions the students should take when setting up a potometer
- dry off leaves
- ensure there are no air bubbles present
- ensure it is airtight
Why does a potometer not truly measure the rate of transpiration
- water used in photosynthesis
- apparatus is not sealed
- water produced in respiration
How does temperature/acidity effect the permeability of the cell surface membrane
Permeability increases
- the proteins in the membrane denature as heat damages the ionic bonds in their tertiary structure
- so more pigment is released
How does conc of ethanol effect the permeability of the cell surface membrane
Increases permeability
- the alcohol starts to dissolve the phospholipid bilayer in the cell surface membrane
- releasing pigment from the cell
When would you use Log10 and why
- when you have a large range of data
- so you can read the data points off the graph accurately
How would you work out the original value from the log value
10^log
How could you reset a potometer
Open the tap
What variable should be controlled if you wanted to do the potometer experiment on two different plants
Surface area of leaves - Number and size of leaves
How would you work out rate of transpiration from a potometer
Volume of cylinder/time taken
Volume = pie x r^2 x distance bubble travelled along tube
What things are required for a good scientific drawing
- 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
Why is a mounted needle needed when using a microscope
To lower the cover slip and prevent air bubbles under the cover slip
What are some safety precautions in the heart dissection practical
- 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
How do you work out mitotic index
No. Of cells undergoing mitosis/total no. Of cells x 100
Why would you stain the cells
To make chromosomes visible
What’s the test for non-reducing sugars?
- heat/with acid
- cool and add an alkali to neutralise
- heat with Benedict’s solution
- turns red
How do you calculate how many times bacteria divides in a certain amount of hours?
- work out the number of divisions
- 1 x 2^no. of divisions
