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Transcript
Lecture 13 Plasma membrane pages 63-68 Plasma Membrane • Separates intracellular fluids from extracellular fluids • Plays a dynamic role in cellular activity • Glycocalyx is a glycoprotein area abutting the cell that provides highly specific biological markers by which cells recognize one another Morphology Extracellular fluid (ECF) = fluid outside the cell Plasma Membrane (cell membrane) = • Membrane composed of proteins & lipids that surrounds the cell •Composition and function vary from region to region Intracellular fluid (ICF), (cytosol, cytoplasm) = •Clear gel inside the cell •Organelles and cytoskeleton suspended within Morphology Intracellular face = side of the membrane that faces the cytoplasm Extracellular face = side of the membrane that faces the outside Fluid Mosaic Model • Double bilayer of lipids with imbedded, dispersed proteins • Bilayer consists of phospholipids, cholesterol, and glycolipids – Glycolipids are lipids with bound carbohydrate – Phospholipids have hydrophobic and hydrophilic bipoles Fluid Mosaic Model Figure 3.3 The Cell Cell Membrane Glycocalyx Extensions 4.5 The structure of membranes correlates with their functions • The plasma membrane controls the movement of molecules into and out of the cell, a trait called selective permeability – The structure of the membrane with its component molecules is responsible for this characteristic – Membranes are made of lipids, proteins, and some carbohydrate, but the most abundant lipids are phospholipids Copyright © 2009 Pearson Education, Inc. Composition • 98% of all membrane molecules are lipids • 2% of all membrane molecules are proteins Lipids • 75% are phospholipids Head – Amphiphilic – Create a bilayer • Head – Hydrophilic – Contain phosphate – Face the water on either side of the membrane Tails Hydrophilic head Phosphate group Symbol Hydrophobic tails Outside cell Hydrophilic heads Hydrophobic region of protein Hydrophobic tails Inside cell Proteins Hydrophilic region of protein 4.5 The structure of membranes correlates with their functions • Phospholipids form a two-layer sheet called a phospholipid bilayer – Hydrophilic heads face outward, and hydrophobic tails point inward – Thus, hydrophilic heads are exposed to water, while hydrophobic tails are shielded from water • Proteins are attached to the surface, and some are embedded into the phospholipid bilayer Copyright © 2009 Pearson Education, Inc. Lipids • 20% of membrane lipids are cholesterol molecules – Found among tails – Low conc. – hold phospholipids still resulting in a stiffened membrane in areas – High conc. – can prevent close packing of phospholipids thus increasing membrane fluidity Lipids • 5% of membrane lipids – glycolipids – Phospholipids with a short carbohydrate chain on the extracellular face of the membrane – Form the glycocalyx – carbohydrate coating on the cell surface with multiple functions Glycocalyx Glycocalyx Glycocalyx • • • • External to the plasma membrane “fuzzy coat” of glycolipids and glycoproteins Present on all animal cells Identification tag for cells Glycocalyx purposes • Protection – Cushions – protects from physical & chemical injury to the plasma membrane • Cell Adhesion – Binds cells together Membrane Proteins • Only 2% of the molecules in the plasma membrane HOWEVER • These proteins are larger than lipids ~ 50% of membrane weight Types Proteins • Transmembrane – pass thru the membrane – Hydrophilic regions in the cytoplasm and extracellular fluid – Hydrophobic regions in contact with the lipid membrane – Free floating or anchored to the cytoskeleton Types Proteins • Peripheral – do not enter the membrane, but adhere to the surface – Tend to be tethered to the cytoskeleton and in association to a transmembrane Types of Proteins • Integral – enter the phospholipid bilayer, but DO NOT cross it completely Functions of these Cell Membrane Proteins Functions of Membrane Proteins • Transport • Enzymatic activity • Receptors for signal transduction Figure 3.4.1 Functions of Membrane Proteins • Intercellular adhesion • Cell-cell recognition • Attachment to cytoskeleton and extracellular matrix Figure 3.4.2 Membrane Junctions • Tight junction – impermeable junction that encircles the cell • Desmosome – anchoring junction scattered along the sides of cells • Gap junction – a nexus that allows chemical substances to pass between cells Membrane Junctions: Tight Junction Figure 3.5a Membrane Junctions: Desmosome Figure 3.5b Membrane Junctions: Gap Junction Figure 3.5c Tight junctions can bind cells together into leakproof sheets – bladder, digestive tract Anchoring junctions link animal cells into strong tissues – heart, skin Gap junctions allow substances to flow from cell to cell – ions to trigger heart contractions Tight junctions Anchoring junction Gap junctions Figure 4.18B Extracellular matrix Space between cells Plasma membranes of adjacent cells Thank you