Chapter 11- Biological Membranes and Transport

Lipids in Water

Structures:

• Micelles

• Membranes

• Liposome

Depends on the type of lipid

Micelle

Forms in the solution of amphipathic molecules that have a larger more polar head than tail

• fatty acids

• SDS

Few dozen tpo thousand molecules

Aggrefation of individual lipids intp the micelle is concentraion dependent

Membrane Bilayer

Consitsts of two leaflets of lipid monolayers

Forms when lipids with polar head groups and more then one lipid tail are in aqueous soluiton

• phospholipids

• sphingolipids

Liposome / Vesicle

Small bilayers will spontaneously seal into spherical vesicles in a concentration-dependent manner

Sythesis vesicle membranes can be made in vitro can contain artifically inserted protiens

the central aqueous cavity can enclose dissolved molecules

They are useful artificial carriers of molecules

Vesicles fuse readily with cell membranes or with each other

What are membranes

complex lipid-based structures that form pliable sheets

composed of a variety lipids and protiens

all cells have a cell membrane which serperates the cell from its surrounding

Eukaryotic cells have various internal membranes that divide the internal space into compartments (organelles)

Integral vs peripheral protiens

integral- firmly associated with the membrane often spanning the bilayer peripheral- weakly associated and can be removed easily

• some are noncovalently attached

• Some are linked to membrane lipids

Composition of Membranes

Lipid composition of membranes varies by: organisms, tissues, organelles

Ratio of lipid to protiens varies

type of phospholipid varies

abundance and type of sterol vaires

lack of sterols in prokaryotes

cholesterol predominant in the plasms membrane but absent in the mitochondria

galactolipids abundant in plant chloroplast but absent in animal

Asymmetry

Two leaflets have different lipid compositions

the outer is often more positivley charged

Phosphatidlylserine on the outside signals something is wrong

• platlets: activates blood clotting

• other cells: marks for destruction

Archaea Membranes

Stability: Isoprenoid tetraethers with unique alcohols

Chirality: L-Glycerol

Fatty Acids: branched isoprene

Linkages: Ether

Topology: Monolayer

Membrane Protiens

Detecting signals from outside

• light- opsin

• Hormones- insulin receptor

• neurotransmitters- acetylcholine receptor

• Pheromones- taste and smell

Channels, gates, pumps

• nutrients- maltoporin

• ions- (K+ channels)

• Neurotransmitter (Serotonin reuptake protien)

Enzyme:

• lipid liosythesis

• ATP Synthesis

Peripheral

Associated with another protien head groups of membrane

relativley loosely associated with membrane

Remove by changing pH or denature the protien

when purified the protiens dont have lipid associates with them

Amphitropic

conditionally attached to the membrane by covalent onteraction with lipids or carbohydrates attached to lipids

biological regulation results in attachment to or clevage form lipids

Lipid Anchors

Covalently linked lipid molecule

Integral Protiens

go into membrane (monolayer) or across the membrane (bilayer)

remove with detergent and the protien will still be associated with detergent when removed

Physical Properties of Membranes

Dynamic and flexible structures

Can exist in various phases and undergo phase transition

not permeable to large pollar solutes and ions

permeable to small plar soltues and nonpolar componds

Permeability can veb artifically increases by chemical treatment

• when DNA wants to get into the cell

Membrane Transport

Transport across a membrane must be energentically favorable

• concentration dependence: the solute moves toward equalibrium across the membrane

• Electrochemical dependence: the solute moves toward charge equalibrium across the membrane

Fluorescence Recovery After Photobleaching (FRAP)

allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescent return

Types of Diffusion

Simple- moves without help down electron gradient

Facilitated- use a protien to move down electrochemical gradient

Primary Active- Use protien and ATP to go against the electrochemical gradient

Secondary active transport- uses protien and a ion gradient to move against the electrochemical gradient

Ion channel- move ions down their graident

Ionophore-Medication Ion- down the electrochemical gradient

Transport protien types

uniport- move one molecule

Cotransport- Move two molecuels

• symport- move in the same direction

• antiport- move in the opposite direction