l02 cell membrane_
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Membrane structureand mechanisms of cell adhesion and
locomotion
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Separate the cell interior from an external environment Take part in cell functions Partition the cytoplasm into compartments - organelles Mediate contacts with other cells or surrounding environment Create a semipermeable barrier – impermeable for macromolecules and selectively permeable for ions
Biomembranes
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BiomembranesDevelopment of membrane concept• 1890: cell surface is of lipidic nature
(Overton)
• 1926: cellular membranes consist of lipid bilayers (Gorder and Grendel)
• 1943: the surface of lipid bilayer is coated by proteins (Davson and Danielli)
• 1960: unit membrane (Robertson)
• 1972: fluid mosaic model (Singer a Nicholson)
• 1980: detailed structure of membrane proteins (Unwin a Henderson)
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Chemical composition of membranes membrane lipids phospholipids, sfingolipids, sterols
membrane proteins (glycoproteins)
Phospholipids:(phosphoglycerides)
phosphatidyl-
cholin
ethanolamine
serine inositol
Sphingolipids -sfingosin
Sterols cholesterol ergosterol
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Physical features of biomembranes
Stability of the bilayer: facilitated by hydrophobic
interaction between fatty acid chains
unsaturated fatty acids decrease the bilayer stability
Sterols increase the bilayer stability
• Polarity of phospholipids hydrophilic ends (PO4 ,, COOH, OH, NH3
hydrophobic ends fatty acid chains • Self-assembly into bilayers
liposomes, myeline structures• Lipid asymmetry• Membrane fluidity
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Mobility of membrane phospholipidsMembrane fluidity rotation lateral migration flip-flop – transversal diffusion transition point
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Heterogeneity of membrane lipids and their asymmetric distribution in bilayer.
Rafts: small islands of sphingolipids and cholesterol creating a separatephase (50 nm in diameter) in outer leaflet of plasma membrane
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Membrane proteins - enzymes
- receptor proteins - transport proteins (pumps,carriers, channels)- linkers
Association of membrane proteins with the lipid bilayer
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Membrane proteins: - have both hydrophobic and hydrophilic regions
- the hydrophobic region extend through the bilayerand is formed by hydrophobic amino acids
hydrophilic regios are exposed to the aqueous environmenton either side of the membrane - Peripheral proteins are attached to the bilayer by lipid groups (dolichol)
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Human red cells in scanning elektron microscope
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Membrane skeletonnetwork of proteins under the plasma membrane. In red cells the membrane skeleton is formed by spectrin, actin filaments and attachment proteins
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Glycocalyx: a coat of poly- a oligosaccharides on the surface of plasma membrane
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Extracelullar matrix (ECM):
• Complex network of polysaccharides and proteins produced by cells of connective tissue (fibroblasts, chondrocytes, osteocytes etc.) Main components:collagens, elastin – structural proteins
fibronectins – fibrous adhesive proteins interconnecting ECM to a and laminins meshwork
proteoglycans – glycoproteins forming a gel
• ECM calcified in bone and teeth• Ropelike in tendon• Transparent in cornea• Plant cell wall – specific type of extracellular matrix
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Cellular interactions and cell adhesioncarry out a structural role, important in cell migration, growth, immunological function, cell recognition, tissue repair, differentiation and embryogenesis
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Cell-to-cell interaction:Cell adhesion molecules (CAM): cadherin, Ig superfamily CAM,
mucin-like CAM, integrinsselectins
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Basal lamina
Thin tough sheet of extracellular matrix.Composition: collagen type IV tensile strength
laminin provides adhesive sites for integrin molecules in the plasma membrane of epithelial cells other proteins
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Structure and function of basal lamina
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Keratinocyte crawling over surface
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Model of focal adhesion
Components:
ECM
Integrin
Vinculin
Talin
Actin filaments
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Plasmamembrane and cell migration
• Cell crawling – formation of filopodia (thin stiff protrusions) and lamelipodia (thin sheet-like extensions) – due to actin polymerization. The new position of the plasma membrane is fixed by focal adhesion complexes
• Contraction of a part of the cytoplasm, invagination of the plasma membrane or invagination of cell layers due to the activity of actin-myosin I or actin-myosin II complexes: actin filaments are anchored to the plasma membrane. Actin filaments slide over each other, the sliding is mediated by myosin motors.
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Key terms1. Biomembranes – overview of the structure and functions 2. Development of membrane concept: lipid layer, lipid
bilayer, phospholid bilayer, localization of membrane proteins
3. Structure of the phospholipid bilayer 4. Hydrophilic head and hydrophobic tails. Self-assembly of
the bilayer. Lipid asymmetry. Membrane rafts. Liposomes.5. Model of the fluid mosaic. Integral a peripheral proteins.6. Hydrophobic regions of integral proteins7. Membrane glycoproteins 8. Glycocalyx 9. Extracelular matrix: main components: collagen, elastin,
fibronectin, laminin, proteoglycans
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Key terms – cont.
10. Basal lamina: components and function
12. Plasmamembrane and cell migration: cell crawling and contractile movements
13. Focal adhesion