Introduction
Osmosis is the process by which water molecules move from a region of low concentration of solute to a region of high concentration of solute through a semipermeable membrane (Van 3). Osmosis therefore can define the movement of water in and out of the cytoplasm of a cell. A solution with a higher concentration of solute molecules than that of the cell cytoplasm is said to be hypertonic to the cell. On the other hand, a solution with a lower concentration of solute than that of the cell cytoplasm is said to be hypotonic to the cell.
Purpose: The purpose of this experiment is to establish the reaction of the onion cells when placed in a hypertonic solution.
Hypothesis: When onion is placed in a hypertonic solution, water molecules will move from the cytoplasm of the onion cell to the solution making the cell membrane of the onion to detach from the cell wall and shrinks inwards towards the nucleus.
The Materials Used:
Purple onion sections
Forceps
Microscope slides
Cover slips
30% concentrated NaCl solution
Procedure
Peal a thin purple layer of the onion tissue.
Place the layer on the slide
Do not put drops of water yet
Place the slide on to the stage of the microscope
Using the revolving nose-piece, direct the low power objective lens to the stage
Using the low power objective lens, scan over the tissue until you spot the centre of the cell, which is purple than the other side.
Using the revolving nosepiece, direct the medium objective lens on to the stage
Use the coarse and fine adjustment knob to focus the view of the tissue on the stage
Observe and draw the cell as shown
Remove the slide and add three drops of concentrated sodium chloride
Leave it for five minutes
Place the slide back on the stage, examine the changes and draw again.
Wait for another five minutes and observe and draw again.
Result:
The diagram below show changes observable changes that took place on the onion cell. The first drawing shows how the cell was before placing concentrated sodium chloride. The next drawing shows how it was after five minutes and the last diagram shows how it was after another five minutes. It was observed that; initially, the purple pigment had a definite shape adjacent to the cell wall. When the concentrated sodium chloride was added, the purple pigment starts to reduce in size, retreat backwards towards the centre of the cytoplasm. After ten minutes, the purple pigment had detached completely from the cell wall and reduces inside at the centre of the cytoplasm.
Diagram 1: Changes on the onion cell when placed in a hypertonic solution (Lab Report)
Discussion
When the cell is first placed on the slide without adding any solution, there is no movement of water in and out the cell. When sodium chloride is added, the solution becomes hypertonic to the cell. Water molecules therefore start to diffuse out of the cell. As a result, the cell becomes flaccid and the cell membrane starts to shrink. Due to the presence of cellulous cell-wall, only the membrane will shrink in size leaving the cell wall in its intact position. After some time the cell membrane will detach completely from the cell wall.
Work Sited
Van Sandt, Vicky ST. "Xyloglucan endotransglucosylase activity loosens a plant cell wall." Annals of Botany 100.7 (2007): 1467-1473.
