H OW M EMBRANES A RE O RGANIZED

Membranes’ Functions give cells their outer boundaries (plasma membranes) and their inner compartments (organelles) Control the movement of substances into and out of cells Regulating the composition of the fluid within individual cells. Control the flow of information between cells Capture and release of energy — (photosynthesis and oxidative phosphorylation)

Membrane Components  lipids, proteins and sugars are three major membrane components  Membrane common general structure: Composed of a phospholipid bilayer with a collage of many different proteins, lipids and carbohydrates.

Membrane Components (cont.).Membrane is highly fluid  most of the lipid and protein molecules can move about in the plane of the membrane  lipid and protein molecules are held together by non- covalent interactions Sugars:  attached by covalent bonds to some of the lipid and protein molecules (glycolipids & glycoproteins)  found on one side of the membrane only

Membrane Lipids  Insoluble in water but soluble in organic solvents  Major types of biological membrane lipid: phospholipids, glycolipids and cholesterol.  The major functions of lipids : 1.serve as energy storage in the body. 2.Small quantities in cell serve as electron carriers, enzyme cofactors (fat-soluble vitamins or their metabolic products), light absorbing pigments, hydrophobic anchors, hormones, intracellular messangers, emulsifying agents.

(A) Phospholipids  consists of glycerol linked to two fatty acid chains, phosphate and choline.  One chain is unsaturated (1-4 cis double bond)  Each double bond puts a ben d in the fatty acid chain.

How to differentiate between saturated and unsaturated lipids  It depends on: number of the hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail

Saturated & Unsaturated Lipids

Saturation &Van der Waals bonds

Is C=C bond cis or trans?

Why Reused Oil is Carcinogenic?  If oil is used only once like when you fry an egg, only a few of cis bonds convert to trans bonds so it's not too bad.  if oil is constantly reused, like in fast food fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up.  fatty acids with trans bonds are cancer-causing.

Types of phospholipids  Two major types: 1-Glycerophospholipids 2- Sphiongophospholipids

Glycolipids  They differ from phospholipids in that glycolipids have a sugar, such as glucose or galactose, instead of the phosphate-containing head.  found on the outer surface of the plasma membrane with their sugars exposed at the cell surface.  Function: Cell – cell recognition.

Cholesterol  -contains a four-ring steroid structure together with a short hydrocarbon side-chain and a hydroxy group.  Plasma membranes have nearly one cholesterol per phospholipid molecule.  Other membranes (like those around bacteria) have no cholesterol.

Arrangment of Cholesterol in Bilayer Membrane  its hydroxyl group oriented toward the aqueous phase forming hydrogen bonds with polar phospholipid head groups.  its hydrophobic ring system adjacent to fatty acid tails of phospholipids

Functions of Cholesterol  immobilize the first few hydrocarbon groups of the phospholipid molecules  makes the lipid bilayer less deformable and decreases its permeability to small water-soluble molecules (bacterium has no cholesterol so it needs cell wall)  prevents crystallization (close packing) of hydrocarbons and phase shifts in the membrane.

Membranes Lipids

Membrane Fluidity

Factors affect Membrane Fluidity 1.Temperature 2. fatty acid composition 3. cholesterol content.

Temperature Effect

Transition Temperature The temperature at which the lipid bilayer melts.  For most biological membranes Trans. Temp. is in the range 10-40°C.

Lipid Composition Effect  if the lipid tails are short or have double bonds Transition Temp is lower & Bilayer is more Fluid Reasons: 1- Short chains will interact less with one another than will long chains (a lower temperature is needed to melt the bilayer containing them) 2- Double bonds put bends in the hydrocarbon tails, (difficult for the phospholipids to pack together & the membrane become more fluid)

Cholesterol Effect  In Eukaryotic animal membranes one cholesterol molecule for every two phospholipid molecules  Cholesterol fits between phospholipids as: 1- hydroxy group close to the phospholipid heads 2- hydrophobic rings & side-chain buried within the fatty acid chains (stiffens them & membrane less fluid)

Cholesterol Effect (cont.)

Membrane Proteins  The amount and types of protein vary considerably from membrane to membrane.  The more active a membrane is in metabolism, the more protein it contains.  proteins with the lipid bilayer form functional membranes.

Protein Interaction Types  single lipid-spanning domain (single pass)  several lipid-spanning domain (multipass)  linked to the membrane by a glycolipid or phospholipid anchor  (protein-protein interactions )

Types of Membrane Proteins  Transmembrane Proteins  Anchored proteins  Peripheral membrane proteins  Glycoproteins

Transmembrane (internisc) Proteins  extend across the lipid bilayer  function as: 1- cellular receptors required for cell-to-cell communication and adhesion 2- transporters that shuttle various particles (including ions, glucose, and proteins) across the membrane  helical, forming rigid 'tubes' studded with hydrophobic amino acid side-chains

Anchored Proteins  do not span the membrane  are covalently attached to the inner surface, by either a fatty acid chain or a phospholipid  can only be removed by treatments disrupt the membrane  called integral membrane proteins  G-proteins involved in sending messages across membranes

Peripheral(extrinisc)Membra ne Proteins  weakly bound to one or other surface of the membrane by non-covalent interactions with other membrane proteins  can be removed by mild treatments leave the membrane intact  Functions as: 1- may affect cell shape 2- facilitate receptor-dependent signaling pathways

Glycoproteins contain sugars -Most of the proteins of the plasma membrane that are exposed to the cell surface have covalently linked sugars -Glycoproteins are carbohydrate-modified proteins

Protein Functions  All known enzymes are proteins  Transport and storage  Contraction  Structural materials  Immune system.  Generation and transmission of nerve impulses  Control elements of DNA  Chaperones: protein folding