Required practicals

• 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 near 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) 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 compared to other microscopes
• Drawings made from an optical microscope should include a magnification scale
• This is a required practical that could come up as a 6-mark essay question in the exam
• 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. Attach lid with adhesive tape to prevent contamination

  4. Incubate plate upside down at 25°C to reduce chances of harmful bacteria growth
• Investigating effect of antibiotics on bacterial growth


  1. Clean bench with disinfectant

  2. Sterilize inoculation loop

  3. Open sterile agar plate near Bunsen burner flame

  4. Use loop to spread chosen bacteria evenly on plate

  5. Place sterile filter paper disks containing antibiotic onto plate

  6. Incubate plate at 25°C for a few days
• Bacterial culture


  • Bacteria can double in number every 20 minutes with enough nutrients and suitable temperature
  • Nutrient broth solution contains all nutrients bacteria need to grow and divide
  • Agar gel plates contain nutrient broth solidified into a jelly, allowing bacteria to grow into visible colonies
• Zone of inhibition


  Region where bacteria are not growing around the antibiotic disk
• Calculating area of zone of inhibition


  Area = π x R^2, where R is the radius of the zone of inhibition
• Plenty of questions on culturing microorganisms and this required practical in the revision workbook
• Osmosis


  Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Plant cells placed in water

  Water moves into the cell by osmosis, causing the cell to expand
• Plant cells placed in concentrated solution


  Water moves out of the plant cell by osmosis, causing the cell to shrink
• Investigating the effect of osmosis on plant tissue


  1. Peel potato

  2. Use cork board to produce cylinders

  3. Trim cylinders to same length

  4. Measure length and mass of cylinders

  5. Place cylinders in test tubes with different solutions

  6. Leave overnight

  7. Remove cylinders, gently roll on paper towel

  8. Measure length and mass of cylinders again
• Distilled water


  Contains no dissolved substances that could affect the weight of osmosis
• Percentage change


  Calculated as: (change in value / original value) x 100
• Graph of percentage changes in mass or length against concentration of sugar solution shows gain in water in dilute solution, loss in concentrated solution, and no change at the concentration inside the 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 (do not filter)

  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
• Ethanol is highly flammable, so no naked flames should be present
• 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