Diffusion and Osmosis

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Diffusion and Osmosis
Intermediate 2
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Diffusion

• Is the movement of liquid or a gas molecules from
  a high concentration to a low concentration until
  the substance is evenly spread.
• Important as it lets food and oxygen into a cell
  and waste products such as carbon dioxide out of
  the cell

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Osmosis
Osmosis is a special type of diffusion involving
WATER
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Definition of Osmosis

• Osmosis is the movement of water molecules,
• from a region of high water concentration
• to a region of low water concentration
• across a selectively permeable membrane.

LEARN THIS !!!!
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Selectively Permeable?

• A selectively permeable membrane is a
• membrane with holes in it small enough for WATER
  to go through it easily.
• Big molecules like starch can not fit through
  easily

• Remember!!
• A membrane controls what goes in and out of the
  cell.
• Permeable means it allows substances to go
  through it.

See next slide
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Model of a Selectively Permeable Membrane
Water
Starch
Small molecules, such as water can pass through
it.
Large molecules like starch can not pass through
the membrane
Diagram shows the net flow of water
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Osmosis what is happening
Osmosis what is happening?
The water molecules pass both ways through the
membrane The overall flow of water molecules is
from a region with lots of water molecules to
region of less water molecules. Water is
described as moving down a concentration gradient
from a high concentration to a low concentration
Water
Starch
Lots of water molecules
Few water molecules
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Osmosis what is happening
Osmosis what is happening?
Water molecules move from the high water
concentration to the low water concentration.
Until both sides have an evenly balanced water
concentration. Water is described as moving down
a concentration gradient from a high
concentration to a low concentration
Water
Starch
Lots of water molecules
Few water molecules
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Summary

• OSMOSIS is the movement of water molecules
  across a selectively permeable membrane.
• The membrane only allows small molecules to pass
  through it, large molecules can not pass through
  the membrane.
• Water molecules move into the region where there
  are less water molecules
• The water acts to even up the concentration of
  water either side of the membrane.

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Osmosis and Plant Cells
In pure water the potato tubes swell, because
water enters their cells by osmosis.
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More Osmosis and Plant Cells
In rich sugar solution the potato tubes shrink,
because water leaves the potato by osmosis.
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Plant Cell in different solutions

• In pure water water will move in swelling
  the vacuole which pushes out against the cell
  wall.
• The plant cell is described as TURGID.
• The cell does not burst because of the cell
  wall.

Plant cell placed in pure water
Water moves Into the cell
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Plant Cell in Strong Sugar Solution
In a strong salt or sugar solution water will
move out of the vacuole causing it to shrink.
The cytoplasm can also shrink away from the cell
wall. The plant cell is described as Plasmolysed
or Flaccid.
Strong Sugar/Salt Solution
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Animal Cells in different solutions

• A red blood cell in pure water, water will move
  into the cell and the cell will BURST (there is
  no cell wall to prevent this happening)
• In a strong salt/sugar solution water will move
  out and the cell will SHRINK (the cell in
  described as CRENATED)

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Some terms when comparing two solutions

• Hypotonic one that has a high water
  concentration compared to surrounding solution
• Hypertonic one that has a low water
  concentration compared to surrounding solution
• Isotonic one that has a water concentration
  equal to the surrounding solution

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More to do

• Click on the link below and go to the website my
  skoool http//kent.skoool.co.uk/keystage3.aspx?i
  d63
• and look over the lessons Cell Structure and
  Function and Plant Cells
• Then go to bitesize revision http//www.bbc.co.uk/
  scotland/education/bitesize/standard/biology/inves
  tigating_cells/index.shtml read over the cells
  topic and answer the questions