biology required pracs

• 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 and slowly lower it until it almost touches the slide

  3. Look through eyepiece and slowly turn coarse focus dial until cells come into focus

  4. Use fine focus dial to bring cells into clear focus

  5. Calculate total magnification by multiplying eyepiece (10x) and objective lens (e.g. 4x) 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
• Drawing of cells should include a magnification scale, measured by placing a ruler on the stage
• Osmosis

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

  • Commonly used plant tissue to investigate osmosis
  • Can also use other vegetables like beetroot or parsnip
• 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
• Percentage change

  Calculated as: (change in value / original value) x 100
• 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 sugar 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 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. Start a stopwatch and count the number of bubbles produced in 1 minute

  6. Repeat the experiment at 20 cm, 30 cm, and 40 cm from the light source
• 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 a measuring cylinder to catch the bubbles and measure the volume of oxygen
• Doubling the distance from the light source


  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