Core practicals A2 Edexcel Biology

Core practicals from A2 biology unit 5 and 6.

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The effect of temperature on Brine Shrimp

  • 1) Use range of temperatures 5-35degreesC
  • 2)Add 2g of salt to 100cm^3 beaker, then add dechlorinated water and stir (till all of the salt is dissolved.)
  • 3) Place tiny pinch off egg cysts on white paper, wet piece of graph paper using DROPS of salt water and then dab paper onto white sheet to pick up around 40 eggs.
  • 4)Use magnifying glass to count number of eggs, then put paper into the beaker (egg side down) and after 3 minutes remove paper with forceps
  • 5) Incubate at temperature
  •  (Control light intensity, pH, oxygen availability as much as possible so abiotic conditions except temperature are always the same for each test)
  • 6) Next day shine bright light near beaker, (hatched larvae move towards light) Then count number of brine shrimp that hatched. 
  • (Once they have hatched use fine glass pippete and place hacthed brine shrimp into beaker with salt water)
  • 7) Repeat at each temperature, record how many hatched compared with how many were used.
  • (You should find tht around 25-30 the brine shrimp hatching success is largest. 28degreesC is optimum)

Evaluation issues: coutning numbers, keeping light intensity same for each temperature is difficult, ethics, fluctuating temperature some brine shrimp eggs may not have been viable to start with and won't hatch.

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PCR - Polymerase Chain Reaction

This is a technique used to amplify DNA so that is can be used  for DNA analysis (in gel elcetrophoresis) Each cycle- DNA doubles.

2^cylcle =amount of dna copies made e.g after 3 cycles therre are 8 copies after 35 cycles there is around 34billion!

 Step 1: Denaturation 94degreesC

  • double standed dna--> Signle stranded DNA (tempalte strand) as H bonds break between nucleotides

Step 2: Anealing 54degreesC

  • Primers added (primers are short complemetary strands)
  • Attaches- bonds form between primers and template strand

Step 3: Extension 72degrees C:

  • heat resistant enzyme called taq polymerase is added( It does not denature at 72degrees unlike human enzymes which denature above 40)
  • placed in correct position makes copy of template strand.
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Gel Electrophoresis

  • DNA cut up into fragments by restriction enzymes(from bacteria).
  • Fragments placed in blots/wells on Agar Gel in solution.
  • (Gene probe) fluorescent Dye added/radioactive isotopes added.
  • Electric current passed through
  • DNA (phosphate in DNA) is negatively charged and moves towards positive electrode.
  • DNA can be seen on photographic paper using under UV light or through x rays.
  • seperates fragements into bands. (6 mark Question)

smaller fragments travel further than larger fragments. (heaviest near wells(start) lightest near opposite end)

used for: 

  • To get a DNA fingerprint or forensic purposes
  • To get a DNA fingerprint for paternity testing
  • To get a DNA fingerprint so that you can look for evolutionary relationships among organisms
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Effect of different antibiotics on bacteria

Wash hands and sterilise desk before practical. Use aseptic technique-flame forceps for each use... this reduces chance of contamination

  • Prepare agar plate seeded with bacteria, label petri dish on base at edge with name, date and type of bacteria.
  • flame forceps, use forceps to pick up mast ring and then place it in the centre of the agar plate.
  • tape dish securely with 2 pieces with adhesive tape (but not all the way as aerobic respiration must occur)
  • turn plate upside down and incubate at 30degrees C for 48 hours (2days) Also we do not use 37 degrees C as pathogens would start to grow and these are dangerous as they are organisms that cause disease!!!!!!!!!
  • Observe zone of inhibition and measure diameter in mm. larger the diameter the greater the effect of antibiotic on bacteria.

Possible evaluation issues:

  • Ensure disks are placed evenly on petri dish
  • Good asceptic technique used
  • Age of antibiotic
  • Repeats
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Observing ecological patterns 1

RANDOM sampling: For flat ground, slow moving organisms and plants:

  • create grid using tape measure
  • Using number table or calculator generate random numbers, this is used to place quadrats on the grid randomly- so there is no bias
  • count number of organisms that fill each square of the quadrat.

Systemic/systematic sampling: For slopes-hills/mountains, and fast moving organisms

  • Line transect
  • often used to study zonation(-distrubution of organisms in zones of-altitude or depth)
  • Several zones along the transect studied, quadrats placed in each zone
  • organisms counted.
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Observing ecological patterns 2

Methods of measuring abundance:

  • Density- number of individuals in several quadrats, take the mean to give number per, e.g out of 4 quadrats there was an averageof 7 woodlouse found in each one.
  • Frequency- number of times organism was present in a quadrat. e.g 4 out of 10 quadrats contained woodlouse.
  • Percentage cover- this is the % of the ground covered by a species within the sampling unit. Count the number of squares within the quadrat that the plant completely covers, then count the ones that arepartly covered and estimate the total number of full squares that would becompletely covered by that species. e.g 25/50 squares within quadrat covered completely i.e 50% of ground covered by dasies

For fast moving organisms and/or large we may have to use capturing techniques:

  • Pooter- container that can capture invertebrates(no back bone e.g mollusk)
  • Pitfall- (quite like a hole in the ground trap you see in movies!!) used to capture invertebrates.
  • Sweep net- Collects invertebrates in long grass usign a net.
  • Tullgren funnel- collects organisms from soil or leaf litter
  • Baermann funnel- Collects orgnanisms from water

Evaluation issues: organisms constantly move, abitioc conditions can affect distrubution of organisms, limitation-only one study produced, disruption to normal habitat, ethics of measuring wild animlas should be considered- should also be released after counted

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Wow, this is so good, as it's simple and concise and includes all the important points . Really helpful! Thanks!

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