Osmosis and Potatoes

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Osmosis and Potatoes!
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What is Osmosis ?

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

Text book definition
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What is Osmosis?

• Osmosis has the following uses
• It allows plants to take in water through their
  roots.
• It is used in kidney dialysis machines to filter
  the blood.

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What is a partially permeable membrane?

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

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

See next slide
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A partially permeable membrane
Water
Sugar
Small molecules, such as water can pass through
it.
Large molecules like sugar can not pass through
the membrane
Diagram shows the net flow of water
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Osmosis - what is actually happening ?
Water
Sugar
The water molecules pass both ways through the
membrane
The overall flow (net flow) of water molecules is
from a region with lots of water molecules to
region of less water molecules.
Lots of water molecules
Few water molecules
The water moves into the sugar - rich region to
fill it up with water. The water moves as if to
even up the concentration either side of the
membrane.
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Summary

• OSMOSIS is the movement of water molecules
  across a partially permeable membrane.
• The membrane only allows small molecules, e.g.
  water to pass through it.
• Large molecules e.g. glucose 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 Potato Investigation
Using the above facts you should now be able to
understand what happens in the potato
investigation

• W

What happens here?
Pure Water
Potato
In pure water the potato tubes swell, because
water enters their cells by osmosis.
Remember, potatoes are rich in starch which is
a complex sugar.
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What happens here?
Sugar Solution
Potato
In rich sugar solution the potato tubes shrink,
because water leaves the potato by osmosis.
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Hope you have got the idea now...

• If you would like to go over some of the points
  again , choose one of the options below.
• BACK TO START
• What is Osmosis?
• What is a partially permeable membrane?
• A partially permeable membrane
• Osmosis what is actually happening?
• Summary
• Potato tubes in pure water
• Potato tubes in sugar rich water