L02 cell membrane_
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Transcript of L02 cell membrane_
Membrane structureand mechanisms of cell adhesion and
locomotion
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
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)
Chemical composition of membranes membrane lipids phospholipids, sfingolipids, sterols
membrane proteins (glycoproteins)
Phospholipids:(phosphoglycerides)
phosphatidyl-
cholin
ethanolamine
serine inositol
Sphingolipids -sfingosin
Sterols cholesterol ergosterol
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
Mobility of membrane phospholipidsMembrane fluidity rotation lateral migration flip-flop – transversal diffusion transition point
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
Membrane proteins - enzymes
- receptor proteins - transport proteins (pumps,carriers, channels)- linkers
Association of membrane proteins with the lipid bilayer
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)
Human red cells in scanning elektron microscope
Membrane skeletonnetwork of proteins under the plasma membrane. In red cells the membrane skeleton is formed by spectrin, actin filaments and attachment proteins
Glycocalyx: a coat of poly- a oligosaccharides on the surface of plasma membrane
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
Cellular interactions and cell adhesioncarry out a structural role, important in cell migration, growth, immunological function, cell recognition, tissue repair, differentiation and embryogenesis
Cell-to-cell interaction:Cell adhesion molecules (CAM): cadherin, Ig superfamily CAM,
mucin-like CAM, integrinsselectins
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
Structure and function of basal lamina
Keratinocyte crawling over surface
Model of focal adhesion
Components:
ECM
Integrin
Vinculin
Talin
Actin filaments
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.
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
Key terms – cont.
10. Basal lamina: components and function
12. Plasmamembrane and cell migration: cell crawling and contractile movements
13. Focal adhesion