rp4 -> Investigation into the effect of a named variable on the permeability of cell-surface membranes.

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. 5cm3)

• Place each test tube in a water bath at a different temperature (eg. 10, 20, 30, 40, 50 oC)

• Leave for same amount of time (eg. 20 mins)

Remove beetroot and measure intensity of colour of surrounding solution:

● Semi-quantitatively:

○ Use a known conc. of extract & distilled water to prepare a dilution series (colour standards)

○ Compare results with colour standards to estimate conc.

● Quantitatively:

○ Measure absorbance (of light) of known concentrations using a colorimeter

○ Draw a calibration curve → plot a graph of absorbance (y) against conc. of extract (x) and draw a line / curve of best fit

○ Absorbance value for sample read off calibration curve to find associated extract conc.

What are the issues with comparing to a

colour standard?

• subjective

● Colour obtained may not match any of colour standards

Why wash the beetroot before placing it

in water?

● Wash off any pigment on surface

● To show that release is only due to [named variable]

Why regularly shake each test tube

containing cubes of plant tissue?

● To ensure all surfaces of cubes remain in contact with liquid

● To maintain a concentration gradient for diffusion

Why control the volume of water?

● Too much water would dilute the pigment so solution will appear lighter / more light passes through in colorimeter than expected

● So results are comparable

How could you 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

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, permeability increases

○ Phospholipids gain kinetic energy and fluidity increases

○ Transport proteins denature at high temperatures as H bonds break, changing tertiary structure

● At very low temperatures, 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 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