Membranes and Transport

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Membranes and Transport

• Chapter 6

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6.1 Membrane Structure

• Biological membranes contain both lipid and
  protein molecules
• Fluid mosaic model explains membrane structure
• Fluid mosaic model is fully supported by
  experimental evidence

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Biological Membranes

• Membrane phospholipids, membrane proteins
• Both have hydrophobic and hydrophilic regions
• Dual solubility properties

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Phospholipid Bilayer

• Membranes are based on fluid phospholipid bilayer
  
• Polar regions of phospholipids lie at surfaces of
  bilayer
• Nonpolar tails associate together in interior

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Phospholipid Bilayer

Fig. 6-2, p. 120
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Cholesterol in Bilayers

Fig. 6-3, p. 121
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Membrane Proteins

• Membrane proteins are suspended individually in
  the bilayer
• Hydrophilic regions at the membrane surfaces
• Hydrophobic regions in the interior

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Structure of Membrane Proteins

Fig. 6-4, p. 121
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The Lipid Bilayer

• Forms the structural framework of membranes
• Serves as a barrier that prevents passage of most
  water-soluble molecules

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Functions of Membrane Proteins

• Proteins embedded in the phospholipid bilayer
  perform most membrane functions
• Transport of selected hydrophilic substances
• Recognition
• Signal reception
• Cell adhesion
• Metabolism

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Types of Membrane Proteins

• Integral membrane proteins
• Embedded deeply in the bilayer
• Cant be removed without dispersing the bilayer
• Peripheral membrane proteins
• Associate with membrane surfaces

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Lipid Bilayer Organization

• Membranes are asymmetric
• Different proportions of phospholipid types in
  the two bilayer halves

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Membrane Structure

Fig. 6-5, p. 122
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Frye-Edidin Experiment

Fig. 6-6, p. 124
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6.2 Functions of Membranes in Transport Passive
Transport

• Passive transport is based on diffusion
• Substances move passively through membranes by
  simple or facilitated diffusion
• Two groups of transport proteins carry out
  facilitated diffusion

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Passive Transport

• Depends on diffusion
• Net movement of molecules with a concentration
  gradient (from region of higher concentration to
  region of lower concentration)
• Does not require cells to expend energy

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Transport Mechanisms

Table 6-1, p. 125
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Simple Diffusion

• Passive transport of substances across lipid
  portion of cellular membranes with their
  concentration gradients
• Proceeds most rapidly for small molecules that
  are soluble in lipids

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Facilitated Diffusion

• Passive transport of substances at rates higher
  than predicted from their lipid solubility
• Depends on membrane proteins
• Follows concentration gradients
• Specific for certain substances
• Becomes saturated at high concentrations of the
  transported substance

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Channel Proteins Aquaporin

Fig. 6-8a, p. 127
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Carrier Proteins

Fig. 6-8b, p. 127
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Transport Control

• Most proteins that carry out facilitated
  diffusion of ions are controlled by gates that
  open or close their transport channels

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6.3 Passive Water Transport and Osmosis

• Osmosis can operate in a purely physical system
• Free energy released by osmosis may work for or
  against cellular life

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Osmosis

• Net diffusion of water molecules
• Across a selectively permeable membrane
• In response to differences in concentration of
  solute molecules

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Osmosis

Fig. 6-9, p. 129
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Tonicity

• Water moves
• From hypotonic solution (lower concentrations of
  solute molecules)
• To hypertonic solution (higher concentrations of
  solute molecules)
• When solutions on each side are isotonic
• No osmotic movement of water in either direction

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Tonicity

Fig. 6-10, p. 130
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Turgor Pressure and Plasmolysis in Plants

Fig. 6-11, p. 131
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6.4 Active Transport

• Active transport requires a direct or indirect
  input of energy derived from ATP hydrolysis
• Primary active transport moves positively charged
  ions across membranes
• Secondary active transport moves both ions and
  organic molecules across membranes

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Active Transport

• Moves substances against their concentration
  gradients requires cells to expend energy
• Depends on membrane proteins
• Specific for certain substances
• Becomes saturated at high concentrations of the
  transported substance

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Active Transport Proteins

• Primary transport pumps
• Directly use ATP as energy source
• Secondary transport pumps
• Energy source Concentration gradient of
  positively charged ions (created by primary
  transport pumps)

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A Primary Active Transport Pump

Fig. 6-12, p. 132
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Secondary Active Transport

• Symport
• Transported substance moves in same direction as
  concentration gradient used as energy source
• Antiport
• Transported substance moves in direction opposite
  to concentration gradient used as energy source

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Coupled Secondary Active Transport

Fig. 6-13, p. 133
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6.5 Exocytosis and Endocytosis

• Exocytosis releases molecules outside cell
• By means of secretory vesicles
• Endocytosis brings materials into cells
• In endocytic vesicles

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Transporting Larger Substances

• Exocytosis and endocytosis
• Move large molecules, particles in and out of
  cells
• Mechanisms allow substances to leave and enter
  cells without directly passing through the plasma
  membrane

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Exocytosis

• Vesicle carries secreted materials
• Fuses with plasma membrane on cytoplasmic side
• Fusion
• Vesicle membrane joins plasma membrane
• Releases vesicle contents to cell exterior

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Exocytosis

Fig. 6-14a, p. 134
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Endocytosis

• Encloses materials outside cell in plasma
  membrane
• Pockets inward and forms endocytic vesicle on
  cytoplasmic side
• Two main forms
• Bulk-phase (pinocytosis)
• Receptor-mediated endocytosis

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After Endocytosis

• Most materials that enter cells are digested into
  molecular subunits
• Small enough to transport across vesicle membranes

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Endocytosis Pinocytosis

Fig. 6-14b, p. 134
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Receptor-Mediated Endocytosis

Fig. 6-14c, p. 134
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Phagocytosis

Fig. 6-15, p. 136