Observing Plasmolysis in Onion Cells (Allium cepa)

OBSERVING PLASMOLYSIS IN ONION CELLS (_ALLIUM CEPA_)

I. INTRODUCTION

Plasmolysis is the process in plant cells in which the cytoplasm is separated from the cell wall as a result of water loss through osmosis.

Osmosis is a type of passive transport involving movement of water molecules across a partially permeable membrane.

1. RESEARCH QUESTION

How does 15% NaCl solution affect the appearance of onion cells?

2. HYPOTHESIS

If onion cells are submerged in solutions of NaCl of different concentrations, then cells will lose water and the plasma membrane will shrink. This is because the process of osmosis will take place. Because of a difference between solute concentrations on either side of a partially permeable membrane, the movement of water molecules will occur.

II. MATERIALS AND METHODS

1. MATERIALS

Light microscope

Slides

Cover slips

Small beakers (50 ± 5 ml)

Forceps

Spatula

Scalpel

Weighing scale (± 0.1 g)

Automatic pipette (100-1000 ±50 µm)

15% NaCl solution, 10 ml

Tap water

Red onion

Yellow onion

2. METHOD

a. Obtain a small piece of the yellow onion using a scalpel.

b. Peel off the outer layer cells using forceps.

c. Mount the tissue in a few drops of tap water.

d. Set up the light microscope and observe the cells in 10 times magnification.

e. Draw the few of these cells seen in distilled water.

f. Weigh 1,5 g of NaCl using the spatula and weighing scale.

g. Add some water to the beaker and make sure that all substance dissolves in water.

h. Add distilled water up to the line 10 ml.

i. Obtain small pieces of both yellow and red onion.

j. Peel off the outer layer cells using forceps.

k. Set up two more slides, this time using a few drops of 15% NaCl solution.

l. Observe the cells of both red and yellow onion in 10 times magnification using the light microscope.

m. Draw the few of these cells seen in the NaCl solution.

III. RESULTS

1. YELLOW ONION CELLS IN TAP WATER.

2. YELLOW ONION CELLS IN 15% NACL SOLUTION.

3. RED ONION CELLS IN 15% NACL SOLUTION.

IV. CONCLUSION

Yellow onion cells in tap water are fully turgid. The cells swell up with water and the cell surface membrane is pushed onto the cell wall.

Yellow onion cells in 15% NaCl solution are partly plasmolysed. The plasma membranes and the cell walls are seen as separate structures.

Red onion cells in 15% NaCl solution are partly plasmolysed. The plasma membranes and the cell walls are seen as separate structures. The cells are filled with a red pigment forming regions of a darker colour.

A cell with a rigid cell wall loses water when mounted in a hyperosmotic solution. The process of plasmolysis takes place. The cytoplasm is separated from the cell wall as a result of water loss through osmosis.

The appearance of onion cells submerged in solutions of NaCl of different concentrations suggest that the rate of plasmolysis is dependent on a concentration. When plant cells are placed in a concentrated solution, the process of plasmolysis occurs. The greater the concentration, the more plasmolysed cells are.

I found in literature (E. P. Solomon, L. R. Berg, D. W. Martin. Biology, Seventh edition. Warsaw. 2005. Page: 105. Published by Brooks Cole, a Cengage Learning Company) that, "Plasmolysis is the process in plant cells in which the cytoplasm is separated from the cell wall as a result of water loss through osmosis. The process of plasmolysis occurs when there are a lot of salts in soils and water, for example due to the presence of fertilisers." Taking into account the outcomes, the statement seem to be verified.

It can be concluded from the results that the hypothesis is supported.

V. EVALUATION

1. EVALUATING ERRORS

TYPE OF ERROR

EXAMPLE

SOLUTION

RANDOM ERRORS

Inconsistency in measured values. The masses of NaCl differed each time they were measured using the same weighing scale.

Recording more data. Using techniques of statistical analysis the averages could be calculated.

SYSTEMATIC ERRORS

Improper calibration of the weighing scale - zero error.

Precise calibration of the weighing scale.

2. IMPROVEMENTS

LIMITATIONS

MODIFICATIONS

Different stages of the plants ' ripeness.

Using plants at the same stages of ripeness.

Poor accuracy of the weighing scale. Preparing precise NaCl solutions was hindered.

Implementation of a higher quality weighing scale, for example a microbalance with readability to 0.001 grams.

The results could be biased owing to the fact that each group used different kinds of plants.

Introducing limitations to the kinds of plants.

VI. CITED REFERENCES

1. BOOKS

E. P. Solomon, L. R. Berg, D. W. Martin. Biology, Seventh edition. Published by Brooks Cole, a Cengage Learning Company, 2005. Pages: 105-106.

A. Damon, R. McGonegal, P. Tosto, W. Ward. Biology Higher Level. Pearson Baccalaureate, 2007. Page: 34.

References: 1. BOOKS E. P. Solomon, L. R. Berg, D. W. Martin. Biology, Seventh edition. Published by Brooks Cole, a Cengage Learning Company, 2005. Pages: 105-106. A. Damon, R. McGonegal, P. Tosto, W. Ward. Biology Higher Level. Pearson Baccalaureate, 2007. Page: 34.