Boundary Lipid bilayer Selectively Permeable Fluid mosaic of lipids and proteins Contains embedded proteins

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2 Boundary Lipid bilayer Selectively Permeable Fluid mosaic of lipids and proteins Contains embedded proteins 2

3 Phosphate head hydrophilic Fatty acid tails hydrophobic Amphipathic Phosphate attracted to water Arranged as a bilayer Fatty acid repelled by water Aaaah, one of those structure function examples

4 Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement (~10 7 times per second) Flip-flop (~ once per month) Phospholipids can move laterally 4

5 hydrocarbon tails affect membrane fluidity Unsaturated hydrocarbon tails: (-C=C-) kinks prevents tight packing incr fluidity Saturated hydrocarbon tails: (-C-C-) tight packing decr fluidity Fluid Viscous Unsaturated hydrocarbon tails (kinks) Saturated hydrocarbon tails 5

6 Cholesterol (a steroid) affects fluidity T fluidity T fluidity Cholesterol Figure 7.5 6

7 Transmembrane proteins embedded in phospholipid bilayer -create semi-permeable channels lipid bilayer membrane protein channels in lipid bilyer membrane

8 1972: Singer and Nicolson membrane proteins are dispersed and individually inserted into the phospholipid bilayer Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein 8

9 fluid structure mosaic of various proteins embedded in it when viewed from the top Membrane proteins can move side-to-side or laterally making the membrane fluid 9

10 Freeze-fracture studies support the fluid mosaic model of membrane structure A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. 10

11 A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Fibers of extracellular matrix (ECM) 11

12 Integral proteins Penetrate the hydrophobic core of the lipid bilayer Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE 12

13 Peripheral proteins Are appendages loosely bound to the surface of the membrane 13

14 Cell membrane separates living cell from aqueous environment thin barrier = 8nm thick Controls traffic in & out of the cell -allows some substances to cross more easily than others -hydrophobic (nonpolar) vs. hydrophilic (polar)

15 Serves as a cellular barrier / border polar hydrophilic heads sugar H 2 O salt nonpolar hydrophobic tails impermeable to polar molecules polar hydrophilic heads waste lipids

16 Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane inside cell H 2 O aa sugar NH 3 salt outside cell

17 Why are proteins the perfect molecule to build structures in the cell membrane?

18 Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein

19 Retinal chromophore H + H + aquaporin = water channel in bacteria, (& plants, mammals) NH 2 Porin monomer H 2 O b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) COOH a-helices in the cell membrane H + + Cytoplasm proton pump channel in photosynthetic bacteria; Retinal chromophores (rhodopsin) in human eyes H 2 O conformational change protein changes shape

20 Water moves rapidly into & out of cells -evidence that there were water channels protein channels allowing flow of water across cell membrane Peter Agre John Hopkins Roderick MacKinnon Rockefeller

21 Proteins determine membrane s specific functions cell membrane & organelle membranes each have unique collections of proteins Classes of membrane proteins: -peripheral proteins loosely bound to surface of membrane ex: cell surface identity marker (antigens) -integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein ex: transport proteins channels, permeases (pumps)

22 Outside Channel Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Antigen Cell surface identity marker Cell adhesion Attachment to the cytoskeleton

23 Transport hydrophilic channel across the membrane; may be selective for a particular solute. (left) transport protein shuttles substance across membrane by changing shape. (right) Some hydrolyze ATP as an energy source to actively pump substances across the membrane. Enzymatic Activity enzyme built into membrane; active site exposed to substances in the adjacent solution. Sometimes, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal Transduction Membrane protein has a binding site with a specific shape that fits the shape of a chemical mssgr, such as a hormone. External mssgr (signal) conformational change in the protein (receptor) relays the message to the inside of the cell. 23

24 Cell-Cell recognition Some glycoproteins serve as ID tags that are specifically recognized by other cells. glycoprotein Intercellular Joining Membrane proteins of adjacent cells may hook together in various kinds of junctions, (Ex. gap junctions/tight junctions) Attachment to cytoskeleton & extracellular matrix (ECM) Microfilaments/other cytoskeletal elements bind to membrane proteins -maintains cell shape -stabilizes localization of certain membrane proteins Proteins adhered to ECM coordinate extra- & intra-cellular changes 24

25 Cell-Cell Recognition: a cell s ability to distinguish one type of neighboring cell from another development - cells tissues organs immune function - ID & rejection of foreign Membrane Carbohydrates: Interact with the surface molecules of other cells, facilitating cell-cell recognition 25

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