Chapter 11 Membrane Structures Plasma Membrane The container

Chapter 11 Membrane Structures

Plasma Membrane • The ‘container’ for the cell – Holds the cytoplasm and organelles together • Barrier for the cell – Bacteria have a single membrane – Eukaryotes have outer plasma membrane and internal membranes • Endoplasmic reticulum • Nuclear membrane • Membrane-bound organelles

Cell Membranes

Cell Membrane Functions • Receives signals from outside the cell for internal cellular activities • Imports and exports molecules • Movement of the cell

General Structure • A lipid bilayer that contains 2 sheets of lipids interdispersed with proteins

Lipid Structure • Hydrophilic head – H 2 O loving – due to polar group in the head • Hydrophobic tail – H 2 O hating – due to the long hydrocarbon tails

Review of Lipids

Lipids • Most abundant lipid is the phospholipid • Phospholipids have a PO 4 group in the 3 rd –OH group of the glycerol instead of hydrocarbon • This can attach a hydrophilic group – Choline – phosphyltidylcholine – Polar amino acids like serine phosphatidylserine

Amphipathic Molecules • Contain both a hydrophilic and a hydrophobic portion to the molecule • Form a bilayer because of this • Other molecules are amphipathic – Steroids – Glycolipids – lipid with a sugar attached rather than a phosphate group

Other Lipid Molecules

Reminder • Hydrophilic molecules can dissolve in H 2 O due to the polarity of both of these molecules – H bonds and other non-covalent interactions may aid in this

Reminder • Hydrophobic molecules will be “caged” by the polar molecules – requires energy • Why when fats or oils are placed in water that they usually sit as a glob on the surface

Membrane • Amphipathic molecules have both components so the hydrophilic head molecules interact with the aqueous solution and the hydrophobic tails will interact with each other

Lipid Bilayer • Due to amphipathic property the membrane can reseal after an ‘injury’ • Bilayer is fluid – the orientation of the lipids and the outer aqueous surroundings keeps the lipids in the bilayer – The lipid can move around the layer – like one person moving in a crowded room • Not the same as flexible – entire membrane bending

Liposomes • Can study membranes by using artificial membrane structures called liposomes • Can follow the movement of lipids in each of the layers

What We Know • Lipids cannot move from one layer to another without the aid of proteins • Lipids can exchange places with neighbors • Lipids can rotate around their axis

Importance of Hydrocarbon • Hydrocarbon tail will determine the fluidity of the membrane just as it does in fats and oils • 2 components are important – Length of hydrocarbon chain • 14 to 24 C but usually 18 to 20 C per tail – Level of unsaturation (# of C=C bonds) • 1 tail has 1 or more C=C bonds (unsaturated) • Other tail is saturated (no C=C bonds)

Unsaturated Hydrocarbons • Each C=C bond causes a kink or bend in the tail – Can’t pack tightly in the layer – More lipids that have unsaturated tails the more fluid the membrane

Membrane Fluidity • Enables the membrane proteins to diffuse rapidly • Simple means of distributing lipids and proteins • Allows membranes to fuse with one another • Evenly distributed during daughter cell formation

Cholesterol in the Membrane • Cholesterol is added to areas that have lots of unsaturated lipids to help fill in the gaps between the tails • Helps to stiffen and stabilize the bilayer – Less fluid – Less permeable

Membranes are Asymmetrical • Inner surface is different from the outer surface – Types of lipids in each layer • Proteins in the bilayer have a specific orientation due to its function

New Membrane • New lipids are added on one side of the membrane • Enzyme called flippase used to put the lipid in the other half of the bilayer – Flippase may be selective for the type of lipids that it puts on either surface

Asymmetry • New membrane comes from the SER • Vesicle buds off the SER and when fuses with the plasma membrane, the orientation is maintained • Membranes have distinct inner and outer surface – Inner – cytosolic face • Adjacent to the cytosol – Outer – non-cytosolic face • Adjacent to the cell exterior or the interior of an organelle

Special Lipids • Glycolipids are found only on the noncytosolic surface – Sugar added in the Golgi apparatus – No flippase to move the glycolipid to the cytosolic surface • Inositol phospholipids are only on the cytosolic surface – Functions to relay signals on cytosolic surface that pass through the membrane

Membranes as Barriers • Because of the hydrophobic interior of the bilayer • Membrane is impermeable to ions and large charged molecules and require special membrane proteins to transport across

Membrane Proteins • Carry out the functions of the membrane (Table 11 -1) – Transporters – Na+ pump to move Na+ across – Linkers – integrins to link intercellular components to extracellular ones – Receptors – to bind a compound that sends a signal to the rest of the cell – Enzymes – perform chemical reactions in the membrane

Association with Membrane • Transmembrane – span the entire membrane • Linked by lipids – on either surface of the membrane • Interaction with transmembrane proteins

Transmembrane Proteins • Protein has hydrophilic and hydrophobic portions – Hydrophilic will interact with the aqueous solutions on either surface – Hydrophobic will be in contact with the hydrophobic interior of the bilayer • Also called integral membrane proteins

Peripheral Membrane Proteins • Proteins that are attached to either surface of the bilayer • Those attached to lipids are covalently linked • Those that interact with other transmembrane proteins are attached by noncovalent interactions – H bonds, hydrophobic and hydrophilic interactions

Membrane Spanning Proteins • Must have hydrophobic side chains in the area that spans the membrane • Peptide backbone is polar – Not real happy in the hydrophobic interior

Helix Span Interior • Interior forces the peptide backbone to form helix • Non-polar R groups are on the outside of the helix • Transmembrane usually span the membrane once – Receptors – collect signal, pass on the inside of cell

Membrane Pores • When protein spans the membrane several times usually form pores that allow molecules to move back and forth through the membrane • Multiple helix span membrane – Hydrophilic on the inside of the channel – Hydrophobic on the outer surface of the channel

Barrel • barrels are made of sheets that are curved into a cylinder • Again the hydrophilic line the inner side and hydrophobic the outer surface • Larger pore than helix pore

Detergents • Used to remove the proteins from the membrane • Amphipathic molecules • Have a single hydrocarbon tail • Form small clusters in aqueous solutions called micelles • SDS and Triton X-100 common in the laboratory

Removal of Proteins

Bacteriorhodopsin – Pumps Out H+

Photosynthetic Reaction Center

Cell Cortex • Membrane is very fragile and support comes from a meshwork on the cytosolic surface • Spectrin is an important protein in the cell cortex – links with transmembrane proteins by an attachment protein

Carbohydrates on Cell Surface • Many of the plasma membrane proteins have sugars attached to them – Short oligosaccharides – glycoproteins – Long polysaccharides - proteoglycans • Sugars on the surface make up the glycocalyx – Keeps cells moist and slippery – Used as cell recognition (lectins) and adhesion molecules

Glycocalyx – Cell Coat

Role of Glycocalyx

Protein Movement • Proteins can move through the layer of the membrane similar to the lipids • Can’t flip from one side to the other

Membrane Domains • Cells can restrict the movement of proteins by – Cell cortex attachment – Extracellular attachment – Attachment to other cells – By diffusion barriers • Tight junction – continuous barrier between adjacent cells

Restriction by Location • Apical side – facing opening • Basal side – bottom of the cell • Lateral sides – side surfaces