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Osmosis In Onion Cell
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The aim of the sixteenth of November experiments was to observe how three different solutions with various sucrose concentration influenced osmosis in relation to three onion cells and the impact on the cells structure.
A small square of a red onion skin (membrane) was observed under a microscope at high power (X40) magnification. The observation showed a large number of onion cells. The structure of one onion cell had a general rectangular shape with a developed cell wall, which gives the rectangular shape to the cell and a cell membrane just beneath it.
The observation under the microscope of a cell of an onion skin soaked for 15 minutes in distilled water showed that the cell membrane was pressed against the cell wall. The cell swelled because of the increased amount of distilled water but the cell wall kept the same rectangular shape. See diagram 1 for sketch. The distilled water moved from outside the cell wall into the cell across the permeable cell wall, through the selectively permeable cell membrane into the vacuoles by osmosis in response to differences in water potential. The water molecules moved down concentration gradient from an area of higher to lower water potential. The water absorption caused the cell membrane to move towards the cell wall and to be pushed against the cell wall. The distilled water moved from an area of high solvent concentration to an area of low solvent concentration. The water was diffused from a hypotonic solution to a hypertonic solution. Because of the amount of water in the vacuoles, the cell became turgid.
The observation under the microscope of a cell of an onion skin soaked for 15 minutes in 1 molar sucrose solution showed that the cell membrane shrunk away from the cell wall in response of the hypertonic environment. The rigidity of the cell wall protects it from collapsing and therefore the onion cell wall kept the same rectangular shape. See diagram 2 for sketch. The water moved from a higher concentration area to a lower concentration area through the selectively permeable cell membrane. Because of its hypertonic environment and the permeable cell wall, the cell membrane was surrounded by the 1 molar sucrose solution and therefore caused a net movement of water molecules to flow out of the cell by osmosis in response to differences in water potential. The lost of water from the vacuoles and cytoplasm pulled the cell membrane away from the cell wall. The water was diffused from a hypotonic solution to a hypertonic solution. Because the water flowed out of the cell by osmosis, the cell became more flaccid and then plasmolysed.
The observation under the microscope of a cell of an onion skin soaked for 15 minutes in a 0.5 molar sucrose solution showed a cell membrane just beneath the cell wall. The cell wall had a rectangular shape. See diagram 3 for sketch. The cell and its surrounding were in an isotonic solution. The two solutions in the cell and out (0.5 molar sucrose solution) of the cell were homogenous. No net movement of water and change in the cell structure was observed. The components of the solute and solvent were evenly intermixed. The concentration of solute and solvent on either side of the cell membrane was equalized. Because the onion tissue didnÐ²Ð‚â„¢t get any water the cell was flaccid and nonturgid.
Those three experiments showed that the way onion cells are dealing with the movement of water in and out of the cell is by osmosis. That Osmosis is the diffusion of water across a membrane into a solution having a greater solute concentration. The cell wall is permeable and the cell membrane is selectively-permeable. In order for the cells not to plasmolysed and wilt the cells need to be in a hypotonic surrounding with turgid cells. Regardless of the amount of water entering and store in the cell, the cell wall keeps the same rectangular shape and prevents the cell from expanding.