Solutions of ions, for example NaCl dissolved in water, can be used to investigate the concentration of solutes in plant tissues. After immersion in solutions of varying concentration, the percentage changes in mass of potato samples were measured. The graph shows the results.

(i) Estimate the osmolarity of the plant tissue.

 .......................moles dm^–3

(ii) Identify which part of the graph represents samples measured in a hypotonic solution.

(iii) State one possible source of error when collecting data during this experiment.

Ions move across the plasma membrane of a neuron during an action potential. The oscilloscope trace shows voltage changes generated in a neuron during three action potentials.

Explain the movement of ions which causes the voltage changes observed during the interval labelled X on the graph.

(i)

0.28 «moles dm^–3» (Allow answers in the range of 0.27 to 0.29 «moles dm^–3»)

0.56 «osmoles dm^–3» (Allow answers in the range of 0.55 to 0.57 «osmoles dm^–3»)

(ii)

«Any» part of the line above 0 percent change in mass (Allow ECF for upper value of molarity.)

0 to 0.28 molarity of NaCl solution

(iii)

Too few samples weighed

Not dried before weighing

Samples from different sources

Not cut all same way so different surface area

Temperature of each sample not the same

Potatoes not left for the same time in the solutions

Error due to the limitation of the apparatus/equipment

Differentiate between errors and mistakes eg: do not accept “balance read incorrectly”

Do not accept mass/weight differences

At the peak the sodium channels close

The potassium channels open

Potassium ions flow out

Repolarization occurs

Delay in closing of potassium channels

Hyperpolarization results

Sodium and potassium pump re-starts to restore ions to resting/previous potentials/concentrations

Accept Na⁺ and K⁺ ions

Award [2 max] if answer refers to part of graph before X

(i) and (ii): Most candidates could estimate the osmolarity of the plant tissue though fewer could identify the samples in hypotonic solution. Credit was given to candidates whose answers considered the dissociation of sodium chloride in water.
(iii) Candidates had difficulty differentiating between a source of error and a mistake through carelessness or improper technique.

Discriminated well with only good candidates scoring full marks. Many referred to ion movement before the interval labelled X on the graph.