osmosis in onion cell

Date: 28/11/13
EVALUATION OF THE EFFECT OF SODIUM CHLORIDE ON ONION EPIDERMAL CELLS

INRODUCTION
A living plant cell will shrinks or swells depending on the solute concentration of the cell in relation to the solute concentration of the fluid surrounding the cell (1). It follows that water will move from a region of high water concentration to a region of low water concentration therefore, if a cell is placed in a hypertonic solution water will move from the cell into the solution until the cell shrinks(1). Further water loss causes can cause the cells protoplasm to peel away from the cell wall leaving a gap between the cell wall and the cell membrane, a process called plasmolysis(1) However if the cell is placed in a hypotonic solution, water will move from the solution into the cell and cause the cell to burst (1).
A cell in an isotonic solution neither swells nor shrinks because there is no concentration gradient for water across the cell membrane (1)
The objective of this experiment is to find out the effect distilled water, 3% sodium chloride solution and 5% sodium chloride solution have on onion epidermal cells.
From the above literature, its hypothesised that the cells in distilled water should maintain their shape, cells in 3% sodium chloride should shrink and the cells in 5% sodium chloride should experience more shrinking than that of the 3% solution.
The onion epidermal cell is transparent with a simple structure so it is a suitable cell for studying the effect of water loss on cells (2) Some specimens can be viewed directly underneath the microscope but putting a drop of water on the specimen can improve how the structures appear under microscope and also prevent the specimen from drying out on the slide (2). Adding water to the specimen is called wet mount.
The liquid used in wet mount fills the space between the slides to support the specimen and also allows light to pass through easily (3).
The liquid is usually water can also be a stain.
Stains are used in microscopy to highlight structures in biological structures for viewing under microscopes (3). There are many types of stains each is used for its own benefits, iodine, eosin are some of the common stains used in the microscopy (3). Iodine is used as a cell stain because it’s absorbed by cell organelles which are mostly transparent and turn them coloured making them more visible under the microscope (3).
It was found that the cells which were placed in 3% solution increased in size contrary to the hypothesis. The cells placed in 5% solution shrank and cells in placed in normal saline kept their normal shape.

MATERIALS
Glass slide
Cover slip
Onion tissue
Dropper
Light miscrope
Iodine
Sodium chloride (NaCl)
Distilled water
Tweezers
Measuring tube
Bunsen tube
Pipette
Blotting paper
Stage micrometer

METHOD
Lay down three different slides labelling the three different solution you intend to use.
Peel a thin layer of tissues (epidermis) using the forceps (tweezers) from the whole onion.
Put the epidermis on the center of each of the slide displayed and labelled.
Drop a few drops of each of the labelled solutions onto the onion membrane, making sure the tissues is flattened and not wrinkled on the slide.
Wait for 3 minutes before putting a cover slip on the slide.
Put the cover slip on the stained tissue and the different solutions involve.
Gently tap out any air bubbles and try to soak out any extra solution around the covered tissues.
Adjust the diaphragm so that the right amount of light is shining on the slide stage.
Put the slide onto the microscope stage and fix with the stage clips
Observe the tissues under the microscope under low, medium and high powers. Also making sketches, writing down observation.
Making sure of an eye piece graticule and stage micrometer scale to measure cells and units.
Be sure to write what magnification for each sketch.
Make sure you label any organelles you see. Cell wall, membrane, nucleus and cytoplasm
Calculate magnification of drawings and actual sizes of specimens

DATA
The eye piece graticule remains constant no matter what magnification the epidermis are looked at.

RESULTS

DISCUSSIONS

CONLUSION

References
1. Sperelakis, Nicholas (2011). Cell Physiology Source Book: Essentials of Membrane Biophysics. Academic Press. p. 288. ISBN 978-0-12-387738-3.
2. Horobin RW, Kiernan JA (2002) Conn 's Biological Stains. A Handbook of Dyes Stains and Fluorochromes for Use in Biology and Medicine. 10th ed. Oxford: BIOS. ISBN 1-85996-099-5
3. Clark G (1981) Staining Procedures, 4th ed., Baltimore: Williams & Wilkins, p. 412, ISBN 0683017071.

References: 1. Sperelakis, Nicholas (2011). Cell Physiology Source Book: Essentials of Membrane Biophysics. Academic Press. p. 288. ISBN 978-0-12-387738-3. 2. Horobin RW, Kiernan JA (2002) Conn 's Biological Stains. A Handbook of Dyes Stains and Fluorochromes for Use in Biology and Medicine. 10th ed. Oxford: BIOS. ISBN 1-85996-099-5 3. Clark G (1981) Staining Procedures, 4th ed., Baltimore: Williams & Wilkins, p. 412, ISBN 0683017071.