Required practicals

• Potato experiment to show osmosis at work

  1. Cut out identical cylinders of potato using cork borer and ruler and measure the mass of each one

  2. Put one into a beaker of pure water and the other into a beaker of very concentrated sugar solution (eg. 1 mol/dm3)

  3. Also put some in a beakers with concentrations between this e.g. 0.2M, 0.4M, 0.6M,0.8M

  4. Leave the potato cylinders in the in each beaker for around 24 hours

  5. Take them out after 24 hours and dry them with a paper towel; then measure the new mass of each
• If the mass of the potatoes has increased

  Water has been drawn in via osmosis
• If the mass of the potatoes has decreased

  Water has been drawn out via osmosis
• Why should we calculate the percentage mass for the potato experiment?

  To better compare the effect of sugar concentration of the cylinders that didn't have the same initial mass
• Potato experiment variables


  • Dependent - Potato mass
  • Independent - Concentration of the sugar solution
  • Control - Volume of solution, temperature, time, type of sugar used, species of potato
• Causes of error in potato experiment

  • Some of the potato cylinders not being fully dried (excess water would increase the mass)
  • Water evaporating from the beakers (the concentrations of the sugar solutions would change)
• How to reduce effect of errors in potato experiment


  Repeat the experiment and calculate a mean percentage at each concentration
• Investigating the effect of pH on enzyme activity


  1. Drop of iodine solution into each well of a spotting tile (dimple tray)

  2. Heat a beaker of water to 35°c (use a thermometer)

  3. Use a syringe to add 1cm³ of amylase solution to 1cm³ of buffer solution with a pH of 5 to a boiling tube

  4. Add 5cm³ of starch solution to the boiling tube

  5. Immediately mix the contents of the tube and start a stop clock

  6. Continuous sampling to record how long it takes for the amylase to break down all the starch - use a pipette to take a fresh sample from the boiling tube every 30 seconds and put a drop in a well
• Investigating the effect of pH on enzyme activity variables

  • Independent - pH of the buffer solution
  • Dependent - time taken for the starch to be fully broken down
  • Control - concentration and volume of amylase solution
• Amylase


  Enzyme that catalyses the breakdown of starch to maltose
• Detecting starch


  Using iodine - if iodine is present, it turns the solution from browny-orange to blue-back
• Calculating rate of reaction

  Rate = 1000/time
• Calculating rate of reaction in an experiment measuring how much something changes over time

  Amount it has changed by / Time taken
• Investigating reaction times


  1. The person being tested should sit with their arm resting against a table

  2. Hold a ruler vertically between their thumb and forefinger. Zero end should be in line with thumb and forefinger. Then, let go without warning

  3. The person being tested should try to catch the ruler as quickly as they can

  4. The reaction time is measure by the number on the ruler where it is caught (from the top of the thumb). The higher the number, the slower the reaction time

  5. Repeat the test the calculate the mean distance that the ruler fell
• Control variables for reaction time test

  • Same person to catch the ruler each time
  • Ruler should always be dropped from the same height
  • Read the ruler from the same place e.g. above the thumb
  • Make sure that the person being tested can't be influenced by anything that may alter their reaction time e.g caffeine
• Independent variable for reaction time test


  Effect that something may have on reaction times e.g caffeine
• Dependent variable for reaction time test

  Reaction times
• Using computers to measure reaction times


  • Use a simple computer test e.g click the mouse as soon as a stimulus is seen on the screen
  • More precise reaction time (removes human error)
  • Can record to milliseconds and give a more accurate measurement
  • Using a computer also removes the possibility that the person can predict when to respond
• Investigating the effect of light on the growth of cress seeds

  1. Put 10 cress seeds into 3 different petri dishes, each lined with moist filter paper

  2. Shine a light onto one of the dishes from above and the other two from different directions

  3. Leave for 1 week until you can observe their responses
• Variables to control when investigating the effect of light on the growth of cress seeds

  • Number of seeds - use the same number of seeds in each dish
  • Type of seed - use seeds that all come from the same packet
  • Temperature - keep petri dishes in a place with a stable temperature
  • Water - Use a measuring cylinder to add the same amount of water
  • Light intensity - Keep distance between the bulb and the dish the same
• Investigating the effect of gravity on plant growth

  1. Place 4 seedlings on damp cotton wool in a petri dish, each with their roots pointing in a different direction

  2. Store the petri dish vertically for a few days in the dark
• Investigating the effect of antibiotics (or antiseptics) on bacterial growth

  1. Place paper discs soaked in different types of antibiotic on an agar plate that has an even covering of bacteria

  2. The antibiotic should diffuse into the agar jelly and kill any bacteria (except resistant bacteria)

  3. A clear zone/area (inhibition zone) will be left where bacteria have died

  4. Incubate for 48 hours at 25 degrees

  5. The bigger the inhibition zone the greater the effect of the antibiotic on the bacteria
• Variables for investigating the effect of antibiotics (or antiseptics) on bacterial growth

  • IV- Different antibiotic/antiseptic
  • DV-Zone of inhibition/Diameter or Area of clear zone
  • CV-Concentration of antibiotic, size of disc, material of disc
• Use a control, a disc soaked in sterile water to allow you to prove that the zone around the disc was due to the effect of the antibiotic alone
• Culture plates are not incubated at temperatures higher than 25 degrees to prevent pathogenic bacteria from growing
• Ways to avoid contamination of cultures

  • Passing inoculating loop through a flame to sterilise
  • Lightly tape the lid of the petri dish shut once the bacteria has been transferred to it to prevent microbes from the air getting in
  • Petri dish should be stored upside down to stop drops of condensation falling onto the agar surface
• Testing food for the presence of starch


  1. Add food to dimple tray

  2. Add iodine

  3. Blue-black colour is a positive result for starch
• Testing for sugars


  1. Add food sample to test tube

  2. Add 5cm3 of benedict reagent (Blue) into the test tube using a pipette

  3. Heat the test tube using a water bath or using a bunsen burner

  4. Brick red shows that sugars are present
• Testing for protein


  1. Add food sample to test tube

  2. Add 5cm3 of copper sulphate and 5cm3 of sodium hydroxide into the test tube using a pipette (This is called the biuret test)

  3. Gently shake the tube

  4. Colour change from blue to purple indicates a protein is present
• Testing for lipids


  1. Add sample of food to a test tube

  2. Using a pipette add 3 drops of Sudan III

  3. Gently shake

  4. Sudan III stains lipids. If the sample contains lipids it will separate out into two layers, the top layer will be red if lipids are present
• Colorimeter


  More accurate than using benedict's, it is quantitative and not qualitative, not subjective
• Investigating the effect of temperature on the rate of decay

  1. Add 5ml of milk, 7ml of sodium carbonate solution, 5 drops of phenolthalein indicator into a test tube

  2. Place into water bath at desired temperature and wait to reach equilibrium

  3. Add 5ml of lipase

  4. Record the time it takes to decompose the milk (colour change from pink to colourless)

  5. Repeat with other temperatures

  6. Calculate rate using Rate = 1000/time
• Phenolphthalein indicator


  Changes colour as the lipase breaks down the milk because the pH changes from alkaline (pH 10) to acidic (pH 2) due to the fat being broken down into fatty acids and glycerol
• Investigating how light intensity effects the rate of photosynthesis

  1. Place lamp at a specific distance from the pondweed

  2. Leave for 1 minute. The oxygen will be collected in a capillary tube. Draw the gas bubble alongside the ruler

  3. Record the volume of oxygen produced. This is proportional to the length of the gas bubble

  4. Repeat at different distances
• Controlling temperature


  • Place in a water bath or use a beaker of water as a heat sink/ to absorb the heat from the lamp
• Ensuring data is reliable


  • Repeat the experiment, remove outliers and calculate a mean
• Preparing an onion slide

  1. Cut up an onion into small squares

  2. Peel the lower epidermis using tweezers

  3. Place onto a SLIDE

  4. Add a drop of iodine to stain the cells/tissue

  5. Place a cover slip onto the specimen, carefully by tilting and lowering. Be careful not to get any air bubbles

  6. View using the lowest-powered objective lens