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