Cell Transport Mechanisms

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Cell Transport Mechanisms

• 4.3

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I. Cell Membranes

1. Cell membranes are semipermeable have control
   over what molecules or ions can enter or leave
   the cell
2. Permeability is influenced by shape and
   lipid/protein organization
3. Cell Membranes help maintain homeostasis

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II. Transport Across Membranes

1. Hydrophobic (water-hating) molecules can pass
   easily through membranes (ex CO2, O2)
2. Hydrophilic (water-loving) molecules do not pass
   through easily (ex ions and polar molecules such
   as Ca2, H2O)
3. Other molecules are just too large to pass
   through the membrane (ex sugars, proteins)

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II. Transport Across Membranes

• Transport proteins within the cell membrane allow
  certain molecules in and out of the cell
• Transport proteins protect the cells internal
  environment, thus maintaining homeostasis
• Two main ways to transport
• Passive transport no energy required
• Active transport uses cell energy

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III. Passive Transport

• No energy is required
• Molecules and ions are sent down the gradient
• Three main kinds
• Diffusion
• Osmosis
• Facilitated diffusion

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III. Passive Transport

• Diffusion molecules move from an area of high
  concentration to low concentration
• Continues until the concentration gradient is
  eliminated
• Concentration gradient difference in
  concentrations between two areas
• Molecules move down the gradient
• The end result is equilibrium an equal
  concentration of molecules on both sides of the
  membrane

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III. Passive Transport

• Osmosis the diffusion of water molecules across
  a selectively permeable membrane from an area of
  high concentration to an area of lower
  concentration
• Three types of solutions
• 1. Hypertonic water moves out of the cell
• 2. Hypotonic water moves into the cell
• 3. Isotonic water moves equally in and out
• Osmostic pressure (turgor) helps support plant
  cells

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v. Osmosis
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III. Passive Transport

• Facilitated Diffusion diffusion of solutes
  through transport proteins in the cell membrane
• No energy is required moving down the
  gradient
• Polar molecules (water) must get through the
  membrane this way
• Transport proteins completely span the membrane
  and are also selective

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IV. Active Transport

• Energy (ATP) is required
• Pumps molecules or ions against (up) the
  concentration gradient
• Particles move from areas of lower concentration
  to areas of higher concentration
• Three main ways
• Sodium-Potassium Pump
• Endocytosis
• Exocytosis

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IV. Active Transport

• Sodium-Potassium Pump exchanges sodium ions for
  potassium ions across the cell membrane
• The carrier protein gets energy from ATP and
  changes shape
• Found in almost every human cell
• Maintains cell potential

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IV. Active Transport

1. Endocytosis bringing a substance into the cell
   by engulfing it with the cell membrane

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IV. Active Transport

• Exocytosis vesicles fuse with the cell membrane
  to release their contents outside the cell
• Used to remove cellular wastes
• Transports newly made proteins and lipids

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V. Homeostasis

1. All types of cell transport, both passive and
   active, are done to maintain an equilibrium
   within each body cell
2. Homeostasis refers to the balance, or dynamic
   (changing) equilibrium, within the body and its
   cells
3. Constant adjustments are needed to maintain a
   stable internal environment