Required practicals

• light microscope

  Used to look at cells on a prepared microscope slide
• light 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) 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
• A magnification scale should be included on drawings made from an optical microscope
• Osmosis


  Diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• 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 sugar solutions or distilled water

  6. Leave overnight

  7. Remove cylinders, gently roll on paper towel

  8. Measure length and mass of cylinders again
• Percentage change


  Calculated as: (change in value / original value) x 100
• Potato cylinder in water


  Gains mass as water moves in by osmosis
• Potato cylinder in concentrated sugar solution


  Loses mass as water moves out by osmosis
• Where line crosses x-axis

  No change in mass, concentration outside cell same as inside
• 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
• Lipid molecules can stick to filter paper, so the lipid test solution is not filtered
• 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 and place it 10 cm away from an LED light source

  2. Fill the boiling tube with sodium hydrogen carbonate solution

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

  4. Leave for 5 minutes to acclimatize

  5. Count the number of bubbles produced in 1 minute
• Repeating the experiment at different distances


  1. Repeat the experiment at 20 cm, 30 cm, and 40 cm from the light source

  2. Calculate the mean number of bubbles produced per minute at each distance
• Problems with counting bubbles


  • Bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• Measuring the volume of oxygen produced

  1. Place the pond weed in a measuring cylinder filled with water

  2. Measure the volume of oxygen produced
• Doubling the distance from the light source


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


  If the distance is doubled, the light intensity falls by a factor of 4, which causes the number of oxygen bubbles to fall by 4 times