BIOLOGY RPS P1

• 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) lens magnifications
• What can be seen under an optical microscope

  • Animal cells: nucleus, cytoplasm, cell membrane, possible mitochondria
  • Plant cells: cell wall, cytoplasm, nucleus, possible vacuole and chloroplasts
• Optical microscopes can only show limited detail, cannot see organelles like ribosomes
• 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
• 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
• 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

  4. Orange colour indicates no 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. Green colour = small amount of sugar

  5. Yellow colour = more sugar

  6. Brick red colour = a lot of sugar
• Reducing sugars


  • Sugars that the Benedict's test works for (e.g. glucose)
  • Non-reducing sugars (e.g. sucrose) do not work with Benedict's test
• Test for proteins


  1. Place 2cm3 of food solution in test tube

  2. Add 2cm3 of Biuret solution (blue)

  3. Purple/lilac colour indicates presence of protein
• 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
• 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, repeat 2x & calculate mean

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

  7. 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
• 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
• Water moves "out" of the cell


  This means that water is moving from an area of higher concentration (inside the cell) to an area of lower concentration (outside the cell)
• Water moves "into" the cell


  This means that water is moving from an area of lower concentration (outside the cell) to an area of higher concentration (inside the cell)