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Membranes
- 1. MEMBRANES
- 2. 2 Membranes function to organize biological processes by compartmentalizing them. Indeed, the cell, the basic unit of life, is essentially defined by its enveloping plasma membrane.
- 3. 3 Moreover, in eukaryotes, many subcellular organelles, such as nuclei, mitochondria, chloroplasts, the endoplasmic reticulum, and the Golgi apparatus are likewise membrane bounded.
- 4. 4 FUNCTIONS OF MEMBRANE 4 Protective barrier Regulate transport in & out of cell (selectively permeable) Allow cell recognition Provide anchoring sites for filaments of cytoskeleton
- 5. 5 Provide a binding site for enzymes Interlocking surfaces bind cells together (junctions) Contains the cytoplasm (fluid in cell)
- 6. 6 Membrane Structure and Function Membrane Structure Lipids and proteins are the main components of the membranes, although carbohydrates are also important.
- 7. 7 The most abundant lipids in most membranes are phospholipids Phospholipids and most of proteins of membrane are amphipathic molecules.
- 8. 8 Amphipathic molecules : A molecule that has both hydrophilic region and a hydrophilic regions. .
- 9. HISTORY OF MEMBRANE STUDIES
- 10. 11 The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water.
- 11. 14 Further investigation revealed two problems. First, not all membranes were alike, but differed in thickness, appearance when stained, and percentage of proteins to lipids.
- 12. 15 Second, measurements showed that membrane proteins are actually not very soluble in water. • Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions. • If at the surface, the hydrophobic regions would be in contact with water.
- 13. 17 THE FLUID MOSAIC MODEL
- 15. 1919 Cell membrane and its surface structure LIPID RAFTS MODEL
- 16. 20 PROTEIN, LIPID AND CARBOHYDRATE COMPOSITIONS OF SOME MEMBRANES
- 17. 21 Depending on the precise conditions and the nature of the lipids, three types of lipid aggregates can form when amphipathic lipids are mixed with water
- 18. 22 • a) Micelle • (b) Bilayer • (c) Liposome
- 19. 23 MICELLES Micelles are spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules. Their hydrophobic regions aggregated in the interior, where water is excluded, and their hydrophilic head groups at the surface, in contact with water.
- 20. 24 BILAYER in which two lipid monolayers (leaflets) form a two-dimensional sheet. Bilayer formation occurs most readily when the cross-sectional areas of the head group and acyl side chain(s) are similar
- 21. 25 The hydrophobic portions in each monolayer, excluded from water, interact with each other. The hydrophilic head groups interact with water at each surface of the bilayer.
- 22. 26 LIPOSOME the bilayer sheet is relatively unstable and spontaneously forms a third type of aggregate: it folds back on itself to form a hollow sphere, a vesicle or liposome.
- 23. 27 By forming vesicles, bilayers lose their hydrophobic edge regions, achieving maximal stability in their aqueous environment. These bilayer vesicles enclose water, creating a separate aqueous compartment.
- 24. 28 BIOLOGICAL MEMBRANES Biological membranes are composed of proteins associated with a lipid bilayer matrix. Their lipid fractions consist of complex mixtures that vary according to the membrane source and, to some extent, with the diet and environment of the organism that produced the membrane
- 25. 29 . Membrane proteins carry out the dynamic processes associated with membranes, and therefore specific proteins occur only in particular membranes.
- 26. 30 MEMBRANE STRUCTURE AND COMPOSITION
- 27. MEMBRANE IS A COLLAGE OF PROTEINS & OTHER MOLECULES EMBEDDED IN THE FLUID MATRIX OF THE LIPID BILAYER Extracellular fluid Cholesterol Cytoplasm Glycolipid Transmembrane proteins Filaments of cytoskeleton Peripheral protein Glycoprotein Phospholipids
- 28. 32 PHOSPHOLIPIDS • Fatty acid tails • hydrophobic • Phosphate group head • hydrophilic • Arranged as a bilayer Fatty acid Phosphate
- 29. 33 PHOSPHOLIPID BILAYER polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads
- 30. 34 MORE THAN LIPIDS… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model!
- 31. 35 MEMBRANE FAT COMPOSITION VARIES • Fat composition affects flexibility • membrane must be fluid & flexible • about as fluid as thick salad oil
- 32. 36 MEMBRANE PROTEINS • Proteins determine membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins
- 33. 37 PERIPHERAL PROTEINS •loosely bound to surface of membrane •cell surface identity marker (antigens)
- 34. 38 • INTEGRAL PROTEINS •penetrate lipid bilayer, usually across whole membrane •transmembrane protein •transport proteins • channels, permeases (pumps)
- 35. 39 Integral protein (hydrophobic domain coated with detergent) Peripheral protein
- 36. 40 PROTEINS DOMAINS ANCHOR MOLECULE • Within membrane • nonpolar amino acids •hydrophobic •anchors protein into membrane Polar areas of protein Nonpolar areas of protein
- 37. 41 • On outer surfaces of membrane • polar amino acids •hydrophilic •extend into extracellular fluid & into cytosol
- 38. 42 NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane H+ Porin monomer b-pleated sheets Bacterial outer membrane proton pump channel in photosynthetic bacteria water channel in bacteria function through conformational change = shape change EXAMPLES
- 39. 43 MANY FUNCTIONS OF MEMBRANE PROTEINS Outside Plasma membrane Inside Transporter Cell surface receptor Enzyme activity Cell surface identity marker Attachment to the cytoskeleton Cell adhesion
- 40. 44 MEMBRANE CARBOHYDRATES • Play a key role in cell-cell recognition • ability of a cell to distinguish one cell from another • antigens • important in organ & tissue development • basis for rejection of foreign cells by immune system
- 42. 2007-2008 MOVEMENT ACROSS THE CELL MEMBRANE
- 43. 47 DIFFUSION • 2nd Law of Thermodynamics governs biological systems • universe tends towards disorder (entropy) Diffusion movement from high low concentration
- 44. 48 DIFFUSION • Move from HIGH to LOW concentration • “passive transport” • no energy needed diffusion osmosis movement of water
- 45. 49 DIFFUSION ACROSS CELL MEMBRANE • Cell membrane is the boundary between inside & outside… • separates cell from its environment IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O OUT waste ammonia salts CO2 H2O products cell needs materials in & products or waste out IN OUT Can it be an impenetrable boundary? NO!
- 46. 50 DIFFUSION THROUGH PHOSPHOLIPID BILAYER • What molecules can get through directly? • fats & other lipids inside cell outside cell lipid salt aa H2Osugar NH3 What molecules can NOT get through directly? polar molecules H2O ions salts, ammonia large molecules starches, proteins
- 47. 51 CHANNELS THROUGH CELL MEMBRANE • Membrane becomes semi-permeable with protein channels • specific channels allow specific material across cell membrane inside cell outside cell sugaraaH2O saltNH3
- 48. 52 FACILITATED DIFFUSION • Diffusion through protein channels • channels move specific molecules across cell membrane • no energy needed “The Bouncer” open channel = fast transport facilitated = with help high low
- 49. 53 ACTIVE TRANSPORT • Cells may need to move molecules against concentration gradient • shape change transports solute from one side of membrane to other • protein “pump” • “costs” energy = ATP “The Doorman” conformational change ATP low high
- 52. 56 HOW ABOUT LARGE MOLECULES? • Moving large molecules into & out of cell • through vesicles & vacuoles • endocytosis • phagocytosis = “cellular eating” • pinocytosis = “cellular drinking” • exocytosis exocytosis
- 53. 57 ENDOCYTOSIS phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal
- 54. 2007-2008 THE SPECIAL CASE OF WATER MOVEMENT OF WATER ACROSS THE CELL MEMBRANE
- 55. 59 • Diffusion of water from high concentration of water to low concentration of water • across a semi-permeable membrane
- 56. 60 CONCENTRATION OF WATER • Direction of osmosis is determined by comparing total solute concentrations • Hypertonic - more solute, less water • Hypotonic - less solute, more water • Isotonic - equal solute, equal water hypotonic hypertonic water net movement of water
- 57. 61freshwater balanced saltwater MANAGING WATER BALANCE • Cell survival depends on balancing water uptake & loss
- 58. 62 AQUAPORINS • Water moves rapidly into & out of cells • evidence that there were water channels 1991 | 2003 Peter Agre John Hopkins Roderick MacKinnon Rockefeller
- 60. 64
- 61. 65
- 62. SUMMARY
- 63. 68 THE COMPOSITION AND ARCHITECTURE OF MEMBRANES
- 64. 69 Biological membranes define cellular boundaries, divide cells into discrete compartments, organize complex reaction sequences, and act in signal reception and energy transformations.
- 65. 70 Membranes are composed of lipids and proteins in varying combinations particular to each species, cell type, and organelle. The fluid mosaic model describes features common to all biological membranes. The lipid bilayer is the basic structural unit.
- 66. 71 Fatty acyl chains of phospholipids and the steroid nucleus of sterols are oriented toward the interior of the bilayer; their hydrophobic interactions stabilize the bilayer but give it flexibility.
- 67. 72 Peripheral proteins are loosely associated with the membrane through electrostatic interactions and hydrogen bonds or by covalently attached lipid anchors..
- 68. 73 Integral proteins associate firmly with membranes by hydrophobic interactions between the lipid bilayer and their nonpolar amino acid side chains, which are oriented toward the outside of the protein molecule
- 69. 74
- 70. 75
- 72. 77 Lipids in a biological membrane can exist in liquid-ordered or liquid-disordered states; in the latter state, thermal motion of acyl chains makes the interior of the bilayer fluid. Fluidity is affected by temperature, fatty acid composition, and sterol content.
- 73. 78 Flip-flop diffusion of lipids between the inner and outer leaflets of a membrane is very slow except when specifically catalyzed by flippases.
- 74. 79 Lipids and proteins can diffuse laterally within the plane of the membrane, but this mobility is limited by interactions of membrane proteins with internal cytoskeletal structures and interactions of lipids with lipid rafts.
- 75. 80 One class of lipid rafts consists of sphingolipids and cholesterol with a subset of membrane proteins that are GPI-linked or attached to several long-chain fatty acyl moieties.
- 76. 81 Caveolin is an integral membrane protein that associates with the inner leaflet of the plasma membrane, forcing it to curve inward to form caveolae, probably involved in membrane transport and signaling.
- 77. 82 Integrins are transmembrane proteins of the plasma membrane that act both to attach cells to each other and to carry messages between the extracellular matrix and the cytoplasm. ■ Specific proteins mediate the fusion of two membranes, which accompanies processes such as viral invasion and endocytosis and exocytosis.
- 78. 83 SOLUTE TRANSPORT ACROSS MEMBRANES Movement of polar compounds and ions across biological membranes requires protein transporters. Some transporters simply facilitate passive diffusion across the membrane from the side with higher concentration to the side with lower.
- 79. 84 Others bring about active movement of solutes against an electrochemical gradient; such transport must be coupled to a source of metabolic energy.
- 80. 85 Carriers, like enzymes, show saturation and stereospecificity for their substrates. Transport via these systems may be passive or active. ■
- 81. 86 Primary active transport is driven by ATP or electron-transfer reactions; secondary active transport, by coupled flow of two solutes, one of which (often H or Na) flows down its electrochemical gradient as the other is pulled up its gradient.
- 82. 87 The GLUT transporters, such as GLUT1 of erythrocytes, carry glucose into cells by facilitated diffusion. These transporters are uniporters, carrying only one substrate.
- 83. 88 Symporters permit simultaneous passage of two substances in the same direction; examples are the lactose transporter of E. coli, driven by the energy of a proton gradient (lactose-H symport), and the glucose transporter of intestinal epithelial cells, driven by a Na gradient (glucose-Na symport)
- 84. 89 Antiporters mediate simultaneous passage of two substances in opposite directions; examples are the chloride-bicarbonate exchanger of erythrocytes and the ubiquitous NaK ATPase.
- 85. 90