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• Mitosis root tip squash


  Required practical where you have to prepare a stained squash of cells and then work out the mitotic index
• Equipment needed


  • Hydrochloric acid
  • Stain (acetic orcein)
  • Mounted needle
  • Root tip (onion or garlic)
• Why use hydrochloric acid

  To soften the root tissues so they can be squashed
• Why use a stain


  To stain the chromosomes in the nucleus so they become visible under the microscope
• Why use a mounted needle

  To lower the cover slip gently and prevent air bubbles
• Why use the root tip

  It is the growing region where cells are dividing and undergoing mitosis
• Method


  1. Grow onion/garlic in water for 3 days

  2. Cut off last 5mm of root tip

  3. Add root tip to hydrochloric acid in water bath at 60°C for 10 mins

  4. Rinse root tip in distilled water

  5. Transfer root tip to slide

  6. Add 2 drops of acetic orcein stain

  7. Carefully lower cover slip

  8. Leave for 10 mins for stain to spread

  9. Gently squash slide
• Cutting with scalpel


  Cut away from yourself to be safe
• Hydrochloric acid


  Wear goggles to protect eyes
• Mounted needle


  Hold from wooden ends, be careful when moving
• Cells observed under microscope

  • Cell in anaphase
  • Cell possibly in metaphase
  • Cells in interphase
• Mitotic index


  Percentage of cells currently undergoing mitosis
• Calculating mitotic index


  Number of cells in mitosis / Total number of cells x 100
• Typical exam questions
• Why use only first 5mm of root tip
• Why press down firmly (gently)
• Why use a stain
• To ensure accurate mitotic index count
• Pedometers can be used to measure the uptake of water as a way to measure the rate of transpiration
• Transpiration


  Water vapour evaporating out of open stomata, mainly found on leaves, which draws up a continuous water column through the xylem of the plant
• Key factors affecting the rate of transpiration

  • Temperature
  • Humidity
  • Air movement or wind
  • Light intensity
• Potometer


  Measures how much water a particular piece of plant is taking up in a period of time
• The amount of water taken up is almost the same as the amount of water which evaporates by transpiration, so the uptake of water is proportional to the rate of transpiration
• Setting up a potometer

  1. Cut plant sample underwater to prevent air entering xylem

  2. Fill potometer equipment with water to remove air bubbles

  3. Place plant sample in potometer through rubber seal

  4. Cover joints with petroleum jelly to make airtight

  5. Introduce one air bubble deliberately
• Measuring transpiration rate with a potometer

  • Look at how far an air bubble moves in the capillary tube over time
  • Calculate volume of water moved using the tube radius and distance travelled
  • Divide volume by time to get rate of water uptake, which represents transpiration rate
• The potometer apparatus has a reservoir and tap to allow resetting and repeating the experiment
• The apparatus must be set up and the plant cut underwater to prevent air entering the xylem and disrupting the continuous water column
• Petroleum jelly is used to cover the joints to prevent air entering and water leaking, maintaining accuracy
• Calculating the volume of water taken up

  1. Use the distance the air bubble moved (15.28mm in 1 minute)

  2. Tube radius is 0.5mm

  3. Volume = π x r^2 x distance = 12 mm^3/min
• To compare transpiration rates between plant species, the leaf surface area must be controlled as a variable
• Respirometer


  An apparatus used to measure the rate of respiration
• Using a respirometer to measure rate of respiration

  1. Set up airtight experimental and control tubes

  2. Add soda lime to absorb CO2

  3. Measure movement of liquid in manometer over time

  4. Calculate volume of O2 absorbed per unit time per unit mass
• Respirometer setup


  • Experimental tube with living organism (e.g. maggots)
  • Control tube with inert object (e.g. glass beads)
  • Manometer to measure pressure changes
  • Soda lime to absorb CO2
• Maggots respiring in experimental tube

  • Oxygen absorbed, CO2 produced and absorbed by soda lime
  • Decrease in gas volume and pressure in experimental tube
  • Liquid in manometer moves towards experimental tube
• Units for rate of respiration


  Volume per unit time per unit mass (e.g. cm3/min/g)
• Calculating rate of respiration

  1. Volume of air moved = π r2 x length

  2. Divide volume by time taken and mass of organism
• Respirometry is one way to measure rate of respiration, linked to topic 5 energy transfers in A-level biology
• The aim of this investigation is to look at whether a named variable, in this case temperature, has an effect on the rate of respiration in a single-celled organism, yeast
• Dehydrogenase


  An enzyme that catalyzes the reaction of removing hydrogen from coenzymes and carbon compounds in the different stages of respiration
• The hydrogen removed by dehydrogenase is normally picked up by coenzymes like NAD or NADH in the final stage of oxidative phosphorylation