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