Summary ALL CORE PRACTICALS Edexcel SNAB A Level Biology

CORE PRACTICALS SNAB A LEVEL BIOLOGY
CP Name Procedure specifics (mark scheme) Control variables

1 Effect of caffeine on HR in Daphnia  Suspend Daphnia in cotton wool to limit movement  Size, age, type of Daphnia
 Determine base heart rate in absence of caffeine  Temperature
 Use a range of caffeine solutions (e.g., made with  pH
dilutions)
 Leave to acclimatise for at least 5 minutes in each
sample
 Specific method to count heart rate (dots on a piece of
paper)
 Repeats/replicates
ETHICAL CONSIDERATIONS TO USING DAPHNIA

2 Vitamin C content of food and drink  Specified volume of DCPIP (indicator) into a conical  Temperature
flask  Volume and concentration of DCPIP
 Burette filled with fruit juice, take a note of start value
and then record the volume required to decolourise
DCPIP (blue to colourless)
 Repeat for each fruit juice
 Determine vitamin C concentration by comparing with
solution of known vitamin C concentration

3 Membrane permeability (beetroot)  Cut at least 5 samples of beetroot  Temperature – keep all in a water bath
 Ensure all samples have the same surface  Age/type of beetroot used (identical for
area/mass/source all samples)
 Leave samples in different dilutions of ethanol for a
suitable time period
 Measure permeability of cell membranes via the use
of a colorimeter
 Calibrate colorimeter initially using distilled water

,  Replicates/repeats to calculate mean

4 Enzyme and substrate concentrations  Range of concentrations of an enzyme (at least 5)  pH
 Ensure that substrate concentration does not become  Temperature
the limiting factor (or vice versa if substrate
concentration is the independent variable)
 Mix the two together
 Measure dependent variable over time (for example,
the collection of gas via an upturned burette in a water
bath)
 Measure initial rate by recording data at specific time
intervals
 Replicates/repeats at each enzyme concentration

5 Observing stages of mitosis  Remove root tip with scalpel and place in hydrochloric  N/a
acid to macerate the tissue
 Rinse in distilled water and add a few drops of a stain,
such as acetic orcein
 Place on a slide and cover with a coverslip
 Calculate the mitotic index by the number of cells
undergoing visible stages of mitosis (chromosomes
present)
 Safety procedure = safety goggles when handling
acid/stain

6 Identifying structures in a stem  Use of forceps to pick out one or two vascular bundles
 Place on microscope slide and use mounted needle to
tease apart
 Add a suitable stain e.g., methylene blue, leave for 5
minutes
 Draw off extra stain with filter paper, add coverslip
 Examine under microscope