paper 1

Question 1

describe how to investigate the effect of antibiotics on bacterial growth

Answer 1

• sterilise glass Petri dishes and agar gel in an autoclave
• pour the sterile agar gel into the petri dish and allow time to set
• sterilise the inoculating loop by passing it through a Bunsen burner flame
• Dip the inoculating loop into the solution of microorganisms and make streaks with the loop on the surface of the agar
• place sterile filter paper discs containing antibiotics onto the plate
• put the lid on the petri dish and secure it with tape. Label the turn and store it upside down
• incubate the culture at 25°C in school laboratories
• measure the zone of inhibitions to find the most effective antiseptic

Question 2

how do you prepare your microscope slide?

Answer 2

• add a drop of water to the middle of a clean slide
• cut up an onion and separate it out into layers. Use tweezers to peel off the epidermal tissue from the bottom of one of the layers
• using the tweezers, place the epidermal tissue onto the water on the slide
• add a drop of iodine solution. Iodine is a stain, which is used to highlight objects in a cell by adding colour to them
• place a cover slip on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there - they’ll obstruct your view of the specimen

Question 3

describe how to observe the effect of sugar solutions on plant tissue

Answer 3

• first, peel the potato - potato skin can affect osmosis
• use a cork borer to produce three cylinders of potato. Using a cork borer makes all of the cylinders the same diameter
• use a scalpel to trim the cylinders to the same length (around 3cm)
• measure the length of each cylinder using a ruler and the mass of each cylinder using a balance
• now place each cylinder into a test tube. Add 10cm3 of a 0.5 molar sugar solution to the first test tube
• add 10cm3 of 0.25 molar sugar solution to the second test tube and 10cm3 of distilled water to the third test tube - distilled water has no dissolved substances which could affect the rate of osmosis
• leave the potato cylinders overnight to allow osmosis to take place
• next, remove the potato cylinders and gently roll them on a paper towel to remove any surface moisture
• measure the length and the mass of the cylinders again
• calculate the percentage change

Question 4

describe how to carry out chemical tests for carbohydrates

Answer 4

• take the food sample and grind this with distilled water using a mortar and pestle.
• transfer the paste to a beaker and add more distilled water. Stir so the chemicals in the food dissolve in the water
• filter the solution
• place 2cm3 of food solution into a test tube
• add a few drops of iodine solution which is an orange colour
• if starch is present then the iodine solution will turn blue-black

Question 5

describe how to carry out chemical tests for proteins

Answer 5

• take the food sample and grind this with distilled water using a mortar and pestle. We want to make a paste
• transfer the paste to a beaker and add more distilled water. Stir so the chemicals in the food dissolve in the water
• filter the solution
• place 2cm3 of a food solution
• add 2cm3 of biuret solution which is a blue colour
• if the protein is present then the biuret solution will change from blue to a purple or lilac colour

Question 6

describe how to carry out chemical tests for lipids

Answer 6

• take the food sample and grind this with distilled water using a mortar and pestle to make a paste
• transfer the paste to a beaker and add more distilled water. Stir so the chemicals in the food dissolve in the water
• do not filter the solution - this is because lipid molecules can stick to filter paper
• transfer 2cm3 of our food solution to a test tube
• add a few drops of distilled water and a few drops of ethanol
• gently shake the solution
• if lipids are present, then a white cloudy emulsion forms
• ethanol is highly flammable
• it is very important that no naked flames are present

Question 7

describe how to carry out chemical tests for sugars

Answer 7

• take the food sample and grind this with distilled water using a mortar and pestle. We want to make a paste
• transfer the paste to a beaker and add more distilled water. Stir so the chemicals in the food dissolve in the water
• filter the solution
• place 2cm3 of food solution in a test tube
• add 10 drops of Benedict’s solution which is a blue colour
• place the test tube containing our solution into a beaker and half-fill the beaker with hot water from a kettle
• now leave this for around five minutes
• if sugars are present, Benedict’s solution will change colour
• the colour of Benedict’s solution gives us an approximate idea of the amount of sugar present - not exact amount
• a green colour tells us that there is a small amount of sugar
• a yellow colour tells us that there is more sugar present
• a brick-red colour tells us that there is a lot of sugar present
• Benedict’s test only works for certain sugars e.g. glucose -> reducing sugars

Question 8

describe how to investigate the effect of pH on the enzyme amylase

Answer 8

• place one drop of iodine solution into each well of a spotting tile
• take three test tubes -> first test tube add 2cm3 of starch solution, second test tube add 2cm3 of amylase solution, third test tube add 2cm3 of pH 5 buffer solution
• place all three test tubes in a water bath at 30°C. leave them for 10 minutes to allow the solutions to reach the correct temperature
• now combine the three solutions into one test and mix with a stirring rod. Return to the water bath and start a stopwatch
• after thirty seconds, use the stirring rod to transfer one drop of solution to a well in the spotting tile which contains iodine
• the iodine should turn blue-black, showing that starch is present
• take a sample every thirty seconds and continue until the iodine remains orange
• when the iodine remains orange, this tells us that starch is no longer present - the reaction has completed
• now repeat the whole experiment several times using different pH buffers e.g. pH 6,7 and 8

Question 9

what are the problems with the amylase practical?

Answer 9

• we are only taking samples every thirty seconds
• this means that we only have an approximate time for the reaction to complete. We could address this by taking samples every ten seconds
• we are looking for the time when iodine does not go blue-black - this is not always obvious
• this is because the colour change tends to be gradual. some wells might have a small amount of blue-back mixed with orange, so it can be difficult to see when the reaction has finished
• one way to address that problem is to ask several people to look at the spotting tile and decide when the reaction has been completed