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INTRODUCTION TO DIFFUSION
Atoms molecules and ions always move
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In solid substances the molecules can’t move very far, because they
are held together by forces between them
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When they move freely
they spread themselves out as evenly as they can
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Diffusion is the movement of particles from an area where they are in high
concentration, to an area where they are in a low concentration
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The corner in which you spray the perfume will be the
area of high concentration
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Therefore, diffusion evens out molecules and results from random movement
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Hence, the lower the temperature the slower the rate of diffusion
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Therefore the
larger the molecule, the slower the rate of diffusion
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For diffusion to occur there must be a concentration gradient (there must be a
concentration of a substance in one area must be higher than it is in another
area)
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DIFFUSION IN LIVING THINGS
Diffusion is important to living organisms because they obtain their requirements
and get rid of their waste products by diffusion
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This creates a gradient into the cell as there is
always more oxygen outside than inside
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The concentration of C02 is always higher on the inside of the cell than on the outside because the
cell produces CO2 all the time
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This is because
there is a lower concentration of carbon dioxide inside the leaf, because cells are using it up
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Therefore carbon dioxide diffuses in the leaf along its

concentration gradient
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There is a higher concentration of oxygen inside the leaf because it is made there
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INTRODUCTION TO OSMOSIS
Osmosis is the movement of water molecules across a selectively permeable
membrane from an area where the molecules are in high concentration to an
area where they are in low concentration
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Osmosis is a passive process, i
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it does not require energy
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OSMOSIS IN ANIMAL CELLS
-When an animal cell is surrounded by a hypertonic solution (a solution that has
less water molecules than the cell’s intracellular solutions), water will leave the
cell by osmosis and the cell will shrink
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Water will enter the cell by
osmosis and the cell will swell and burst
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The amount of water that enters the
cell is the same amount that exits
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The cytosol of the cytoplasm is also a dilute solution containing many dissolved
nutrients and gases
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The tonoplast (vacuolar membrane) and the cell membrane both provide the
selectively permeable barrier for osmosis to occur
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OSMOSIS IN PLANT CELLS
When a plant cell is surrounded by a hypotonic solution, water will enter the cell
and the cell will become turgid
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The cell doesn’t burst because the cell wall protects it, because the cell wall
is not elastic like the cell membrane so it can only stretch a little
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The cell then becomes bigger and has
a round shape
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If a plant cell is surrounded by a hypertonic solution, water will leave the cell by
osmosis and the cell will become plasmolysed
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FLACCID PLANT CELLS
This occurs when there is just enough water inside to keep the cell membrane
against the cell wall, but the vacuole is not full enough to exert a pressure against
the wall and keep out more water

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