Cells Practicals

Question 1

aim of required practical 4

Answer 1

-investigation into the effect of a named variable
on the permeability of cell-surface membranes

Question 2

describe a method to investigate the effect of a named variable (eg.
temperature) on the permeability of cell-surface membranes

Answer 2

1. cut equal sized / identical cubes of plant tissue (eg. beetroot) of same age / type using a scalpel
2. rinse to remove pigment released during cutting or blot on paper towel
3. add same number of cubes to 5 different test tubes containing same volume of water (eg. 5 cm3)
4. place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50oC)
5. leave for same length of time (eg. 20 minutes)
6. remove plant tissue and measure pigment release by measuring intensity of colour or concentration of surrounding solution semi-quantitatively / quantitatively

Question 3

describe a semi-quantitative method to estimate pigment concentration in a solution

Answer 3

-use a known concentration of extract and distilled water to prepare a dilution series
-compare results with these ‘colour standards’ to estimate concentration

Question 4

describe a quantitative method to estimate pigment concentration in a solution

Answer 4

-use a known concentration of extract and distilled water to prepare a dilution series
-compare results with these ‘colour standards’ to estimate concentration

Question 5

explain why the beetroot is washed before placing it in water

Answer 5

-wash off any pigment on surface
-to show that release is only due to [named variable]

Question 6

explain why each test tube containing cubes of plant tissue is regularly shaken

Answer 6

-to ensure all surfaces of cubes remain in contact with liquid
-to maintain a concentration gradient for diffusion

Question 7

explain why the volume of water needs to be controlled

Answer 7

-too much water would dilute the pigment so solution will
appear lighter / more light passes through in colorimeter
than expected
-so results are comparable

Question 8

explain how you could ensure beetroot cylinders were kept at the same temperature throughout the
experiment

Answer 8

-take readings in intervals throughout experiment of
temperature in tube using a digital thermometer /
temperature sensor
-use corrective measure if temperature has fluctuated

Question 9

describe the issues with comparing to a colour standard

Answer 9

-matching to colour standards is subjective
-colour obtained may not match any of colour standards

Question 10

what does a high absorbance suggest about the cell-membrane

Answer 10

-more permeable / damaged
-as more pigment leaks out making surrounding solution more concentrated (darker)

Question 11

explain how temperature affects permeability of cell-surface membranes

Answer 11

-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

Question 12

explain how pH affects permeability of cell-surface membranes

Answer 12

-high or low pH increases cell membrane permeability
○ transport proteins denature as H / ionic bonds break, changing tertiary structure

Question 13

explain how lipid-soluble solvents eg. alcohol affect permeability of cell-surface membranes

Answer 13

-as concentration increases, cell membrane permeability increases
-ethanol (a lipid-soluble solvent) may dissolve phospholipid bilayer (creating gaps)