Transport Processes Ex

To investigate the effect of distilled water and 1 mol dm-3 sucrose solution on the plant tissue (leaf petiole / leaf stalk), two plant specimens were cut and secured horizontally to the base of the Petri-dishes with plasticine as shown in the diagrams below:


Distilled water is poured into a Petri-dish and sucrose solution into the other. The dishes are completely covered and left for 10 minutes.

(a) Write down the expected observations of the above investigation. [2]

(b) Account fully for the expected observations in (a), relating the behaviour of the pieces of tissue to the nature of the liquid bathing them and to the differences in structure of the cells. [8]

(c) Explain briefly how you could extend the investigation procedure to obtain an estimate of the water potential of this plant tissue. [4]

ANSWERS
(a) Specimen in water: curved more outward / distance between the cut end increased;
Specimen in sucrose solution: curved inward / distance decreased;

(b) In water: Distilled water has a higher water potential than cell sap, water potential gradient is established, water enters into vacuoles of cells by endosmosis;
Vacuoles become larger, push cytoplasm against the cell wall, causing cells to swell and become turgid;
More water enters into cortex and pith cells compared to epidermis, due to presence of the waterproof cuticle surrounding the epidermis;
Cells in cortex and pith swell more than epidermal cells and causes strip to bend outwards;

In sucrose solution: Sucrose solution has a lower water potential than cell sap, water potential gradient is established, water leaves vacuoles of cells by exosmosis;
Vacuoles shrink in size, cytoplasm shrinks away from cell wall, cells become flaccid;
More water leaves the cortex and pith cells compared to epidermis, due to presence of cuticle;
Cells in cortex and pith shrink more than epidermal cells and causes strip to bend inwards;

(c) Use a range of different conc. of sucrose solution, quote at least 5 values eg: 0, 20, 40, 60, 80, 100 g/dm3 sucrose solution
Measure value A just after cutting, and then 30 minutes into immersing the strips, use at least 5 strips per concentration;
Calculate percentage change of A ([Final A- Initial A]/Initial A) and plot a graph of average percentage change of A (y axis, responding variable) against concentration of sucrose solution (x axis, controlled variable);
Concentration of sucrose solution that would produce zero percentage change (where curve crosses the x axis) will have water potential that is equal to water potential of the plant tissue;