Demonstrate Osmosis using Banana Peel

A straight portion of a fresh, raw banana fruit, at least 6cm long, labelled specimen D, Two Petri dishes, a scalpel blade, 2 glass beakers containing liquids E and F (about 40 cm3 of distilled water in a beaker, labelled “liquid E“ and about 30 cm3of concentrated glucose solution, in a beaker, labelled “liquid F“, A glass measuring cylinder, A stop watch and Means of labelling (2 labels)

(i) Label the two Petri dishes E and F

(ii) Place 30cm3 of liquid E into Petri dish E and 30cm3 of liquid F into Petri dish F

(iii) Using the scalpel, p rep are four thin, straight, flat strip s from the raw banana fruit peel

(iv) Each strip should measure about 4cm by 2mm as illustrated below:

Note: To get a straight, flat, thin strip, remove all the banana flesh (endocarp), leaving only the peel (epicarp- the green part +mesocarp- the white part that bears thread-like structures)

(v) Immerse two strip s in Petri dish E and the other two in Petri dish F and leave the set-ups undisturbed for 10 minutes

(a) (i) State your observations in Petri dishes E and F after 20 minutes

Petri dish E

• Strips curved outwards; /green part on the inner
• The strips were hard/ rigid/ firm;

Petri dish F

• Strips curved inwards; (green part on the outer side)
• The strips were soft/ flabby/ flexible;

(ii) Account for the observations mad e in (a)(i)

Petri dish E

• Inner/mesocarp cells draw in water molecules by osmosis hence become more turgid/longer than the outer/ epicarp cells;

Petri dish F

• Inner/mesocarp cells lose water molecules by osmosis hence become more flaccid/shorter than the outer/epicarp cells;

(b) Describe the nature of liquids E and F in relation to the cell sap of the cells in the banana peel used in the experiment

Petri dish E

• Hypotonic;

Petri dish F

• Hypertonic

(c) With reference to the observations made, compare the nature of the outer (epicarp) and inner (mesocarp) surfaces of the banana peel

• Outer surface/ Epicarp: Impermeable/ Less permeable/ Waterproof
• Inner surface/ Mesocarp: Permeable;

(d) (i) Name the cell structure responsible for the observations mad e in this experiment

• Cell membrane/ Plasma membrane

(ii) Explain how the cell structure named in (d)(i) above works to bring about the observations made

• Semipermeable/Allows selective movement of only some materials through it; since it has(small) pores

Demonstrate Osmosis using Dandelion Stem section

A freshly obtained dandelion stem measuring 5 cm long was split lengthwise to obtain two similar pieces. The pieces were placed in two different solutions of different concentrations in petri dishes (L₁ and L₂) for 20 minutes. The appearance after 20 minutes is as shown.

(a) Account for the appearance of the pieces in solutions L₁ and L₂ (6 Marks)

L₁

• Inner (cortex) cells draw in water by osmosis, hence (turgidity) increased in length; epidermal cells did not gain water because they are covered by a waterproof cuticle leading to curvature.

L₂

• Inner (cortex) cells lost water by osmosis, leading to (flaccidity) decrease in length; epidermal cells did not lose water because they are covered by a waterproof cuticle leading to curvature.

(b) State the significance of the biological process involved in the experiment. (2 marks)

• Absorption of water by the roots
• Opening and closing of the stomata.

Demonstrate Osmosis using Stem Stoppered bottle

In a certain experiment, a plastic bottle full of water was stoppered using a piece of stem obtained from a young maize plant whose epidermis had been peeled off. Initially the stopper was loose but after 24 hours, it was observed that the stopper closed the bottle tightly.

(a) (i) Name the physiological process that was being demonstrated in the above experiment. (1 mark)

• Osmosis;

(ii) Account for the observation made after 24 hours. (2 marks)

• The cell sap of the cells of the stem are hypertonic; to the water in the bottle. The cells therefore draw in water by osmosis; become turgid; hence stoppered the bottle tightly.

(b) The above experiment was repeated using a plastic bottle full of honey. Suggest the likely observation that was made after 24 hours. (1 mark)

• The stopper would be loose;