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 – H2O loving – due to polar group in the head

• Hydrophobic tail – H2O hating – due to the long hydrocarbon tails

Review of Lipids

Lipids• Most abundant lipid

is the phospholipid• Phospholipids have

a PO4 group in the 3rd –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 H2O 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 non-cytosolic 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 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