Membrane Transport

biological membrane

bilyaer structure composed of lipids and proteins that surround cells and organelles

5 major functions of biological membranes

1. Barrier separating biological molecules and processes.

2. Signal transduction proteins embedded in the membrane allow signals to pass from exterior to interior of the cell.

3. Selective pumps embedded in the membrane regulate

4. transport of ions and biological molecules, for example a proton (pH) gradient used for synthesizing ATP.

5. Provides a surface for organizing biological reactions and pathways.

Membranes assemble by

non-covalent, non-polar interactions

lipid bilayers are

amphiphatic

Two key forces in lipid monoloayers

1. Hydrophobic effect: association of non-water tails to release the associated water 

2. Van de waals interactions between cloesly packed acyl tails

between closely packed tails

van der waals interactions exist

Lateral movement

ipids and proteins diffusing side-to-side within a single layer

transverse movement

molecules moving from one leaflet of the bilayer to the other

lateral movement is assoaicated with

low energy cost

degree of membrane fluidity is determined by the

proportion of saturated unsat and cholestorl

transverse diffusion is associated with

large energy barried for the polar head of lipids to travel thorugh two layers of tails

leaflet

one lipid layer of the bilayer

fluidity of a membrane is also

temperature dependent

membranes are rigid at

LOW T

phase transition temperature

temp at which the fluidity increases abruptly

shorter fatty acids have lower

TM

more double bonds means

lower TM

CHOLestorl does not change the TM but it

broadens the transition

Integral membrane (transmembrane) proteins

compeltely span the bilayer so they have surfaces exposed to both the interior and exterior of the cell

transmembrane proteins have a

Hydrophobic bundle or sheets that will make favoable interactions with the interior of the lipid

Lipid anchored membrane proteins

have a hydriphobic tail that can insert into the membrane to hold the protein in place with hydrophobic interactions

proteins are kept in the membrane via

hydrophbic interactions

peripheral membrane proteins are

associated with the membrane throigh contracts with the polar head groups

peripheral membrane proteins are associated with the membrane due to what kind of interactions

charge-charge and h bonding

What are the key structural rules of membrane proteins

1. Satisfy the main chain hydrogen bonds

1. Transmembrane a-helices: (i+ithrough4 H bonding) 

2. B-barrels (inter-strand H bonding) 

3. Exposed loops are hydrophilic

Three types of membrane-protein anchors

1. protein joined to fatty acid for a tail

2. protein joined to prenyl group for a tail

3. protein joined to a phosphotidylinositol through a glycan for a tail

passive transport

no energy is recquired

• can occur without or with protein mediation

Active transport

energy required

-will always involved a protein

simple diffusion

only hydrophobic molecules

pore

1. located in bacterial outer membrane

2. narrow channel, blockes larger sized molcules

aquaporins

rapid movement of water acorss membranes

aquaporins are a

narrow channels that use H bodning to compensate for loss of H bonds with water

Primary active transport

 the exergonic reaction generating the energy is directly coupled to the endergonic transport reaction

Secondary active transport

the endergonic reaction of one transported substrate is coupled to the exergonic reaction of some other substrate.

uniport

passing one susbtrate at a time

synport

passing two substrates at a time

antiport

passing one substrate while at the sime time another passes in the other direction 

Describe how a membrane can be used to generate concentration and charge gradients

1. Protons pumped out of membrane, takes energy: 

2. protons flow back over the membrane, generates energy, makes atp 

3. Concentration gradients also contain free energy 

the cell interior usually has a higher

anion concentration and lower cation concentration compared to the outside of the cell

Hormones

signal responses in different tissues

epinephrine

fight or flight response

insulin

glucose uptake

glucagon

glycogen breakdwon

Glycogen

complex carbohydrate used to store glucose

what is the 3 step model for signal transduction

1. Cells respond to external stimuli 

2. Signals are transmitted through the membrane without transporting a molecule through the membrane 

3. Cellular response 

first messenger

external stimulus initiates release of a hormones (ei epinephrine)

second messenger

typically a small molecule that diffuses through the cell

agonists

stimulates a response

antagonists

blocks a response

G-Proteins depends on

GTP/GDP

G proteins have three suunits called

alpha beta gamma

the alpha and gamma subunits of g-proteins are

lipid anchored membrane proteins

TO TURN SIGNAL ON

G-GDP interacts with recptor complex

after g-gdp interacts with receptor complex

the GDP is swapped with GTP through GEF assisted activation

GDP to GTP causes a

change in the subunit

the change in the subunits cauess

beta and gamma subunits to dissocociate

Once b and G dissociate

G-GTP propagates a signal by interacting with an effector protein

to turn a signal off

the alpha subunit slowly hydrolyzes GTP to GDP assited by GAP

GTP to GDP causes

dissacoaition from the effector protein

after the dissociaiton

G-GDP associates with the beta and gamma subunits