Diffusion and Osmosis

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Diffusion and Osmosis

• The selective permeability of the cell membrane

2
The movement of molecules

• Heat causes the random motion of all molecules
• This movement cannot be seen directly, but its
  effects can be seen under a microscope
• The movement of particles resulting from
  collisions with other particles is called
  Brownian movement

3
Solutions

• A solution is a homogenous mixture in liquid form
• Usually we think of solutions as some substance
  (the solute) dissolved in water (the solvent)
• Almost all biological systems are suspended in
  water

4
How do solutions form?

• Solutions form through the process of diffusion
• Diffusion is the net movement of substances from
  areas of high concentration, heat, or pressure,
  to areas of low concentration, heat, or pressure
• In most biological systems, heat and pressure are
  constant (most of the time), so we will discuss
  movement along a concentration gradient

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The Formation of a Sugar-Water Solution
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Diffusion

• The rate of diffusion is affected by the
  steepness of the gradient
• The rate is also affected by several
  characteristics of the molecule, including size,
  polarity, and solubility
• Which do you think will diffuse faster? A
  substance with a HIGH molecular weight, or a
  substance with a LOW molecular weight? Why?

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Diffusion and Membranes

• Cells are surrounded by membranes composed of a
  phospholipid bilayer
• The nature of this membrane allows some molecules
  to pass more easily than others, making it
  selective and differentially permeable
• Small, non-polar molecules can pass easily
  through the membrane, while large, polar
  molecules cannot

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Diffusion and Membranes

• However, there are some openings in the membrane
  (i.e. protein channels, ion channels, etc.)
• These channels allow polar molecules to cross the
  membrane, but are still selective on the basis of
  size and shape
• In this lab, we can simulate the membrane and its
  differential permeability (based on size) with
  dialysis tubing

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Differential Permeability
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Dialysis

• Dialysis is the separation of dissolved
  substances by means of their unequal diffusion
  through a differentially permeable membrane
• Particles below a certain size can pass through,
  while those above a certain size cannot
• Dialysis tubing DOES NOT select based on charge
  and solubility like cells do, ONLY size

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Osmosis

• Osmosis is the diffusion of water across a
  differentially permeable membrane (Diffusion
  refers to ANY substance, Osmosis refers ONLY to
  water)
• We can predict the net movement of water based on
  concentration gradients relative to the
  environment
• We can describe solutions in one of three ways
  (relative to their environment) hypotonic,
  isotonic, or hypertonic

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Osmosis

• A hypotonic solution has a LOWER concentration of
  solutes
• A hypertonic solution has a HIGHER concentration
  of solutes
• An isotonic solution is a solution that has a
  concentration of solutes equal to its environment
• So how does this affect cells?

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Osmosis

• The net movement of water is always FROM a
  hypotonic solution TO a hypertonic solution
• If you look at it as simply a diffusion of water,
  the net movement is always from the higher
  concentration of water (the hypotonic solution)
  to the lower concentration of water (the
  hypertonic solution)
• There is NO NET MOVEMENT in isotonic solutions

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Effects of Osmosis
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Effects of Osmosis