Cell Membranes and Transport Chapter 4 Part 1

Cell Membrane What does it do for cell? Controls what goes in and out Regulates molecules moving from one liquid side of the cell to the other liquid side of the cell Protects Supports

Cell Membrane Lipid bilayer A cell membrane is made of two What are lipids? What does bi- mean? What’s a layer? A cell membrane is made of two layers of lipid molecules

Micelles Stable, ball-like structure made when phospholipids are placed in water Hydrophilic heads face outward (toward water) and hydrophobic tails face inward

Bilayers Two layered structures Made of phospholipids Can form in sheets Found in the cell membrane as a phospholipid bilayer

Liposomes Artificially prepared membrane-bound compartments (vesicles) Made by breaking up biological membranes and allowing them to reseal themselves Used to deliver drugs, cosmetic uses (lotions), food supplements

Cell membrane Phospholipids bilayer Made of a negatively charged phosphate “head” PO43- Attracts water because the phosphate is charged (-) Water is a polar , slightly positive ends and slightly negative ends Attached to the phosphate group are 2 fatty acid chains Hydrophobic= don’t like water So the inside of the cell membrane doesn’t let water in but the outside allows cells to be dissolved in aqueous environments Amphipathic molecule – has hydrophilic & hydrophobic regions

Other things in the membrane… Proteins embedded in lipid bilayer Carbohydrates attached to proteins Cholesterol scattered inside FLUID MOSAIC MODEL Singer and Nicolson, 1972 Fluid b/c molecules within the membrane can move around in their own layers Mosaic b/c protein and cholesterol are scattered throughout (as well as carbohydrates) Model no one has seen the cell membrane in detail….model explain behavior of molecules we have observed in experiments

Membrane Structure and Function Collagen proteoglycan Fibronectin

Membrane Structure and Function (con’t) Plasma Membrane boundary that separates living cell from its non-living surroundings + 7 nm thick + selectively permeable + unique structure relates to function

Membrane Structure and Function (con’t) Fluid Mosaic Model (Singer and Nicolson, 1972) The Fluid Quality of Membranes + held together by hydrophobic interactions - lipids/proteins drift about laterally + unsaturated hydrocarbon tails - maintain fluidity at low temperatures + cholesterol - stabilizes the membrane restrains movement at high temp. hinders close packing at low temp.

Proteins Proteins help things get across membrane Different Types Channels Pumps Carriers

Intrinsic proteins Extrinsic proteins Integral proteins Found in inner layer, outer layer or spanning the entire membrane “transmembrane” protein Hydrophobic portion that crosses membrane is made of one or more alpha-helical chains Hydrophilic portion made from hydrophilic amino acids Mobile “icebergs” in the phospholipid bilayer Extrinsic proteins Peripheral proteins Found on inner or outer surface of cell membrane Many bound to intrinsic proteins Others bound to other molecules or phospholipids

Different types of membrane proteins

Transport Proteins Carrier proteins Channel Proteins Can be 1 single protein or many working together Transport Proteins Carrier proteins Channel Proteins Constantly flip between two shapes Binding site opening alternates Water filled pores Fixed shape Allow charged substances to diffuse across Usually “gated” Part of membrane can move to open or close Two examples Na ion pump K+ ion pump BOTH are HIGHLY specific BOTH provide HYDROHILIC CHANNELS for ions and polar molecules to pass through (this means that the amino acids of these proteins lining the inside of them must be hydrophilic R-groups) Many are GATED part of protein molecule on the inside surface of the membrane can move to open or close the pore

Carbohydrates… Chemical identification cards “ID” card of cell Helps individual cells id each other Glycoproteins and glycolipids Receptor molecules Consist of short carbohydrate chains

Carbohydrates Glycoproteins Glycolipids Short carbohydrate chain attached to intrinsic protein Important in cell to cell recognition Form sugary coating of cell called “glycocalyx” In animal cellsmade of glycoproteins Short carbohydrate chains attached to phospholipids Good for adhesion to other cells...form tissues Form sugary coating of cell called “glycocalyx” In plant cellsmade of glycolipids Both Glycoproteins and Glycolipids can act as cell markers or ANTIGENS, allowing cell-to-cell recognition

Carbohydrate receptor molecules: Cell markers Antigens Cell-cell recognition Signaling receptors Help coordinate activities of animal cells Recognize hormones and neurotransmitters Endocytosis receptors Ligand activated endocytosis Binding receptors Cell adhesion Binds cells to other cells in tissues and organs

Plasma membrane synthesis

Functions of Membrane Proteins Transport Enzymatic Activity Signal Transduction Intercellular joining Cell-cell recognition Attachment to the cytoskeleton and ECM

Cell Walls are RIGID but PERMEABLE! Not selective!!! Outside cell membrane (does NOT replace membrane!!!) Plants, algae, fungi, prokaryotes Have pores to allow things in Function: SUPPORT and PROTECT

Cell Walls are RIGID but PERMEABLE! Cell walls made of… TOUGH CARBOHYDRATES RIGID for support but very permeable Plant Cell Walls CELLULOSE Fungi Cell Walls CHITIN Bacterial Cell Walls PEPTIOGLYCAN Cell Walls are RIGID but PERMEABLE! Not selective!!!

Types of Membrane Proteins

Glycoprotein

Glycolipid

Cholesterol Reduces membrane fluidity by reducing phospholipid movement Hinders solidification at low temperatures

Cholesterol is a steroid