Chapter 5 Homeostasis and Transport

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Chapter 5Homeostasis and Transport

• Section 5.1

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Passive Transport

• The movement of substances across a cell membrane
  without any input of energy from the cell

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Diffusion

• Simplest type of passive transport
• Molecules move from an area of higher
  concentration to an area of lower concentration
• Driven by kinetic energy

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Equilibrium

• When the concentration of the molecules of a
  substance are the same throughout a space

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Diffusion Across Membranes

• Cell membranes allow some molecules to pass
  through, but not others
• Depends on size and type of molecule
• Also depends on the chemical nature of the
  membrane

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Osmosis

• Water molecules diffuse across a cell membrane
  from an area of higher concentration of H2O to an
  area of lower concentration

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Direction of Osmosis

• Hypotonic- the concentration of solute molecules
  outside the cell is LOWER than the inside of the
  cell
• Water moves INTO the cell

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• Hypertonic- the concentration of solute molecules
  outside the cell is HIGHER than the inside of the
  cell
• Water moves OUT OF the cell

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• Isotonic- the concentrations of solutes outside
  and inside the cell are equal
• No movement of water occurs

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How Cells Deal with Osmosis

• Contractile vacuole- organelles to remove water
• Turgor pressure- pressure of water against the
  cell wall
• Plasmolysis- cells shrink away from cell walls

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Facilitated Diffusion

• The movement of molecules across a cell membrane
  through the use of carrier proteins
• First, a carrier protein binds to a molecule on
  one side of the cell membrane

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• Second, the carrier protein changes shape
  shielding the molecule from the interior of the
  membrane
• Finally, the molecule is released on the other
  side of the membrane

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Diffusion Through Ion Channels

• Ion channels provide small passages for ions to
  diffuse across the cell membrane
• Channels may have gates that respond to
  stretching of the cell membrane, electrical
  signals, or chemicals in the cytosol

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• Ions such as Na, K, Ca2, and Cl- are important
  for a variety of cell functions