Movement of Substances through a Cell Membrane

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Movement of Substances through a Cell Membrane

• BIOLOGY 2

GCSE Additional Science
Chapter 3
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How do Substances Enter and Leave Cells?

• Molecules move quickly, at random, through a
  liquid this is called DIFFUSION. Substances
  move from a region of high to low concentration.

GCSE Additional Science
Chapter 3
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• Here the pink molecules will move from the left
  to the right until there are the same number of
  pink molecules on both sides. The orange
  molecules move in the opposite direction. This is
  how substances such as oxygen and carbon dioxide
  enter and leave cells.

Flow
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Chapter 3
Flow
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The Following Factors Affect the Rate of
Diffusion

1. Concentration the larger the difference between
   the concentration of molecules, the faster they
   will diffuse.
2. Temperature the higher the temperature, the
   faster diffusion of the molecules.
3. Pressure the higher the pressure on the
   molecules, the faster the movement (from the
   region of higher pressure to the region of lower
   pressure)

GCSE Additional Science
Chapter 3
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The Effect of the Cell Membrane on Diffusion

• If a cell membrane was totally permeable (allows
  everything in and out), the cell would die in no
  time. Even though useful substances would enter
  the cell easily, the cells content would diffuse
  out. In order to survive therefore, the cell
  membrane has to be selectively permeable (allows
  only some substances to enter and leave)

GCSE Additional Science
Chapter 3
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Osmosis
Higher

• Here we can see the water diffusing through a
  selectively permeable membrane, from a region of
  high concentration to a region of lower water
  concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
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Osmosis
Higher

• Here we can see the water diffusing through a
  selectively permeable membrane, from a region of
  high concentration to a region of lower water
  concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
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Osmosis
Higher

• Here we can see the water diffusing through a
  selectively permeable membrane, from a region of
  high concentration to a region of lower water
  concentration.

Selectively permeable Membrane
water
GCSE Additional Science
Chapter 3
sugar
High water concentration Low sugar concentration
Low water concentration High sugar concentration
Weak solution
Strong solution
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Practical Work

• We can use a special plastic called Visking
  tubing to demonstrate osmosis.
• Here we can see that the water molecules can
  move through the tubing but the sucrose molecules
  are prevented. The reason for this is to do with
  the size of the molecules. The sucrose molecule
  is larger than the water and cannot fit through
  the tiny holes in the wall of the tubing. Water
  can move into the sucrose solution, but the
  sucrose cannot go into the water.
• After about half an hour, the solutions level
  in the tube will have risen, and the water level
  will have dropped.

capillary tube
tight knot
GCSE Additional Science
Chapter 3
sucrose solution
Visking tube
water
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Practical Work

• Potatoes can be used to demonstrate osmosis in
  living tissue.
• Potato cylinders of the same length and diameter
  are each placed in different concentrations of
  sucrose solutions for 20 minutes.
• The length and mass of each cylinder should
  then be re-measured. The potatoes that have been
  in strong sucrose solutions will have reduced in
  length and mass, whilst the ones placed in weak
  sucrose solutions will have gained length and
  mass.
• Cylinders showing no change will have cells with
  cytoplasm of equal strength to the sucrose
  solution that they were placed in.

Distilled Water
0.1M
0.2M
1M
0.5M
Labelled Petri dishes
5mm
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Chapter 3
50mm
Potato cylinder
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The Importance of Osmosis to Cells
Higher

• If a red blood cell was in a solution containing
  less water than itself (hypertonic), then the
  cell would lose water. It would shrivel and die.

GCSE Additional Science
Chapter 3
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Homeostasis
Higher

• It is very important therefore to keep the blood
  constant so that the cells in it do not gain or
  lose too much water.
• The term for keeping these internal conditions
  constant is Homeostasis.

GCSE Additional Science
Chapter 3
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The importance of Osmosis to Cells
Higher

• If a red blood cell was in a solution that had
  more water in it than was in the cell
  (hypotonic), then it would gain water. It would
  expand, burst and die.

GCSE Additional Science
Chapter 3
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Active Transport
Higher

• In many cases substances are moved into or out
  of cells against a diffusion gradient i.e.
  opposite to the movement of the molecules.
• Energy is needed for this process, and so its
  called active transport.

GCSE Additional Science
Chapter 3
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Examples of Active Transport
Higher

• In the body we see active transport in the small
  intestine, when digested food is absorbed into
  the blood in the opposite direction to diffusion.
  
• In a plant, rare minerals from the soil are
  pulled into the roots opposite to the direction
  of diffusion.

GCSE Additional Science
Chapter 3
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Summary

• Diffusion is the movement of particles down a
  concentration gradient.
• Osmosis is the diffusion of water molecules
  through a selectively permeable membrane.
• Active transport is the movement of molecules
  against a concentration gradient this requires
  energy.

GCSE Additional Science
Chapter 3