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Chapter 13 Diffusion & Osmosis
A Selectively Permeable Membrane allows
some but not all molecules to pass through e.g.
the cell membrane in plants/animals or visking
tubing (artificial membrane).
If the salt molecules cannot move the water molecules have to move instead
until the water molecules have the same concentration on both sides of the
membrane.
Osmosis in Plant Cells – Turgor and Plasmolysis
Plant cell wall is fully permeable. The cell membrane, cytoplasm and
vacuole membrane together act as a selectively permeable barrier.
The vacuole contains a solution of sugars and salts known as cell
sap.
Turgor is the pressure of the cytoplasm and vacuole against the cell
wall.
If a plant cell is placed in a beaker of water,
the water passes into the cell by osmosis.
The vacuole of the cell now exerts an
outward pressure called turgor pressure.
The cell is prevented from
bursting by an inward pressure
exerted by the inflexible cell wall.
The cell wall is fully stretched and the cell is said to
be turgid.
If a plant cell is placed in a beaker of
concentrated sugar solution (higher conc. of water
molecules in the vacuole than in the conc. Sugar
solution in the beaker.
Water from the vacuole moves out of the cell by
osmosis into the beaker. The vacuole inside the cell
shrinks and this pulls the cytoplasm away from the
cell wall. Turgor pressure is lost and the cell
becomes flacid or flabby.
Loss of water from the vacuole and the pulling away
of the cytoplasm from the cell wall is called
plasmolysis and this causes wilting in plants.
Active Transport
Diffusion is a passive process and does not require energy.
Osmosis is a passive process and does not require energy.
Sometimes molecules e.g. salts like potassium are actively moved
against the concentration gradient (i.e. they are forced from an area
of low concentration to an area of high concentration). This requires
energy from the breakdown of ATP (adenosine tri-phosphate) and is
called active transport.