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Unformatted text preview: Plasma Membrane
Chapter 13: The Plasma Membrane P529-P565
5 4 3 2 1 7 8 6 10 9 Structure of Plasma Membrane
Fluid mosaic model Lipid Bilayer
- Forms a stable barrier between two aqueous compartments; - Impermeable to water-soluble molecules including ions and most biological molecules; - The bilayer lipids are mobile, not solid. Proteins
- Selective transport of molecules; - Cell-cell, Cell/Matrix interactions; - Cell signaling. Lipid Bilayer
The outer leaflet and inner leaflet are asymmetrical SM PC + Outer leaflet: SM & PC, GlycoLipid, cholesterol Inner leaflet: PS & PE, PI, cholesterol PS Phospholipid PI PE Sphingomyelin - SM Phosphatidylcholine - PC Phosphatidylserine - PS Phosphatidylethanolamine - PE Phosphatidylinositol - PI - Phospholipids - Account for more than half of the lipid in most membrane; - Responsible for the basic function of membranes as barriers; - The long fatty acid chains can move freely in the interior of the membrane. Glycolipids - Constitute about 2% of the lipids of most plasmas membrane; - Exclusively in the outer leaflet; - Its carbohydrate portions exposed on the cell surface play protective role, as well as involved in cell-cell recognition. Cholesterol - A major constituent of animal cell membrane: about equal molar amounts as the phospholipids; - It fills the gap between kinked long fatty acid chains; Help control membrane fluidity: at high Tm, reduces membrane mobility and permeability; at low Tm, prevents membrane from freezing. Synthesis of Phospholipid - All major lipids of eukaryotic membranes are synthesized by the smooth ER or Golgi; - Phospholipids are synthesized from water soluble cytosolic precursors on the cytosolic side of the ER memrane; - New phospholipids are added only to the cytosolic half of the ER membrane; - Smooth ER is also the major site for the synthesis of Cholesterol and Ceramide. Ceramide is converted to glycolipids or sphingomyelin. How are newly synthesized phospholipids transferred to the lumenal half of the ER membrane in order to maintain a stable membrane?
- Phospholipid flippase catalyze the translocation of phospholipids across the ER membrane resulting in even growth of both halves of the bilayer; - Lipids are exported from ER in vesicles to Golgi, and subsequently to the plasma membrane. Inside -out Inside -in Membrane Proteins
- Roughly 30% of all animal proteins are membrane proteins. - Proteins make up from 25-75% of membrane mass with a typical membrane containing 50% of its mass as protein. Integral proteins Peripheral and lipid anchored proteins - Released by membrane disruption - Peripheral proteins are proteins attached to other membrane proteins by ionic bonds and can be released by extreme PH or high salt concentration Transmembrane proteins
- The transmembrane domains are usually alpha helices of 20-25 hydrophobic amino acids Extracellular domain Transmembrane domain Intracellular domain Glycosylation review - The process of adding carbohydrates to protein peptides is called glycosylation; - Glycosylated proteins are called glycoprotein and most ER Proteins Are glycosylated; - Glycosylation is initiated in ER before translation is completed; - A common oligosaccharide unit is added to acceptor asparagine residues in the consensus sequence Asn-X-Ser/Thr by a membrane-bound enzyme called oligosaccharyl transferase; Oligosaccharide unit - The transmembrane domains in some cases are beta-sheets. One such example is the porin family proteins, which form barrel-like channels. Lipid or Glycolipid Attached Proteins
- Covalent bond - Lipid attached proteins are synthesized on free cytosolic ribosomes and modified by the addition of lipids; - Glycosylphosphotidylinositol (GPI) anchored proteins are synthesized in the ER and transported to the outside of the plasma membrane. GPI anchored Lipid anchored Attachment of Glycolipid review • Glycolipid (lipid linked to oligosaccharides), also named as glycosyl-phosphotidylinositol (GPI) due to the presence of phosphatidylinositol, is synthesized inside the ER; • The pre-assembled GPI is added to the carboxy terminus of some proteins that are retained in the membrane by a C-terminal hydrophobic sequence; • The C-terminal sequence is cleaved and exchanged for the GPI anchor, so the proteins remain attached to the membrane only by the GPI anchor; • Glycolipid modification helps to attach some proteins to the external face of the PM. Glycocalyx
- A carbohydrate coat on the outer face of the plasma membrane, formed by the oligosaccharides of glycolipids and glycoproteins; - Protects cell surface, also involved in cell-cell recognition.
cytosol glycocalyx nucleus Plasma membrane Are membrane proteins mobile? - protein mobility can be restricted by membrane protein-protein interaction, protein interaction with cytoskeleton, interaction with extracellular matrix and interaction with proteins of neighboring cells. (Larry Frye and Michael Edidin, 1970) Transportation across the plasma membrane (no external source of energy used) Passive diffusion Facilitated Diffusion - Through the lipid bilayer - No external energy required Passive Diffusion - Concentration gradient dependent Facilitated Diffusion
- Concentration gradient dependent (from high to low); - No external energy required; - Membrane proteins mediated. Carrier proteins
-Transport specific molecule by conformational change Channel proteins
- Allow free diffusion of any molecules of appropriate size and charge through its open pore Glucose Transporters GLUT1-13 Porins Gap junctions Ion channels
Ligand gated Voltage gated Voltage gated Ligand gated Presynaptic cell (-40mV) Postsynaptic cell Active Transport
- Transportation against concentration gradient ATP hydrolysis driven Ion gradient driven Na+-K+ pump (energy derived from coupled transport of a second molecule in the energetically favorable direction) Symport
(Apical membrane of intestinal or kidney epithelial cells) Antiport
Na+/Ca+ antiporters Na+/H- exchange protein Co-existence of active transport and facilitated diffusion Endocytosis
- The process of a cell taking up macromolecules and particles from its surrounding medium Phagocytosis - Ingestion of large particles such as bacteria Pinocytosis - Ingestion of fluids or macromolecules in small vesicles pinocytosis Endocytosis vs. Exocytosis Phagocytosis Human macrophages destroy ~ 1 x 1011 RBC every day Receptor Mediated Pinocytosis 1500 cholesteryl esters 800 phospholipid 1 apoprotein B100 LDL: low density lipoprotein Cholesterol uptake LDL receptor pathology: Hypercholesterolemia The children were born with blood cholesterol levels over 1000 mg/dl, and they began having heart attacks before age 5. (Chapter 2 The Composition of Cells p43-p63) Cellular Composition
Water Inorganic ions Organic molecules Carbohydrates Lipids
Monosaccharide Oligosaccharide Triacylglycerols Polysaccharide Polypeptide Phospholipids Cholesterol Glycolipids Energy storage Membrane structure Cell signaling
Cell structure Cellular transportation Cell signaling Cellular defense Enzymatic activities …… Fatty acid Proteins
Amino acid Nucleic Acids
Energy storage Cell structure Cell recognition and interaction Oligonucleotide Polynucleotide DNA RNA Genetic information ...
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This note was uploaded on 03/04/2011 for the course BIO 310 taught by Professor Junjunliu during the Spring '08 term at Cal Poly Pomona.
- Spring '08