membranes and transport chapter 6. 6.1 membrane structure biological membranes contain both lipid...
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Membranes and Transport
Chapter 6
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
Biological Membranes
Membrane phospholipids, membrane proteins • Both have hydrophobic and hydrophilic regions
• Dual solubility properties
Phospholipid Bilayer
Membranes are based on fluid phospholipid bilayer
Polar regions of phospholipids lie at surfaces of bilayer
Nonpolar tails associate together in interior
Phospholipid Bilayer
Fig. 6-2, p. 120
Cholesterol in Bilayers
Fig. 6-3, p. 121
Membrane Proteins
Membrane proteins are suspended individually in the bilayer
Hydrophilic regions at the membrane surfaces
Hydrophobic regions in the interior
Structure of Membrane Proteins
Fig. 6-4, p. 121
The Lipid Bilayer
Forms the structural framework of membranes
Serves as a barrier that prevents passage of most water-soluble molecules
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
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
Lipid Bilayer Organization
Membranes are asymmetric• Different proportions of phospholipid types in the
two bilayer halves
Membrane Structure
Fig. 6-5, p. 122
Frye-Edidin Experiment
Fig. 6-6, p. 124
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
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
Transport Mechanisms
Table 6-1, p. 125
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
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
Channel Proteins: Aquaporin
Fig. 6-8a, p. 127
Carrier Proteins
Fig. 6-8b, p. 127
Transport Control
Most proteins that carry out facilitated diffusion of ions are controlled by “gates” that open or close their transport channels
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
Osmosis
Net diffusion of water molecules• Across a selectively permeable membrane
• In response to differences in concentration of solute molecules
Osmosis
Fig. 6-9, p. 129
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
Tonicity
Fig. 6-10, p. 130
Turgor Pressure and Plasmolysis in Plants
Fig. 6-11, p. 131
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
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
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)
A Primary Active Transport Pump
Fig. 6-12, p. 132
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
Coupled Secondary Active Transport
Fig. 6-13, p. 133
6.5 Exocytosis and Endocytosis
Exocytosis releases molecules outside cell• By means of secretory vesicles
Endocytosis brings materials into cells• In endocytic vesicles
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
Exocytosis
Vesicle carries secreted materials• Fuses with plasma membrane on cytoplasmic side
Fusion • Vesicle membrane joins plasma membrane
• Releases vesicle contents to cell exterior
Exocytosis
Fig. 6-14a, p. 134
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
After Endocytosis
Most materials that enter cells are digested into molecular subunits• Small enough to transport across vesicle
membranes
Endocytosis: Pinocytosis
Fig. 6-14b, p. 134
Receptor-Mediated Endocytosis
Fig. 6-14c, p. 134
Phagocytosis
Fig. 6-15, p. 136