Required practical 4

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

How should plant tissue cubes be prepared at the start of an investigation into the effect of temperature on membrane permeability?

• Use a scalpel to cut plant tissue into identical cubes.

What should be done to the plant tissue cubes after cutting, and why?

• Rinse the cubes to remove pigment released during cutting.

How should the plant tissue cubes be added to test tubes, and what volume of water should be used?

• Immerse the same number of cubes in each test tube.

• Each test tube should contain the same volume of water.

At what temperatures should the test tubes be placed, and for how long?

• Place each test tube in a water bath at a different temperature, for example 10, 20, 30, 40, and 50°C.

• Leave them for the same length of time.

After removing the plant tissue, what should be measured to determine the effect of temperature on membrane permeability?

• Measure pigment release by measuring the intensity of colour or the concentration of the surrounding solution.

• This can be done either semi-quantitatively or quantitatively.

How can pigment concentration be estimated semi-quantitatively?

• Use a known concentration of extract and distilled water to prepare a dilution series of known concentrations.

• Compare the results of the experiment with these colour standards to estimate the concentration.

How can pigment concentration be estimated quantitatively using a colorimeter?

• Measure the absorbance of light of known concentrations using a colorimeter.

How is a calibration curve created for quantitative pigment concentration measurement?

• Draw a calibration curve by plotting a graph of absorbance (y-axis) against concentration of extract (x-axis).

• Draw a line or curve of best fit.

How is the concentration of an unknown sample determined from a calibration curve?

• Read the absorbance value of the unknown sample.

• Find the associated concentration by locating this absorbance value on the calibration curve.

Why is the beetroot washed before placing it in water? (First reason)

• To wash off any pigment on the surface.

Why is the beetroot washed before placing it in water? (Second reason)

• To show that any pigment release is only due to the named variable (for example, temperature).

Why should each test tube containing cubes of plant tissue be regularly shaken? (First reason)

• To ensure that all surfaces of the cubes are exposed (in contact with the liquid).

Why should each test tube containing cubes of plant tissue be regularly shaken? (Second reason)

• To maintain a concentration gradient for diffusion.

Why does the volume of water need to be controlled? (Effect of too much water)

• Too much water would dilute the pigment, so the solution will appear lighter or more light will pass through in the colorimeter than expected.

Why does the volume of water need to be controlled? (Reason for control)

• Controlling the volume ensures that results are comparable.

How could you ensure beetroot cylinders were kept at the same temperature throughout the experiment? (Method of monitoring)

• Take temperature readings at intervals throughout the experiment.

• Use a digital thermometer or a temperature sensor.

How could you ensure beetroot cylinders were kept at the same temperature throughout the experiment? (Corrective action)

• Use a corrective measure if the temperature has fluctuated.

What is one issue with comparing results to a colour standard?

• Matching to colour standards is subjective.

What is another issue with comparing results to a colour standard?

• The colour obtained may not match any of the colour standards.

What does a high absorbance reading suggest about the state of cell-surface membranes?

• The membranes are more permeable.

• This is because more pigment leaks out, making the surrounding solution more concentrated and darker.

How does an increase in temperature affect the permeability of cell-surface membranes, and what happens to the phospholipids?

• As temperature increases, cell membrane permeability increases.

• Phospholipids gain kinetic energy, so their fluidity increases.

How do high temperatures affect transport proteins in cell-surface membranes?

• Transport proteins denature at high temperatures.

• This is because hydrogen bonds break, changing their tertiary structure.

How do very low temperatures affect the permeability of cell-surface membranes, and why?

• At very low temperatures, cell membrane permeability increases.

• Ice crystals can form, which pierce the cell membrane and increase permeability.

How do high or low pH levels affect the permeability of cell-surface membranes?

• High or low pH increases cell membrane permeability.

Why do high or low pH levels increase the permeability of cell-surface membranes?

• Transport proteins denature as hydrogen or ionic bonds break, changing their tertiary structure.

How does the concentration of lipid-soluble solvents such as alcohol affect the permeability of cell-surface membranes?

• As the concentration of a lipid-soluble solvent increases, cell membrane permeability increases.
  

How does ethanol, a lipid-soluble solvent, increase the permeability of cell-surface membranes?

• Ethanol may dissolve the phospholipid bilayer, creating gaps.