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