2. Cells Practicals Flashcards
aim of required practical 4
-investigation into the effect of a named variable
on the permeability of cell-surface membranes
describe a method to investigate the effect of a named variable (eg.
temperature) on the permeability of cell-surface membranes
- cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel
- rinse to remove pigment released during cutting or blot on paper towel
- add same number of cubes to 5 different test tubes containing same volume of water (eg. 5 cm3)
- place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50oC)
- leave for same length of time (eg. 20 minutes)
- remove plant tissue and measure pigment release by measuring intensity of colour or concentration of surrounding solution semi-quantitatively / quantitatively
describe a semi-quantitative method to estimate pigment concentration in a solution
-use a known concentration of extract and distilled water to prepare a dilution series
-compare results with these ‘colour standards’ to estimate concentration
describe a quantitative method to estimate pigment concentration in a solution
-use a known concentration of extract and distilled water to prepare a dilution series
-compare results with these ‘colour standards’ to estimate concentration
explain why the beetroot is washed before placing it in water
-wash off any pigment on surface
-to show that release is only due to [named variable]
explain why each test tube containing cubes of plant tissue is regularly shaken
-to ensure all surfaces of cubes remain in contact with liquid
-to maintain a concentration gradient for diffusion
explain why the volume of water needs to be controlled
-too much water would dilute the pigment so solution will
appear lighter / more light passes through in colorimeter
than expected
-so results are comparable
explain how you could ensure beetroot cylinders were kept at the same temperature throughout the
experiment
-take readings in intervals throughout experiment of
temperature in tube using a digital thermometer /
temperature sensor
-use corrective measure if temperature has fluctuated
describe the issues with comparing to a colour standard
-matching to colour standards is subjective
-colour obtained may not match any of colour standards
what does a high absorbance suggest about the cell-membrane
-more permeable / damaged
-as more pigment leaks out making surrounding solution more concentrated (darker)
explain how temperature affects permeability of cell-surface membranes
-as temperature increases, cell membrane permeability increases
○ phospholipids gain kinetic energy so fluidity increases
○ transport proteins denature at high temperatures as hydrogen bonds break,
changing their tertiary structure
-at very low temperatures, cell membrane permeability increases
○ ice crystals can form which pierce the cell membrane and increase permeability
explain how pH affects permeability of cell-surface membranes
-high or low pH increases cell membrane permeability
○ transport proteins denature as H / ionic bonds break, changing tertiary structure
explain how lipid-soluble solvents eg. alcohol affect permeability of cell-surface membranes
-as concentration increases, cell membrane permeability increases
-ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (creating gaps)
aim of required practical 2
-preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these
stained squashes and calculation of a mitotic index
describe how to prepare squashes of cells from plant root tips
- cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide
- soak root tip in hydrochloric acid then rinse
- stain for DNA (eg. with toluidine blue)
- lower coverslip using a mounted needle at 45 degrees
without trapping air bubbles - squash by firmly pressing down on glass slip but do not push sideways
explain why root tips are used in practical 2
-where dividing cells are found / mitosis occurs
explain why a stain is used in practical 2
-to distinguish chromosomes
-chromosomes not visible without stain
explain why the cover slip needs to be squashed / pressed down in practical 2
- (spreads out cells) to create a single layer of cells
-so light passes through to make chromosomes visible
explain why the cover slip should not be pushed sideways in practical 2
-avoid rolling cells together / breaking chromosomes
give two reasons why the roots should be soaked in acid in practical 2
-separate cells / cell walls
-to allow stain to diffuse into cells
-to allow cells to be more easily squashed
-to stop mitosis
describe how to set-up and use an optical microscope
- clip slide onto stage and turn on light
- select lowest power objective lens (usually x 4)
- a. use coarse focusing dial to move stage close to lens
b. turn coarse focusing dial to move stage away from lens until image comes into focus - adjust fine focusing dial to get clear image
- swap to higher power objective lens, then refocus
what are the rules of scientific drawing
✓ look similar to specimen / image - draw parts to the same scale / relative size
✓ no sketching - only clear, continuous lines
✓ no shading / hatching
✓ include a magnification scale (eg. x 400)
✓ label with straight, uncrossed lines
explain how the stages of mitosis can be identified
-prophase
● chromosomes visible / distinct → because condensing
● but randomly arranged → because no spindle activity / not attached to spindle fibre
-metaphase
● chromosomes lined up on equator → because attaching to spindle
-anaphase
● chromatids (in two groups) at poles of spindle
● chromatids V shaped → because being pulled apart at their centromeres by spindle fibres
-telophase
● chromosomes in two sets, one at each pole
what is a mitotic index
-proportion of cells undergoing mitosis (with visible chromosomes)
-mitotic index = number of cells undergoing mitosis / total number of cells in sample
explain how to determine a reliable MI from observed squashes
-count cells in mitosis in field of view
-count only whole cells / only cells on top and right edges → standardise counting
-divide this by total number of cells in field of view
-repeat with many / at least 5 fields of view selected randomly → representative sample
-calculate a reliable mean
suggest how to calculate the time cells are in a certain phase of mitosis
- identify proportion of cells in named phase at any one time
○ number of cells in that phase / total number of cells observed - multiply by length of cell cycle
aim of practical 3
-production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue
describe how a dilution can be calculated
- calculate dilution factor = desired concentration / stock concentration
- calculate volume of stock solution = dilution factor x final desired volume
- calculate volume of distilled water = final desired volume - volume of stock solution
describe a method to produce of a calibration curve with which to identify the water potential of plant tissue (eg. potato)
- create a series of dilutions using a 1 mol dm-3 sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8,
1.0 mol dm-3) - use scalpel / cork borer to cut potato into identical cylinders
- blot dry with a paper towel and measure / record initial mass of each piece
- immerse one chip in each solution and leave for a set time (20-30 mins) in a
water bath at 30 oC - blot dry with a paper towel and measure / record final mass of each piece
-repeat (3 or more times) at each concentration
describe processing data in practical 2
- calculate % change in mass = (final - initial mass)/ initial mass
- plot a graph with concentration on x axis and percentage change in mass
on y axis (calibration curve)
○ must show positive and negative regions - identify concentration where line of best fit intercepts x axis (0% change)
○ water potential of sucrose solution = water potential of potato cells - use a table in a textbook to find the water potential of that solution
explain why % change in
mass is calculated in practical 2
-enables comparison / shows proportional change
-as plant tissue samples had different initial masses
explain why the potatoes
are blotted dry before
weighing in practical 2
-solution on surface will add to mass (only want to measure
water taken up or lost)
-amount of solution on cube varies (so ensure same amount of solution on outside)
explain increase in
mass in practical 2
-water moved into cells by osmosis
-as water potential of solution higher than inside cells
explain decrease in
mass in practical 2
-water moved out of cells by osmosis
-as water potential of solution lower than inside cells
explain no change in mass in practical 2
-no net gain/loss of water by osmosis
-as water potential of solution = water potential of cells
