1. Membrane Structure and Function

2. Overview: Life at the Edge
Plasma membrane is boundary that separates living cell from surroundings
Plasma membrane is selective permeability
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3. 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.
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4. WATER Hydrophilic head Hydrophobic tail WATER Figure 7.2
Figure 7.2 Phospholipid bilayer (cross section). 4

5. 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

6. 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
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7. 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

8. 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
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9. 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

10. 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

11. 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

12. 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

13. 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
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14. 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

15. Six major functions of membrane proteins
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
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16. (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

17. (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

18. (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

19. 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
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20. 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