Membrane Permeability

Topics: Osmosis, Cell, Cell wall Pages: 5 (1476 words) Published: August 9, 2012
Experiment 5
Title: Membrane Permeability
1.Define solvent, solute, solution, selectively permeable, diffusion, osmosis, concentration gradient, equilibrium, turgid, plasmolyzed, plasmolysis, turgor pressure, tonicity, hypertonic, isotonic, hypotonic; 2.Describe the effects of hypertonic, isotonic, and hypotonic solutions on Elodea leaf cells and onion scale leafs.

Membrane permeability is a quality of a cell’s plasma membrane that allows substances to pass in and out of the cell, so that the cell can expel waste products and ship out the chemicals it assembles for the body. At the same time, the nutrients that the cell needs can pass through the membrane to the inside. Cell membranes have selective permeability, meaning that the membrane will allow certain substances to pass while forming a barrier against others. Cells are like microscopic factories: they design, produce, and package the substances the body needs to survive each day. Just like a factory, a cell needs a way to bring the raw materials for its products — such as nutrients from food — inside its workshop. Once the cell has assembled and packaged a substance, the cell needs a way to ship the finished product out into the bloodstream so that the body can make use of it. The cell membrane is a flexible plasma that envelopes the exterior boundaries of the cell. It separates the intracellular fluid — the fluid within the cells — from the extracellular fluid, which is the fluid outside of the cells. The cell membrane is not a passive or insurmountable wall however, as there is a constant and dynamic exchange of substances between the two fluids. If a plant cell is placed in a hypertonic solution, the plant cell loses water and hence turgor pressure, making the plant cell flaccid. Plants with cells in this condition wilt. Further water loss causes plasmolysis: pressure decreases to the point where the protoplasm of the cell peels away from the cell wall, leaving gaps between the cell wall and the membrane. Eventually cytorrhysis – the complete collapse of the cell wall – can occur. There are some mechanisms in plants to prevent excess water loss in the same way as excess water gain, but plasmolysis can be reversed if the cell is placed in a weaker solution (hypotonic solution). Stomata help keep water in the plant so it does not dry out. Wax also keeps water in the plant. The equivalent process in animal cells is called crenation. The liquid content of the cell leaks out due to diffusion. The cell collapse and cell membrane pulls away from the cell wall (in plants). Most animal cells consist of only a phospholipid bilayer and not a cell wall, therefore shrinking up under such conditions. Materials: Refer to lab manual

Methods: Refer to lab manual
Elodea leaf in distilled waterThe cells appear turgid. The chloroplasts are pushed to the peripheral of the cell. Elodea leaf in 20% NaCl solutionThe cell membrane pulls away from the cell wall. The chloroplasts are pushed to the centre of the cell. Elodea leaf in 20% NaCl solution then added with distilled waterThe cells appear turgid. The chloroplasts are pushed to the peripheral of the cell.

When Elodea leaf is placed in distilled water, the solute concentration of cell is higher than water, therefore water diffused into the cell via osmosis. This causes the cell to appear turgid and chloroplasts are pushed to the side of the vacuole due to high turgor pressure. The Elodea cell did not burst or undergoes lysis because it contains cell wall which will prevent cell from bursting due to high turgor pressure. When 20% NaCl solution is added, the cell membrane shrunk and the chloroplasts were centralized in the middle of the cell. This is because water diffuse out from cell via osmosis. This process is called as plamolysis of cell, the cell becomes flaccid but its shape will not change because of strong cell wall. The Elodea leaf...
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