Membrane Structure and Function

Overview: Life at the Edge Plasma membrane is boundary that separates living cell from surroundings Plasma membrane is selective permeability © 2011 Pearson Education, Inc.

Cellular membranes are fluid mosaics of lipids and proteins Phospholipids most abundant lipid in plasma membrane amphipathic molecules Fuid mosaic model: membrane is a fluid structure with “mosaic” of various embedded proteins For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files. © 2011 Pearson Education, Inc.

WATER Hydrophilic head Hydrophobic tail WATER Figure 7.2 Figure 7.2 Phospholipid bilayer (cross section). 4

Phospholipid bilayer Hydrophobic regions of protein Figure 7.3 Phospholipid bilayer Figure 7.3 The original fluid mosaic model for membranes. Hydrophobic regions of protein Hydrophilic regions of protein 5

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is specialized preparation technique that splits membrane along the middle of phospholipid bilayer © 2011 Pearson Education, Inc.

Inside of extracellular layer Inside of cytoplasmic layer Figure 7.4 TECHNIQUE Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer RESULTS Figure 7.4 Research Method: Freeze-fracture Inside of extracellular layer Inside of cytoplasmic layer 7

The Fluidity of Membranes Phospholipids membrane can move within bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane © 2011 Pearson Education, Inc.

Figure 7.5 Fibers of extra- cellular matrix (ECM) Glyco- protein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Figure 7.5 Updated model of an animal cell’s plasma membrane (cutaway view). Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE 9

Lateral movement occurs 107 times per second. Figure 7.6 Lateral movement occurs 107 times per second. Flip-flopping across the membrane is rare ( once per month). Figure 7.6 The movement of phospholipids. 10

Mixed proteins after 1 hour Figure 7.7 RESULTS Membrane proteins Mixed proteins after 1 hour Mouse cell Figure 7.7 Inquiry: Do membrane proteins move? Human cell Hybrid cell 11

Unsaturated hydrocarbon tails Saturated hydrocarbon tails Figure 7.8 Fluid Viscous Unsaturated hydrocarbon tails Saturated hydrocarbon tails (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane Figure 7.8 Factors that affect membrane fluidity. Cholesterol 12

Membrane Proteins and Their Functions Peripheral proteins bound to surface of membrane Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices © 2011 Pearson Education, Inc.

EXTRACELLULAR SIDE N-terminus  helix C-terminus CYTOPLASMIC SIDE Figure 7.9 EXTRACELLULAR SIDE N-terminus  helix Figure 7.9 The structure of a transmembrane protein. C-terminus CYTOPLASMIC SIDE 14

Six major functions of membrane proteins Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) © 2011 Pearson Education, Inc.

(b) Enzymatic activity (c) Signal transduction Figure 7.10 Signaling molecule Receptor Enzymes ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Figure 7.10 Some functions of membrane proteins. Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) 16

(b) Enzymatic activity (c) Signal transduction Figure 7.10a Signaling molecule Receptor Enzymes ATP Figure 7.10 Some functions of membrane proteins. Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction 17

(d) Cell-cell recognition (e) Intercellular joining Figure 7.10b Glyco- protein Figure 7.10 Some functions of membrane proteins. (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) 18

The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual © 2011 Pearson Education, Inc.

Receptor (CD4) but no CCR5 Co-receptor (CCR5) Plasma membrane Figure 7.11 HIV Receptor (CD4) Receptor (CD4) but no CCR5 Co-receptor (CCR5) Plasma membrane Figure 7.11 Impact: Blocking HIV Entry into Cells as a Treatment for HIV Infections HIV can infect a cell that has CCR5 on its surface, as in most people. HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals. 20