A Closer Look at Cell Membranes

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A Closer Look at Cell Membranes
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Key Concepts

• Cell membrane consists of a bilayer of
  phospholipids and embedded proteins
• Transport proteins span the bilayer
• Open channels, gated channels, carriers, and
  pumps
• Receptor proteins receive chemical signals
• Recognition proteins are used for identification

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Key Concepts

• Diffusion is the movement of ions from a region
  of higher concentration to one of lesser
  concentration
• Osmosis is the movement of water across a
  selectively permeable membrane to a region where
  its concentration is lower
• Some membrane proteins function in passive
  transport whereas some function in active
  transport

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Plasma Membrane

• Bilayer of phospholipids
• Hydrophilic phosphate head
• Hydrophobic tails of fatty acids
• Fluid-Mosaic Model
• Phospholipids
• Glycolipids
• Sterols
• Proteins

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Fluid Mosaic Model

• Specialized proteins and enzymes embedded in the
  membrane

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How Substances CrossCell Membranes

• Diffusion
• Movement of substance from a region where it is
  more concentrated to a region where it is less
  concentrated

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How Substances CrossCell Membranes

• Water soluble substances and water diffuse
  through channel proteins
• Passive and Active Transport

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Factors Influencing the Rate and Direction of
Diffusion

• Concentration gradient
• Until equilibrium is reached
• Molecular size
• Small molecules move faster
• Temperature
• Faster at higher temperatures
• Electric or Pressure gradient
• Electrical charge difference across membrane
• Pressure differences

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Mechanisms By Which Solutes Cross Cell Membranes

• Diffusion of small non-polar molecules and water
• Passive transport
• Facilitated diffusion
• Polar substance transport through proteins
• Active transport
• Movement of substance against the concentration
  gradient
• Requires ATP
• Examples Exocytosis and Endocytosis

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Mechanisms By Which Solutes Cross Cell Membranes

• Exocytosis and Endocytosis

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Directional Movement of Water Across Membranes

• Osmosis
• Diffusion of water due to a water concentration
  gradient between two regions that are separated
  by a selectively permeable membrane
• Osmotic movement
• Dependent on concentration of solutes in the
  water
• Side with more solutes has a lower concentration
  of water

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Osmosis
Effect of solute concentration on water movement
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Effects of Tonicity

• The solute concentrations of two fluids
• Hypotonic
• Water diffuses in
• Cell swells
• Hypertonic
• Water diffuses out
• Cell shrinks
• Isotonic
• No net change

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Effects of Fluid Pressure

• Volume of fluid exerts hydrostatic pressure
• Force against a wall or membrane

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Protein Mediated Transport

• Passive transport
• Allows polar molecules to move from one side of
  membrane to the other

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Passive Transport

• Solute transport through transport protein
• Movement
• From higher to lower concentration
• No ATP required

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Active Transport

• ATP required
• Movement is against the concentration gradient
• Sodium-potassium pump
• Calcium pump

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Exocytosis and Endocytosis

• Exocytosis
• Vesicle moves to cell surface and fuses with
  plasma membrane
• Endocytosis
• Substances move in when plasma membrane balloons
  inward

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Phagocytosis

• Form of endocytosis
• Cell engulfs microbes, large particles, and
  cellular debris
• Amoebas and white blood cells

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Membrane Cycling

• Cycling of membrane by endocytosis and exocytosis
  
• Vesicles from ER and Golgi bodies contribute
• Receptor proteins and lipids recycled

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In Conclusion

• Plasma membrane is structural and functional
• Cell membrane is a bilayer of lipids with
  proteins embedded
• Fluid mosaic model of cell membrane with diverse
  lipids and proteins within
• Transport proteins allow water-soluble
  substances to pass through membranes

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In Conclusion

• Receptor proteins bind substances
• Recognition proteins allow cells to be recognized
  
• Adhesion proteins allow cells to adhere to each
  other
• Molecules tend to move from regions of higher to
  lower concentration

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In Conclusion

• Diffusion rates are influenced by concentration
  gradients, temperature, molecular size,
  electrical charge and pressure
• Osmosis is the diffusion of water across a
  selectively permeable membrane in response to
  concentration gradients
• Water moves from hypotonic solution to a
  hypertonic solution

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In Conclusion

• Small non-polar molecules diffuse across the
  membranes bilayer
• Ions and polar substances move across by passive
  or active transport
• Transport proteins shunt solutes across membrane
• Passive transport allows movement down a
  concentration gradient

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In Conclusion

• Active transport pumps a solute across the
  membrane against the concentration gradient using
  ATP
• Exocytosis entails movement of a vesicle to the
  plasma membrane and release of particles
• Endocytosis entails infolding of the plasma
  membrane to engulf particles
• developed by M. Roig