5. osmosis and plant cells

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8 Terms

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WATER ENTERING PLANT CELL

  • water enters the cell by osmosis and the cell begins to swell

  • vacuole and cytoplasm enlarge until they press on the cell wall- inward pressure of the cell wall (pressure potential) increases until it allows no more water molecules to move in

  • cell is said to be at turgor

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WATER LEAVING FROM CELL

  • water leaves the cell by osmosis and turgor is lost

  • the cell membrane starts to pull away from the cell wall- incipient plasmolysis

  • if water continues to leave the cell the vacuole is reduced in size and protoplasm shrinks away from the cell wall completely- cell suffering from plasmolysis

  • measure incipient plasmolysis by using serial dilutions (the point where 50% of cells are plasmolysed)

  • cell shape will not change much either way because of the cell wall, only contents change

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TWO VARIABLES AFFECTING WATER POTENTIAL

  • osmotic potential (π)​

  • turgor pressure (P)

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OSMOTIC POTENTIAL

  • potential of water to move across a partially permeable membrane from a solution of low conc of solutes (hypotonic) to one of high conc of solutes (hypertonic)

  • the greater the conc of solutes, the lower (more negative) the osmotic potential

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TURGOR PRESSURE

  • the hydrostatic pressure generated as the cell contents push against the cell wall as a result of water entering by osmosis

  • this will rise until the osmotic force pulling water into the cell is balanced by the turgor pressure opposing further entry

  • at this balancing point:​ P = π

  • ​turgor pressure is a pos value and can be measured using a pressure probe

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WATER POTENTIAL EQUATION

  • the combined effect of osmotic potential pulling water INTO the cell and turgor pressure forcing OUT

  • when turgor pressure and osmotic potential are equal, the cell is at full turgor and the water potential of the cell is 0

  • if water potential is different between two regions that are separated by a partially permeable membrane, then water will move from the area of higher water potential to the area of lower water potential

<ul><li><p>the combined effect of osmotic potential pulling water <mark data-color="red" style="background-color: red; color: inherit">INTO </mark>the cell and turgor pressure forcing <mark data-color="red" style="background-color: red; color: inherit">OUT</mark></p></li></ul><p></p><ul><li><p>when <mark data-color="red" style="background-color: red; color: inherit">turgor pressure</mark> and <mark data-color="red" style="background-color: red; color: inherit">osmotic potential</mark> are equal, the cell is at <mark data-color="red" style="background-color: red; color: inherit">full turgor</mark> and the water potential of the cell is <mark data-color="red" style="background-color: red; color: inherit">0</mark></p></li></ul><p></p><ul><li><p>if water potential is different between two regions that are separated by a <mark data-color="red" style="background-color: red; color: inherit">partially permeable membrane</mark>, then water will move from the area of <mark data-color="red" style="background-color: red; color: inherit">higher water potential</mark> to the area of <mark data-color="red" style="background-color: red; color: inherit">lower water potential</mark></p></li></ul><p></p>
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MOVEMENT OF WATER IMAGE

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another osmosis definition.

  • Net movement of water molecules from a region of higher osmotic potential to a region of lower osmotic potential through a partially permeable membrane.