RPAs - Paper 1

• Optical microscope


  Used to look at cells on a prepared microscope slide
• Optical microscope


  • Has a stage to place the microscope slide
  • Has a light source (lamp or mirror) to illuminate the slide
  • Has objective lenses with different magnifications (4x, 10x, 40x)
  • Has an eyepiece lens with 10x magnification
  • Has coarse and fine focusing dials
• Using an optical microscope to view a prepared slide


  1. Place slide on stage and secure with clips

  2. Select lowest power (4x) objective lens

  3. Slowly turn coarse focus dial to lower lens until it almost touches slide

  4. Look through eyepiece and turn coarse focus dial to bring cells into focus

  5. Use fine focus dial to sharpen focus

  6. Calculate total magnification by multiplying eyepiece (10x) and objective (4x, 10x, 40x) lens magnifications
• What you might see under an optical microscope

  • Animal cells: nucleus, cytoplasm, cell membrane, possible mitochondria
  • Plant cells: cell wall, cytoplasm, nucleus, possible vacuole and chloroplasts
• An optical microscope can only show limited detail, cannot see organelles like ribosomes
• Magnification scale


  Measure diameter of field of view in mm, draw scale bar on diagram with magnification value
• Preparing uncontaminated bacterial culture using aseptic technique

  1. Sterilize petri dishes, bacterial nutrient broth, and agar to kill unwanted microorganisms

  2. Transfer bacteria using a sterilized inoculating loop

  3. Seal plate with adhesive tape to prevent contamination

  4. Incubate plate upside down at 25°C to prevent moisture disrupting colonies
• Investigating effect of antibiotics on bacterial growth

  1. Clean bench with disinfectant

  2. Sterilize inoculation loop

  3. Spread chosen bacteria evenly on sterile agar plate near Bunsen burner flame

  4. Place sterile filter paper discs containing antibiotics on plate

  5. Incubate plate at 25°C

  6. Measure zone of inhibition around antibiotic discs to calculate area using formula: Area = π x r^2
• Bacteria can double in number every 20 minutes with enough nutrients and suitable temperature
• Osmosis

  Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Nutrient broth


  Solution containing all the nutrients bacteria need to grow and divide
• Effect of osmosis on plant tissue

  1. Place plant cell in water

  2. Water moves into cell by osmosis, cell expands

  3. Place plant cell in concentrated solution

  4. Water moves out of cell by osmosis, cell shrinks
• Agar gel plate


  Nutrient broth gelled using a chemical called agar, allows bacteria to grow into visible colonies
• Potato


  • Commonly used plant tissue to investigate osmosis
  • Can also use other vegetables like beetroot or parsnip
• Contamination from other microorganisms in the environment must be avoided when culturing bacteria
• Investigating effect of osmosis on plant tissue

  1. Peel potato

  2. Use cork board to produce potato cylinders of same diameter

  3. Trim cylinders to same length (around 3cm)

  4. Measure length and mass of each cylinder

  5. Place cylinders in test tubes with different solutions (0.5M sugar, 2.5M sugar, distilled water)

  6. Leave overnight to allow osmosis

  7. Remove cylinders, gently roll on paper towel to remove surface moisture

  8. Measure length and mass of cylinders again
• The zone of inhibition around antibiotic discs indicates the effect of the antibiotic on bacterial growth
• Percentage change


  Calculated as: (change in value / original value) x 100
• Calculating percentage change


  • Potato cylinder starting mass 1.56g, increases by 0.25g - percentage increase is 16.03%
  • Potato cylinder starting mass 1.32g, decreases by 0.19g - percentage decrease is 14.39%
• Concentration of sugar solution


  Affects percentage change in mass or length of potato cylinder
• Graph of percentage change vs sugar solution concentration shows cylinder gains mass in water, loses mass in concentrated solution, no change at concentration inside cell
• Carrying out chemical tests for carbohydrates, proteins and lipids

  1. Grind food sample with distilled water using mortar and pestle to make a paste

  2. Transfer paste to beaker and add more distilled water

  3. Stir to dissolve chemicals

  4. Filter solution to remove suspended food particles
• Carbohydrates


  Include starch and sugars such as glucose
• Test for starch


  1. Place 2cm3 of food solution in test tube

  2. Add a few drops of iodine solution

  3. Blue-black colour indicates presence of starch
• Test for sugars (e.g. glucose)

  1. Place 2cm3 of food solution in test tube

  2. Add 10 drops of Benedict's solution

  3. Heat test tube in hot water bath for 5 minutes

  4. Colour change indicates amount of reducing sugars present
• Benedict's test


  Only works for reducing sugars, not non-reducing sugars like sucrose
• Test for proteins


  1. Place 2cm3 of food solution in test tube

  2. Add 2cm3 of Biuret solution

  3. Purple/lilac colour indicates presence of proteins
• Test for lipids/fats


  1. Grind food with distilled water using mortar and pestle

  2. Transfer 2cm3 of solution to test tube

  3. Add a few drops of distilled water and ethanol

  4. Shake gently

  5. White cloudy emulsion indicates presence of lipids
• All chemicals used in these tests are potentially hazardous, so safety goggles must be worn
• Investigating the effect of light intensity on the rate of photosynthesis

  1. Take a boiling tube

  2. Place 10 cm away from an LED light source

  3. Fill boiling tube with sodium hydrogen carbonate solution

  4. Put a piece of pond weed into the boiling tube

  5. Leave for 5 minutes to acclimatize

  6. Start stopwatch and count bubbles produced in 1 minute

  7. Repeat 2 more times and calculate mean

  8. Repeat experiment at 20 cm, 30 cm, and 40 cm
• Problems with the practical

  • Number of bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• Solving the problems


  1. Measure the volume of oxygen produced instead of counting bubbles

  2. Use equipment to catch the bubbles in a measuring cylinder filled with water

  3. Use the measuring cylinder to measure the volume of oxygen produced
• Doubling the distance from the light to the pondweed

  Number of bubbles per minute falls by a factor of 4
• Inverse square law


  If we double the distance, the light intensity falls by 4 times, which causes the number of oxygen bubbles to fall by 4 times
• Higher tier students need to understand the inverse square law
• You'll find plenty of questions on this required practical in the revision workbook