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Essential A level Biology Practical Skills

What you need to be able to do:

1. Carrying out basic laboratory techniques and understanding the principles thatunderlie them

2. Working safely, responsibly and legally in the laboratory, with due attention toethical aspects

3. Designing, planning and conducting biological in estigations!. "btaining, recording, collating and analysing biological data#. $sing data in se eral forms e.g. numerical, te%tual, erbal and graphical&. ' aluating your e%perimental technique

(eneral )pproach to *ractical Work +ead handouts in ad ance, where possible, so you understand why you are doing a

particular practical and the principles behind it e aware of the time in which you ha e to work Consider safety ha-ards before you begin "rganise your working bench space Write up work as soon as possible after the practical

)ccuracy and *recision in techniques

)ccuracy the closeness of a measured data alue to its true alue *recision the closeness of repeated measurements to each other

/o0 a balance with a fault in it could gi e precise i.e. ery repeatable results butinaccurate untrue results. $nless there is a bias fault in the measuring system,precision will lead to accuracy.

ias can be due to 4 5ncorrectly calibrated instruments e.g. faulty water bath '%perimental manipulations e.g. using a thermometer to measure temperature can

itself decrease the temperature

/ub6ecti e ideas by the e%perimenter e.g. 6udging when an end4point is reached,or fi%ing results to fit those e%pected

7inimising 'rrors

When designing an e%periment 4 'nsure that the independent ariable is the only ma6or factor that changes

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Essential A level Biology Practical Skills

5ncorporate a control e%periment to show it is only the independent ariablewhich causes the measured effect

Where appropriate, select e%perimental sub6ects randomly to cancel out ariationarising from biased selection this is important in ecological in estigations

8eep the number of replicates as high as possible 'nsure the same number of replicates is done for each alue of the independent

ariable 5dentify other factors which could affect the dependent ariable and keep them

constant control ariables e.g. temperature, olumes of solutions, light intensity,time for reaction

7inimising 7easurement 'rror Common source is carelessness e.g. reading a scale in the wrong direction9 reduce

this by more careful recording think how you can do this and by repeating themeasurement

)ccuracy depends on using suitable equipment with care .

)ccurate measurement of liquids :igh iscosity liquids are difficult to transfer9 allow time for all the liquid to

transfer "rganic sol ents or hot liquids may e aporate quickly, making measurements

inaccurate9 transfer these liquids quickly and co er containers

;iquids likely to froth e.g. yeast or protein solutions are difficult to measure9transfer slowly /uspensions e.g. yeast or cell cultures may sediment9 mi% well before transferring $se measuring cylinders on a le el surface so the scale is hori-ontal9 fill to below

the desired mark, then add liquid slowly e.g. by pipette to reach desired le el 7ake sure there are no air bubbles in syringes when measuring olumes. '%pel

liquids slowly and touch the end of the syringe on the essel to remo e any liquidstuck to the end

*otential errors inaccurate measurements for reasons gi en abo e. What effects

would this ha e on the results< $sing balances

=e er weigh anything directly onto a balance>s pan9 this will contaminate it forother users. $se a weighing boat or slip of aluminium foil, or paper

=ote reading to 2 decimal places. =. . When calculating a mean a erage of

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Essential A level Biology Practical Skills

readings, the a erage should also be to 2 decimal places *otential errors samples spilt onto the pan will be measured but not used9 as will

samples left if the weighing boat is not scraped clean. :ow would the results beaffecteds test @ heat for 5-10 minutes in a water bath at 80 oCo $se insulation if necessary or possible

*otential errors has the temperature aried from the stated alue< :ow wouldthis affect the results