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    Water, Osmosis, Cellular Function, and Turgor Pressure

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    Review physical and chemical properties of water. How might changes in temperature affect cellular function? How might water pollution affect cellular function? How might it affect water movement in cells and plants?


    What are roles of osmotic potential or turgor pressure in regulating water potential of plant cells? Explain your answer. Do plant cells have a negative turgor pressure? Why or why not?

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    Key Terms
    1) Hypertonic - Having a higher osmotic pressure than another fluid.
    2) Hypotonic - A solution with a lower osmotic pressure than another fluid.
    3) Isotonic - Having equal osmotic pressure.
    4) Osmotic potential ("Pi") - Measure of the tendency of water to move into a region due to the presence of solutes.
    5) Pressure potential (P) - Measure of the physical pressure exerted by cell walls upon a cell.
    6) Water potential ("Psi") - Measure of the overall tendency of water to move into a region.

    Plant cells are surrounded by rigid cellulose walls and take in water by osmosis when placed in pure water. However, plant cells do not burst because their cellulose cell walls limit how much water can move in. The cell walls exert pressure, called turgor pressure, as the cells take up water. These physical forces can be described by a simple mathematical equation:

    Psi = P + Pi,

    where ("Psi") = Water potential, a measure of the overall tendency of water to move into a cell;
    P = Pressure potential, a measure of the turgor pressure exerted by the cell walls;
    Pi = Osmotic potential. Water always moves from regions of higher Psi to areas of lower Psi. Animal cells do not have cellulose cell walls, so P = 0 and Psi= Pi for these cells.

    The significance of osmosis to plant function is best described by its role in the regulation of guard cells. Guard cells are specialized cells scattered across the surface of plant leaves. Each pair of guard cells surround a special pore, termed a stoma (plural stomata), and control its opening. Guard cells have a special arrangement of microfibrils in their walls, so that when the guard cells swell the stomata open. When the stomata of a plant leaf are open, this increases photosynthetic gas exchange and movement of water out of the plant by transpiration.
    In many plants, certain environmental stimuli, such as sunlight, stimulate the guard cells to take up potassium from surrounding cells. This causes their osmotic potential (Pi) to decrease and water moves in by osmosis. Thus, the guard cells swell, the stomata open, and the rate of gas exchange through the stomata increases. This increases the rate of photosynthesis and plant growth.

    Other environmental factors such as water shortage, cause plants to transmit chemical signals to the guard cells, causing them to release potassium, which increases their osmotic potential, and to lose water by osmosis. This causes the guard cells to shrink, so closing the stomata, and decreasing the rate of water transpiration through the stomata. This reduces water loss and prevents wilting of the plant.
    Plants rely upon other environmental signals to regulate the osmotic movement of water into their guard cells and the opening of the stomata, so that the advantage of increased photosynthesis is balanced against the disadvantage of increased water loss.

    Negative turgor pressure remains little-studied area of plant water relations. While it does not appear to form in cells such as the leaf mesophyll, that can deform easily, it exists in living cells with either lignified cell walls or where the cell is embedded in a matrix of lignified tissues . Advantages of negative turgor pressures in preventing mechanical and osmotic damage associated with severe desiccation make it worthy of further research.

    Reference- http://5e.plantphys.net/article.php?ch=t&id=387.

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