REQUIRED PRACTICALS

• Explain how you could use a clear plastic ruler to measure the sizes of cells as viewed under the microscope.

  Place a Perspex ruler on top of your prepared slide so that it sits above a layer of onion epidermal tissue.

  · Place the ruler and the slide back on to the stage of the microscope.

  · Use medium power objective (totalling x100 magnification).

  · Adjust the position of the ruler until it lines up with a continuous group of cells across 1 mm of the ruler. 1mm = 1000 microns (µm)

  · Count the number of cells across the 1000 µm sample.

  · Calculate the size of a single onion cell in microns using the formula by dividing the number of cells counted by the length of the tissue sampled. In this case, the length of the tissue sampled is 1000 µm: Length of cell in µm = Number of cells counted in sample

  1000 µm

  · Write the actual length of the cell on the diagram you have drawn.
• Calculate the magnification of an onion slice seen using an eyepiece lens of x10 and an objective lens of x50

  10 x 50 = x500
• What is the equation used to calculate magnification?

  Actual size = image size / magnification
• what are the hazards in the microscope experiment?

  knife - could cut yourself

  iodine - irritant
• how to reduce risk of hazards in microscopy experiment?

  • carry knife safely
  • use knife on white tile
  • use goggles when using stain
• what are potential ways to modify the microscopy method to investigate different independent variables?

  Could use different tissues or try using different stains
• what are ways of improving the microscopy method to reduce sources of error?

  Lower coverslip slowly to reduce the chances of bubbles
• what are possible sources of error in microscopy method?

  Might confuse air bubbles for structures within

  the cell
• what is the aim of the microscopy experiment?

  Use a light microscope to observe, draw and label a selection of plant and animal cells
• equipment needed for the microscopy experiment?

  · Onion

  · Knife

  · White tile

  · Microscope slide

  · Coverslip

  · Forceps

  · Iodine

  · Microscope
• what mag do you start with in magnescope practical?

  lowest posible magnification lens

  x40
• what do you use to gently lower cover slip on microscopes?

  a mounted needle
• what magnification lenses do you then use in the microscope experiment?

  first x 40

  then x100

  then x400
• how do you focus the microscope?

  Use the x40 magnification to focus the onion cells. Turn the course focus dial until the slide

  is almost in focus, then use the fine focus dial to make the image clear

  Now use x100 and then x400 to view individual cells.
• how do you safely hold a microscope?

  one hand on handle

  one hand under base of stage
• define magnification

  The amount that an image of something is scaled up when viewed through a microscope.
• what is the equation for the magnification of the microscope?

  Magnification of the microscope = magnification of eyepiece × magnification of objective
• what are the 2 lenses in a microscope?

  the eyepiece and the objective lens
• what is the role of iodine in the microscopy experiment?

  will dye the starch in the cells, to help visualise the onion cells
• how many micrometres are in a single mm?

  1000
• how to estimate average cell size?

  count how many full cells can fit in the width across the lese

  measure this distance across lens in mm with clear ruler

  divide this distance by number of cells, in micrometres
• aim of osmosis required practical?

  : Investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue
• equipment needed for osmosis required practical

  Potato

  · Corer or knife

  · Ruler

  · White tile

  · Electronic balance

  · Distilled water

  · Salt solutions of different

  concentrations

  · Timer

  · Paper towel
• what is the independent variable in osmosis practical?

  concentration of salt solution
• what is dependant variable in osmosis required practical?

  percentage change in mass
• what are the control variables in osmosis practical?

  • Time left needs to be the same for all
  • Temperature
  • Surface area of potatoes
  • use the ruler to ensure same dimensions
  • Type of potato
• what is hazard in osmosis practical?

  knife - could cut yourself

  so only use knife on white tile and carry knife carefully
• what are possible sources of error in osmosis method?

  Potatoes not dried thoroughly – making

  mass too high

  · Potatoes squeezed too much during drying

  – making mass too low

  · Potatoes not same starting mass / surface

  area
• Ways of improving the osmosis method to reduce sources of error?

  Only dab potatoes lightly to remove surface moisture

  · Ensure potatoes have same mass and/or surface area
• Ways to modify the osmosis method to investigate different independent variables?

  Could investigate different types of vegetables

  · Could investigate wider range of concentrations

  · Could investigate different solutes
• how to calculate change in mass?

  % change in mass = change in mass / original mass x 100
• why is % change better than change in mass>

  Allows better comparison if starting mass is not the same for all potatoes
• why do we use a corer to cut potato sample?

  ensures sample pieces have same diameter, so can be compared
• why do we cut of potato ends/skins?

  these are impermeable, won't allow water to travel
• what is the relationship between concentration and mass?(osmosis practical)

  increasing concentration increases mass lost
• method for osmosis experiment?

  • use corer to cut potato into same size cylinders
  • measure mass off cylinders
  • place each sample into test type of different salt concentration solutions (O%=distilled water, 5%,10%),
  • leave for at least 10 minds
  • remove + gently dab to remove excess surface liquid
  • record new mass and size of sample
  • calculate percentage change of mass, plot this on graph with concentration on x axis and change in mass on y axis
• what happens when low salt concentration?

  potatoes gain mass
• what happens when medium salt concentration?

  little change in mass
• what happens when high salt concentration?

  potatoes loose mass
• why do potatoes gain mass in low salt solutions?

  higher concentration of water outside potato cells, so water moves into cell