Diffusion and Osmosis

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Diffusion and Osmosis
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Diffusion

• Particles in a fluid (ex. Air, water) move
  randomly and constantly.
• Particles will move from areas of high
  concentration to areas of low concentration until
  the concentration is uniform. (ex. A drop of food
  dye in water)
• Equilibrium the balance point when the number
  of particles moving one way is equal to the
  number moving the opposite direction.

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Passive Transport

• Cells have selectively permeable membranes some
  substances can cross the membrane easily, others
  cannot
• Passive transport diffusion that does not
  require energy across a membrane dependent only
  on random movement of particles

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Facilitated Diffusion

• Because cell membranes are selectively permeable,
  some molecules cannot pass through the membrane
  easily.
• Facilitated Diffusion transport proteins provide
  channels for molecules to move across the
  membrane.
• For example, sugars and small, polar molecules
  use transport proteins to enter a cell.
• http//www.sumanasinc.com/webcontent/animations/co
  ntent/carrier_proteins.html

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Facilitated Diffusion
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Osmosis

• Osmosis passive transport of water across a
  selectively permeable membrane
• When an area inside a membrane has a higher
  concentration of particles (and the particles
  cannot cross the membrane), water will diffuse
  into the area to balance the concentration.
• When an area inside a membrane has a lower
  concentration of particles, water will leave the
  area inside the membrane in order to balance the
  concentration.

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Osmosis Continued

• Hypertonic a solution with a higher
  concentration of a particle (hyper above)
• Hypotonic a solution with a lower concentration
  of a particle (hypo below)
• Isotonic solutions with equal concentrations of
  a particle

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Osmosis in Cells

• Osmosis occurs in cells when the concentration of
  a particle outside the cell is different from the
  concentration inside the cell.
• A cell in a hypotonic solution will swell as
  water enters the cell, and a cell in a hypertonic
  solution will shrivel as it loses water.

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Osmosis in Plant Cells

• Unlike animal cells, hypotonic solutions are best
  for plant cells. The flow of water into the cell
  keeps the cell firm, and the cell wall prevents
  the cell from bursting.
• In isotonic solutions, the cell does not apply
  pressure to the cell wall, and the plant will
  wilt.
• In hypertonic solutions, the plasma membrane will
  separate from the cell wall.

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Active Transport

• Active Transport a cell uses energy to move
  particles across a membrane
• A transport protein pumps particles across the
  membrane against the concentration gradient
  (opposite the direction the particles would
  travel in passive transport)
• Used to maintain chemical environments
• Ex animal cells require higher concentrations of
  potassium than their surroundings, so potassium
  is pumped into the cell.

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Active Transport
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Endocytosis and Exocytosis

• Large particles cannot diffuse across a membrane.
• To cross the membrane, they are packaged in
  membrane sacs called vesicles
• The vesicle fuses with the membrane wall, either
  releasing the particle into the cell
  (endocytosis) or depositing the particles outside
  the cell (exocytosis).

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Exocytosis and Endocytosis