AS Practical

Question 1

How fast product of a reaction is made (RP1)

Answer 1

EG Catalase and H2O2
Set up 5 boiling tubes containing same volume and concentration of hydrogen peroxide
Add equal volumes of suitable buffer solution to keep pH constant
Attach bung with delivery tube to upside down measuring cylinder in a trough filled with water. Cylinder should also be filled with water
put each boiling tube into a water bath set at a different temperature (10/20/30/40/50) - FIVE MINIMUM
Use a pipette to add same volume and concentration of catalse to each boiling tube and quickly attach bung with delivery tube
Record o2 volume prouced in the first minute using a stopwatch
Repeat each temperature 3 times MINIMUM
Calculate mean of each temperature and present data using a graph

Question 2

Adjustments for enzyme control reactions

Answer 2

Have the same temperature and vary:
pH in buffer solutions
concentration of substrate
concentration of enzyme
NEGATIVE CONTROLS SHOULD ALSO BE TAKEN using  heat-treated enzyme for each experiment/temp/conc to show the effects of the enzyme on the reaction

Question 3

How fast a substrate is broken down (RP1)

Answer 3

=> A drop of iodine in potassium iodide is put into each well of a spotting tile
=> known conc of amylase and starch mixed together in test tube
=> pipette is used to put a drop of mixture into wells containing iodine solution
=> iodine solution goes blue-black when starch present, no change when not
=> record the time for iodine solution to no longer turn blue-black
=> repeat using different concentrations of amylase (repeating each concentration 3 times)

Question 4

Interpreting Graphs

Answer 4

LOOK AT START RATES - Do they start from the same point? Are some rates faster than others? Compare these.
REST OF GRAPH - plateauing at different heights - why?All of substrate used up?- optimum temperature
Denaturing?- temp too high
Not all of substrate used up?- temp too low, slow and stead increase (NO PLATEAU)

Question 5

Observing Mitosis

Answer 5

=>Cut 1cm from tip of growing root (has to be tip, where mitosis takes place)
=>Transfer root tip to boiling tube containing 1M HCl in 60C water bath to incubate for 5 minutes
=>Use pipette to rinse the root tip wih cold water and leave to dry
=>Place root tip on microscope slide and cut 2mm from it (get rid of rest)
=>Break tip open and spread cells out evenly with mounted needle
=>Add a few drops of stain (i.e. toluidine blue) and leave for a few minutes
=>Place cover slip over sample and apply pressure to squash tissue (makes it thinner for light to pass through))
=>DON’T SMEAR - you’ll damage the chromosomes
observe and calculate mitotic index :)

Question 6

AQA alternative (enzyme controlled reaction)

Answer 6

Appearing cross experiment (or disappearing…)

Question 7

mitotic index

Answer 7

number of cells with visible chromosomes
_
Total number of cells

Question 8

Graticule and micrometer

Answer 8

=> Line up eyepiece graticule with stage micrometer
(stage micrometer division = 0.1mm long)
=> Count how many divisions of eyepiece graticule match with stage micrometer (i.e. 4.5 divisions with graticule = 1 division with micrometer)
=> Work out size of 1 division on graticule (0.1/4.5 = 0.022mm)
=> Compare size of [cell] to number of divisions e.g. if cell is 4 graticule divisions long and 1 graticule division = 0.022mm then cell length = 4 X 0.022 = 0.088mm
ALTERNATIVE - just use AIM (magnification equation)

Question 9

Artefacts

Answer 9

Things that you see in your specimen that aren’t part of it - bubbles, dust, fingerprints, inaccuracies in squashing and staining
Careful preparation minimises creation of artefacts

Question 10

Calibration curve (RP 3)

Answer 10

=> Produce 5 different solutions of sucrose either using serial dilution or scale factor/percentage techniques
=> Use a cork borer to cut potatoes into identically sized chips
=> Divide chips into groups of three and measure mass of each group using a mass balance
=> Place one group into each of your five sucrose solutions
=> Leave groups for 20 minutes
=> Remove chips and pat dry using a paper towel
=> Measure mass of each group again and record
=> Calculate %change in mass
=> Plot %change against sucrose concentration and make calibration curve by joining plotted points
=> Point where graph meets x axis is where potatoes lost NO mass therefore sucrose solution has same water potential as potato (water potential for specific concentrations can be found in data books)

Question 11

Calibration curve (RP4)

Answer 11

=> Using stock solution of beetroot extract, create at least 5 standard solutions using percentage dilution (including pure water and pure extract)
=> Take samples of each solution and put into colorimeter
=> Use data to plot absorption against concentration and produce a calibration curve
=> Produce at least 5 different solutions of alcohol using either serial dilution or percentage dilution technique (including pure water and pure alcohol)
=> Leave in water bath (with bung on test tubes) until alcohol temperature at 30C
=> Blot beetroot discs with paper to remove excess water
=> Put two discs into each test tube using forceps and replace bung
=> Leave for 5 minutes, then remove discs and place test tubes into colorimeter
=> Use calibration curve to compare beetroot concentrations between alcohol solutions and standards
=> Higher absorbance = more pigment released = higher permeability of membrane