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AQA A-Level Biology all practical notes
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AQA A-Level Biology all practical notes
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BIO PRACTICALS★ CONTROL = proving the independent variable is causing the dependent variable to change
★ STANDARD = comparisons
PRACTICAL 1: INVESTIGATION INTO THE EFFECT OF A NAMED VARIABLE ON THE
RATE OF AN ENZYMES CONTROLLED REACTION
It‘s important to measure the initial rate of reaction rather than the average rate over a longer time
period as comparisons can only be made at the start of the reaction, when controlled variables such as
substrate concentration are the same for all values of the independent variable
Systematic error = an error that isn’t determined by chance but it’s introduced by an inaccuracy
Random error = error in measurement caused by factors which vary from one measurement to another
If the surface of the cuvette is scratched, it can result in a greater absorbance of light. If the cuvette used
for the reaction was scratched, but the reference cuvette was not, this would give a systematic error
because it would cause absorbance readings to be higher than the true value for every measurement
● PH = buffer could be used to maintain PH at a suitable level
● Temperature = a water bath & thermometer could be used to maintain a suitable temperature
How is a control set up in a practical measuring enzyme activity?
- Replace the enzyme solution with distilled water or boiler enzyme solution
How can the results of the practical measuring the enzyme activity be used to find the initial rate of
reaction?
- Plot your results on a graph of ‘rate of reaction’ against ‘time’
- Draw a tangent at time = 0 to find the initial rate
How is the rate of reaction calculated from time? = Rate of reaction = 1/time
PRACTICAL 2: PREPARATION OF STAINED SQUASHES OF CELLS FROM PLANT ROOT
TIPS; SET UP & USE OF AN OPTICAL MICROSCOPE TO IDENTIFY THE STAGES OF
MITOSIS IN THESE STAINED SQUASHES & CALCULATION OF THE MITOTIC INDEX
➔ Hydrochloric acid = softens & loosens the root tissues
➔ Acetic orcein stain = turns chromosomes a purple-red colour
➔ Mounted needle = to lower the cover slip & prevent air bubbles under the cover slip
➔ Root tip of onion or garlic = the tip is the growing region so mitosis should be occurring here
Risk assessment:
Hazard Risk Prevention
Scalpel Cut your self Cut on top of a white tile to help prevent
slipping. Cut away from your body when slicing
Hydrochloric acid Irritant [corrosive depending on concentration] Wear goggle & wash hands if any spills on you
Stain May cause irritation to eyes or in cuts Wear goggles & avoid contact with skin
Mitotic index = the number of cells in mitosis x 100
the total number of cells
, ★ The first 5 mm from the tip of an onion root should be used because this is where you will find
the cells that are undergoing mitosis
★ Press down firmly on the cover slip to get a thin layer of cells so light can pass through
★ Stain should be used to make chromosomes visible
★ When counting the cells to calculate the mitotic index, examine large number of fields of
view/many cells to ensure representative sample
Method for calibrating eyepiece graticule:
- Line up eyepiece graticule with stage micrometre
- Use stage micrometre to calculate the size of divisions on eye piece graticule at a particular
magnification
- Take the micrometre away & use graticule to measure how many divisions make up the object
- Calculate the size of the object by multiplying the number of divisions by the size of division
- Recalibrate eyepiece graticule at different magnifications
❖ The root tip was heated with acid to break up the tissues into individual cells. The cellulose cell
walls of plant cells are held together by pectins such as calcium pectate. Treatment with
hydrochloric acid breaks this down
❖ Effect of maceration & pressing the slide preparation on the dividing cells = maceration &
pressing the slide preparation will separate the cells in the meristem tissue into individual cells in
a single layer. This makes it easier to see the chromosomes & identify the stages of division.
Pressing down also makes the slide preparation as thin as possible, allowing light to pass
through. This is important as a light microscope is being used
❖ What information do the cell counts give you about each stage of mitosis? = the cell counts show
the relative duration of each stage in the cell cycle. The longer a phase, the more cells are likely
to be going through that phase at any point in time
❖ The root tips are immersed in acetic orcein stain because the stain binds to chromosomes so
they can be seen more easily
PRACTICAL 3: PRODUCTION OF DILUTION SERIES OF A SOLUTE TO PRODUCE
CALIBRATION CURVE TO IDENTIFY THE WATER POTENTIAL OF A PLANT TISSUE
● Use a large fresh potato - if the potato is not fresh because as the potato dries up, the water
content of the potato will decrease and the concentration of solute will increase which means
that it has a higher negative water potential.
● Distilled water: to make the dilution solutions, as it has a water potential of 0 Kpa.
● Syringe: for transferring precise volume of solution
● tile: allows to cut the potato without causing harm to your fingers or damaging the table surface
● Scalpel: trim any skin on potato chip. Skins are waterproof = osmosis wouldn’t occur
● Kitchen towels: to absorb excess water on the potato chips as excess water will be present in
varying amounts in the initial mass of the potato chips
● Markers & labels: to mark concentration of sucrose solution on the test tubes = avoid confusion
● Wet the cork borer with water as it helps lubricate it. This makes it easier to push the potato chip
out [which can be done using a pencil]
● Dry each chip using a paper towel to remove excess water to ensure that you are not also
measuring the mass of any moisture that may be attached to the potato chips. However, do not
squeeze the potato chips as this would damage the cells.
Control variables:
● Use the same potato to cut all the 6 chips as if all the chips are made from the same potato, they
will all have cells with the same water potential = valid results
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