membranes and transport chapter 6. 6.1 membrane structure biological membranes contain both lipid...
TRANSCRIPT
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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
• Can’t 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