RPAs

• 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
• 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. 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 inoculating 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 equation: Area = π x r^2
• Bacteria can double in number every 20 minutes with enough nutrients and suitable temperature
• Nutrient broth


  Solution containing all the nutrients bacteria need to grow and divide
• Agar gel plate


  Nutrient broth gelled using a chemical called agar, allows bacteria to grow into visible colonies
• Contamination from other microorganisms in the environment must be avoided when culturing bacteria
• Zone of inhibition


  Region around antibiotic disc where bacteria do not grow
• 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 3cm length

  4. Measure length and mass of cylinders

  5. Place cylinders in test tubes with 0.5M sugar solution, 2.5M sugar solution, and 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
• Percentage change calculations


  • Potato cylinder starting mass 1.56g, increases by 0.25g - 16.03% increase
  • Potato cylinder starting mass 1.32g, decreases by 0.19g - 14.39% decrease
• 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