Diffusion and Osmosis

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Diffusion and Osmosis
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Selective Permeability of Cell Membranes(CM)

• The CM determines what gets in and out of cells.
• Some molecules pass through easily including
  lipids that dissolve in the lipid layer of the
  CM.
• Small molecules such as water, glucose and amino
  acids also pass through easily.

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Selective Permeability of Cell Membranes(CM)
continued.

• Some molecules do not pass through easily
  including large molecules (starch, protein) and
  those that carry charges (attract to opposite
  charge on CM).
• How cells move substances across the cell
  membrane involves a number of different processes.

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Diffusion

• Molecules of liquids and gases are always in
  motion. These molecules collide with each
  other,and thus undergo diffusion.
• Diffusion Movement of molecules from an area of
  high concentration, to one of low
  concentration(Ex. Perfume,coffee)

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Concentration Gradient

• The difference in concentration between a region
  of greater concentration and a region of lesser
  concentration is called a concentration gradient
  (shown below).

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Low O2 level High CO2 level
High O2 level
Low CO2 level
Movement of gases in cellular respiration involves
concentration gradients.Note the direction of
movement from high to low.
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Facilitated Diffusion

• This is a fast form of diffusion.
• Molecules get some help crossing the membrane.
• Evidence shows that there are proteins in the
  cell membrane with hollow channels.
• Molecules pass through these channels along a
  concentration gradient.

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Facilitated Diffusion
Animation
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Active Transport

• This is another way to move materials into and
  out of cells.
• In this case the cell must use energy to move
  substances against a concentration gradient.
• This allows the cell to maintain internal
  conditions that are chemically different from the
  external environment.

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Active Transport
Animation
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Osmosis

• Osmosis is a type of diffusion.
• In this case, water moves across a semipermeable
  membrane from an area of high water concentration
  to one of low water concentration.
• An example is the Naked egg demonstration.