A-Level AQA Biology Practicals

Practicals in Biology

Potometer (binary/octet-stream)

Food Tests

Reducing Sugars: 1) add blue Benedict's reagent and heat in a water bath 2) will form coloured precipitate if there is a sugar present (precipitate goes from blue to green to yellow to orange to brick red) 3) the further the colour change goes, the more sugar is present in the solution

Non-Reducing sugars: 1) complete reducing sugars test, only continue if negative 2) add dilute hydrochloric acid and heat to break down into monosaccharides 3) add sodium hydroxide to neutralise 4) carry out the Benedict's test again

Food Tests

Test for Starch: add iodine to the test sample and if starch is present, the iodine will turn from orange to blue-black

Test for Lipids: 1) add ethanol to food sample and shake 2) add solution to water 3) if positive, a milky emulsion will form

Food Tests

Food Tests (binary/octet-stream)

Test for Protein: 1) add sodium hydroxide to make test solution alkaline 2) add Biurets solution (containing copper(II) sulphate) 3) if positive, reaction sample will go from blue to purple

Measuring Reaction Rates

Measuring Product Formed: 1) set up 5 boiling tubes of equal volume and concentration hydrogen peroxide and add an equal volume of buffer to each tube and put each tube into a water bath at tempratures at 10 degree intervals 2) set up the apparatus opposite 3)add the same volume of catalase to the solution, and quickly insert the bung and start the stopwatch 4) record the amount of oxygen formed in the first 60 seconds  5) repeat for each temperature

Gas Production (binary/octet-stream)

Measuring Reaction Rates

1 (binary/octet-stream)

1) put a drop of iodine into each well on a spotting tile 2) mix starch and amylase (breaks down starch) in a test tube and start the stop watch 3) at regular intervals, drop some of the mixture into the spotting tile until the mixture no longer turns blue in the iodine, indicating the reactant has all been used up 4) repeat the experiment using different amylase concentrations

Cell Fractionation

1) homogenise to break up the cell membrane and release organelles into solution. conditions of the solution must be ice cold (prevent actions of enzymes which break down organelles), isotonic (prevent osmosis causing damage to organelles) and pH buffered. 2) the solution must them be filtered through a gauze to remove any large cell debris or tissue debris (organelles pass through the gauze as they are much smaller than the debris)  

3) ultracentrifugation:    i) homogenate is put into a test tube and placed into a centrifuge at a low speed. The heaviest organelles are pulled into a pellet at the bottom of the tube    ii) supernatant is drained off the top of the pellet and centrifuged again at a higher speed, and slightly heavier organelles form another pellet    iii) this continues until all of the organelles are separated off

Serial Dilution (binary/octet-stream)

Serial Dilution

serial dilution by a factor of 2: 1) line up 5 test tubes in a rack 2) add 10cm3 of the solution to the firs test tube and 5cm3 of distilled water to each of the following test tubes to the other 4 tubes 3) take 5cm3 of the fist solution to the second test tube and shake 4) take 5cm3 from the second test tube and add to the third and shake 5) continue for all the test tubes

Measuring Water Potential Using a Potato

1) use a cork borer to get equally sized chips of potato 2) find the original mass of groups of three potato chips 3) put each group of chips into a different concentration of serial diluted sucrose solution 4) leave the solutions for an equal amount of time (around 20 minutes) 5) measure the final mass of the groups of potato chips after removing any solution from the surface 6) measure the percentage change in mass and draw a calibration curve 7) where the calibration curve crosses the x axis is the water potential of the potato

Water Potential Curve (binary/octet-stream)

ELISA Test (HIV)

Elisa Test (binary/octet-stream)

1) HIV antigen is bound to the bottom of the well in a well tray 2) blood sample is added to the well, and if HIV is present, antibodies will bind to the antigen on the bottom of the well. well is rinsed to remove unbound antigens 3) secondary HIV antibody is put into the well which has an enzyme attached to it. the well is then washed out to remove any of the secondary antibody which is unbound 4) a substrate is added to the well which changes colour if the enzyme is present (intermediate washing is important as it prevents any false positive results due to residual antibody)  

Effects of Antibiotics

1) use a sterile pipette or metal loop to spread the bacterial culture over the agar 2) use sterile forceps to place paper discs soaked in antibiotics onto the agar jelly 3) lightly tape on the lid leaving air gaps, invert the agar plate and incubate at 25 degrees for 48 hours 4) measure the size of the clear patch (inhibition zone) around the antibiotic discs, the largest inhibition zone shows the most effective antibiotic

Antibiotic Tests (binary/octet-stream)

Aseptic Techniques

- regularly disinfect the work surface - place any used utensils directly into a beaker of disinfectant - use sterile equipment - work near a Bunsen flame as the hot air makes any microbes in the air rise away from the culture you are using - minimise the time with the lid off the agar plate to reduce the amount of airborne microorganisms reaching the jelly - flame the neck of the container of the bacterial broth after opening and before closing moving air out of the container preventing unwanted microorganisms from falling in

Thin Layer Chroatography

Comparing Plant Pigments: 1) grind up leaves from several different plants with anhydrous sodium sulphate, then add a few drops of propanone 2) transfer the liquid to a test tube and add petroleum ether and transfer the top layer formed into another test tube 3) draw a horizontal line at the bottom of the TLC in pencil and slowly build up a concentrated dot of the pigment by applying a drop then letting it dry and repeating around 10 times 4) once the origin point is totally dry put the TLC plate into a beaker containing solvent below the origin point and leave the solvent to move up the plate until the solvent has almost reached the top of the TLC plate

5) take the plate out of the solvent ad mark the slovent front and leave to dry in a well ventilated area 6) repeat for other plant types 7) calculate Rf values by dividing distance travelled by spot by the distance travelled by the solvent

Investigating Photosynthesis

1) cut a few leaves into small pieces removing tough stalks 2) grind with a pestle and mortar in isotonic solution 3) filter this liquid into a beaker through a muslin cloth 4) transfer the liquid into centrifuge tubes and centrifuge for 10 minutes to form chloroplast pellets in the bottom  5) remove the liquid from the top of the tubes 6) resuspend the chloroplasts in fresh isotonic solution and store on ice 7) set up a colorimeter using a red filter and calibrate using the chloroplast suspension and distilled water

8) set up a test tube rack a set distance from a lamp 9) in each tube, put a set amount of chloroplast extract and a set volume of DCPIP and mix the contents of the tube 10) immediately take a sample of the solution and put into the colorimeter and record the absorbance 11) repeat every 2 minutes for 10 minutes 12) repeat steps 8 to 11 for every distance under investigation

Respiration Experiments

Aerobic Respiration: 1) put a known volume and concentration of substrate in a test tube and add a pH buffer 2) place test tube in a water bath of one of the temperatures under investigation for 10 minutes 3) add a known mass of dry yeast and stir 4) after the yeast is dissolved add a bung with a delivery tube into a gas syringe set to 0 and start the stopwatch 5) CO2 will form and fill the gas syringe which should be recorded at regular time intervals for around 10 minutes 6) a control should be set at each temperature with no yeast present to prove no reaction takes place without it 7) repeat the experiment at each temperature and find an average reaction rate at each temprature

Anaerobic Respiration: 1) complete steps 1 to 3 on the left 2) after yeast is dissolved, add a layer of liquid paraffin to the top of the solution. this will prevent oxygen reaching the solution making the yeast respire anaerobically 3) repeat steps 4 to 7 on the left

Respiration Experiment

Respirometer (binary/octet-stream)

1) set up the apparatus on the right at optimum temperature for the investigated organisms 2) control is set up on the opposite side containing the same mass of object as the specimen 3) for 10 minutes leave the tap open to allow the specimen to settle 4) close the tap and use the syringe to level off the liquid in the central tube 5) as the specimen respires, O2 is used and CO2 is absorbed, pulling the liquid in the tube towards the side of the specimen due to volume of air decreasing  6) after around 10 minutes record the level on the manometer, then repeat 4 times so that an average can be taken 

Investigating Animal Responses

Choice Chambers (binary/octet-stream)

1) construct a choice chamber as shown to the right 2) place 10 woodlice in the choice chamber and place the lid. 3) after 10 minutes, take off the lid and record the number of woodlice in each of the 4 quaters 4) repeat by placing the woodlice in the centre again and leaving for 10 minutes

Investigating Glucose Concentration

1) set up five serial dilutions of glucose solution 2) do a quantitative Benedict's test by using a colorimeter and make a calibration curve 3) use a colorimeter in the specimen samples after doing the Benedict's test on each 4) use the calibration curve to find the glucose concentrationof the specimen samples (usually urine samples)

Glucose Test (binary/octet-stream)