A. Membrane Functions

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1

2 A. Membrane Functions

3 Biological Membranes are composed of

4 Membrane Lipid Protein Myelin Sheath 80% 20% Plasma Membrane 50% 50% Mitochondrial 25% 75% Inner Membrane

5 Fig. 5.12: Phospholipids Hydrophilic head 2 Hydrophobic tails Phospholipds are amphipathic molecules (contain both hydrophilic and hydrophobic parts)

6 Phospholipids form Membrane Bilayers Bilayer consisting of two inverted phospholipid layers (leaflets) Hydrophobic Interior ~30 Å (3 nm) ~45 Å (4.5 nm) Hydrophobic interior is an impermeable barrier to passage of hydrophilic molecules, but not to hydrophobic molecules

7 Cholesterol has profound effects on membrane fluidity

8 Fig 7.8: Membrane (a) Phospholipid molecules move side-to-side within leaflet easily (lateral diffusion) but do not flip-flop across bilayer (transverse diffusion) (b) Phospholipids containing unsaturated acyl chains increase membrane fluidity by reducing packing efficiency (c) Cholesterol reduces membrane fluidity at normal temperatures (reduces phospholipid movement) At low temperatures it keeps membrane fluid (disrupts packing) Fluidity

9 Membrane Proteins can Move Laterally Within the Lipid Bilayer Membrane proteins labeled with different color fluorescent dyes Supports fluid-mosaic model of a dynamic membrane structure

10 Three Types of Membrane Proteins 1. Integral membrane proteins (transmembrane proteins) span the bilayer transmembrane domain has hydrophobic surface cytosolic and extracellular domains have hydrophilic surfaces Extracellular domain Transmembrane domain Cytosolic domain 2. Lipid-anchored membrane proteins - anchored via a covalently attached lipid 3. Peripheral membrane proteins - interact with hydrophilic lipid head groups or with integral membrane proteins

11 How do proteins cross lipid bilayer membranes? δ- δ+ δ- δ+ Even if the R-groups are hydrophobic, the peptide bond atoms are hydrophilic (polar) and will want to form Hydrogen Bonds; there are no H-bond donors or acceptors in the middle of a lipid bilayer.

12 Fig 7.9: α-helices Are Commonly Found in Membrane Proteins Polar peptide bond atoms H-bond with each other. α-helix of 20 amino acids is long enough to cross the bilayer. N-terminus α helix C-terminus EXTRACELLULAR SIDE CYTOPLASMIC SIDE

13 Fig 7.12: Membrane Synthesis & Sidedness

14 Fig. 7.9: Functions of Membrane Proteins Enzymes Signal ATP Receptor Transport Enzymatic activity Signal transduction Glycoprotein Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extra-cellular matrix (ECM)

15 Fig 7.7: Overview of the Plasma Membrane Fibers of extracellular matrix (ECM) bind to some membrane proteins Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton linked to some membrane proteins Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

16 Fig. 7.22: Endocytosis Phagocytosis Pinocytosis Receptor-Mediated Endocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Plasma membrane Ligand Receptor Coat proteins Food or other particle Coated pit Coated vesicle Food vacuole Vesicle CYTOPLASM

17 Membrane Transport

18 Energetics of Diffusion Why do molecules diffuse? A difference in concentration contains chemical potential energy. Molecules diffuse to try to equalize concentrations. [A] [A] [A] [A]

19 Fig. 7.13: Diffusion of Solutes Across A Membrane Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion (b) Diffusion of two solutes Net diffusion Equilibrium

20 Water Movement If a membrane is permeable to water but impermeable to a solute with different concentrations in two compartments, water will move to try to equalize the concentrations on the two sides of the membrane. Membrane permeable to water but impermeable to solute

21 Fig. 7.15: Water Balance of Living Cells (a) Animal cell Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled (b) Plant cell H 2 O Cell wall H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed Osmosis