Plasma (cell) membrane

Transcription

1 Plasma Membrane 1. Chemical composition of the plasmalemma 2. Models of cell membrane structure 3. Structure and functions of membrane proteins carriers (transporters) channels receptors 4. Cell coat (glycocalyx) 5. Cell surface specializations 6. Intercellular contacts types

2 Plasma (cell) membrane Plasma membrane, synonym: plasmalemma (Gr. lemma = 'husk') thickness 8-10 nm (EM) asymmetrical Chemical composition: lipids 20-40% phospholipids steroids (cholesterol) glycolipids 2-10% proteins ~50% hydrophobic structural hydrophilic receptors and enzymes >30 carbohydrates 10% glycoproteins glycolipids glycocalyx 2

3 Membrane models Ernest Overton ( ) Overton s concept (1880) the first comprehensive theory of lipid membrane structure: lipid-impregnated barrier layer Gorder-Grendel model (1925) phospholipids lipid bilayer Danielli-Davson sandwich model two protein layers peripherally lipid bilayer centrally hydrophilic (outside) pole hydrophobic (inside) pole Robertson model (1959) EM studies unit membrane Fluid mosaic model Singer & Nicolson (1972) Atomic model (2001) model of lipid bilayer atomic structure of integral membrane proteins 3

4 Fluid mosaic model Seymour Jonathan Singer (1924-) Lipid-protein mosaic model structural skeleton phospholipid double layer globular proteins: integral membrane proteins, (single-pass and multi-pass transmembrane proteins) peripheral proteins freeze-fracture: P-face (protoplasmic) Е-face (extracellular) Garth L. Nicolson (1943-) 4

5 Membrane proteins structure and functions Membrane proteins: ~50% w/w in the plasmalemma synthesized in the rough endoplasm reticulum their molecules are completed in the Golgi apparatus transported in vesicles to the cell surface Functional categories: transport proteins structural proteins (membrane-anchored) receptor proteins enzymes 5

6 Transport proteins pumps: transport ions Na + -K + ATPase metabolic precursors of macromolecules amino acids, sugar carriers (transporters, permeases): uniport (facilitated diffusion) symport (cotransport) carriers antiport (exchange diffusion) channels allow the passage of: small ions and molecules open and closed conformations voltage-gated ligand-gated mechanically gated 6

7 Structural proteins Cell adhesion molecules: typically transmembrane receptors homophilic and heterophilic binding 5 protein families 50 types: Ca 2+ -dependent: cadherins E-, P- and N- selectins E-, L- and P- integrins 24 members Ca 2+ -independent: immunoglobulin superfamily neural cell adhesion molecules intercellular cell adhesion molecules vascular cell adhesion molecules platelet-endothelial cell adhesion molecule 7

8 Receptor proteins Membrane receptors: neurotransmitter receptors ionotropic channels metabotropic hormone receptors peptide hormone steroid hormone immune receptors pattern recognition receptors complement receptors Fc receptors B-cell receptors T-cell receptors endocytosis receptors coated pits clathrin-dependent endocytosis internalization of iron some growth factors LDL receptors 8

9 Membrane-bound enzymes ~30 different enzymes: Alkaline phosphatase 5 I -Nucleotidase АТPase digestive enzymes disaccharidases dipeptidases 9

10 Cell coat (glycocalyx) First description Yamada (1955) Synonym = glycolemma thickness up to 100 nm renewal 6-8 h PAS-positive Chemical composition: glycolipids cerebrosides gangliosides glycoproteins proteoglycans hyaluronic acid Functions: defense absorption immunologic role cell recognition cell adhesion 10

11 Cell surface modifications 4 types structural specializations: (Gr. mikros, small + Lat. villus, tuft of hair) Microvilli: cytoplasmic processes, 1 µm high;0.08 µm wide brush (striated) border core of actin filaments terminal web Basal cell surface folds: interdigitations Stereocilia: non-motile microvilli of unusual length ductus epididymis hair cells of the ear Cilia & Flagella 11

12 Cilia motile cytoplasmic processes Gr. kinesis, movement + Lat. cilium, eyelash in respiratory epithelia capable of moving fluid and particles rapid back-and-forth movement ATP is the source of energy for ciliary motion movement rate 1/25 s 12

13 Cilia length: 2-10 µm diameter: µm axoneme (central core): 9 doublets + 2 singlets 9 x transitory part: microtubule organizing center basal bodies (kinetosome): 9 triplets 9 x

14 Flagellum much longer: µm limited to one flagellum per cell in the human body only in spermatozoa 14

15 Kartagener s syndrome immotile ciliary syndrome of Kartagener a defect in the action of the cilia lining Manes Kartagener ( ) the respiratory tract and fallopian tube due to the absense of dynein armes in them Chronic respiratory infections and male infertility: chronic rhinitis chronic sinusitis chronic bronchitis bronchiectasis sterile males females may be fertile 15

16 Intercellular junctions 3 types intercellular junctions: Barrier (impermeable) junctions: tight junction, zonula occludens occluding strip, fascia occludens occluding spot, macula occludens Adhering (anchoring) junctions: punctum adhaerens belt desmosome, zonula adhaerens spot desmosome, macula adhaerens (Gr. desmos, band + soma, body) Communicating junctions: gap junction, nexus synapse Junctional complex 16

17 Thank you 17