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
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•
•
•
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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