Membrane Transport

differences in membrane composition differentiates organelles by

dictating shape, curvature, function

examples of membrane composition differentiating organelles

mitochondria (cardiolipin rich —> supports the ETC)

ER: phosphaticlycholine- rich (flexible synthesis site)

Plasma membrane= cholesterol and sphingolipids (rigidity and signaling)

rafts

regions of the membrane that are less fluid, holding proteins together for localized interactions

what makes up rafts

cholesterol and glycosphinolipids

cholesterol helps rafts by

modulating fluidity and permeability (decreasing) and thickening the eukaryotic membrane and causes an increase order

lipid distribution in a leaflet

not random or even

why is lipid distribution in a leaflet not random nor even

dictated by things like cholesterol and different interactions like the hydrophobic effect and it isn’t even because it is asymmetric

glycosphingolipids

increase rigidity by having longer chain tails

job of rafts (proteins)

rafts allow for separation of proteins in the membrane

functions of biological membranes

define boundaries of the organism or cell

allow for selective import/export AKA selective permeability

maintain metabolites and ions within the cell

sense external signals and transmit information into the cell

provide compartmentalization within the cell

produce and transport nerve signals

store energy as a proton gradient and support synthesis of ATP

how does selective permeability work

import of nutrient (e.g. lactose)

export of waste and toxin (e.g. antibiotics)

provide compartmentalization within the cell

separate energy= producing rxns from energy consuming rxns

keep proteolytic enzymes away from important cellular proteins

membrane fusion

special property of the fluid mosaic model which states that membranes can fuse separately without losing continuity (no loss of contents)

fusion preserves the hydrophobic barrier while mixing membranes

vesicles can also be removed from the surface membrane

spontaneous fusion

can occur between lipid bilayers under favorable conditions

protein mediated diffusion

required for specific events- viral entry into host cell, neurotransmitter release, and vesicle trafficking

double membranes

can create an inner-membrane space useful for gatekeeping or energy storage

examples, mitochondria and E.coli inner membrane

mitochondria double membrane

serves as an energy reservoir—> proton gradient

Ecoli double membrane

outer membrane- often contains porin (beta barrel proteins) for diffusion

inner membrane- highly selective, houses energy conservation enzymes

chemical potential

depends on the concentration difference

electrical potential

depends on charge imbalance

electrochemical potential

combined force of an electrical difference and a chemical concentration difference across a membrane, which influences the movement of charged ions.

integral membrane proteins

span the bilayer—> facilating some kind of support

membrane proteins are either all helical or all beta sheet within the membrane

alpha helical proteins

dominate eukaryotic membranes (transports, receptors)

beta barrel proteins

found in bacterial outer membranes

helical

transport, signaling, and energy production

Trp and tyr in integral proteins

stabilize protein at the hydrophilic and hydrophobic interface because of their amphipathic side chains.

porins

beta sheet structures with a hole in the middle that allows small molecules to pass through

general functions of integral membrane proteins

solute transport, signal transduction, cell adhesion, and energy conversion

membrane protein insertion

membranes integrated into a membrane must be added to the membrane during translation

how do membrane proteins insert themselves onto the membrane

insert co translationally during synthesis on ribosomes bound to the rough ER

Sec 61

translocon acts as a protein- conducting channel, integrating hydrophobic transmembrane domains into the lipid bilayer.

transport for catalysis

integral membrane proteins catalyze diffusion of molecules across the membrane

polar molecules and ions don’t move spontaneously across the membrane because

unfavorable to desolvate or dehydrate them

how do transporters reduce the activation energy

provide polar binding sites that shield ions from the hydrophobic core

membrane proteins accelerate ion transport by how much

10^12 s^-1

simple diffusion

small nonpolar molecules down the concentration gradient

facilitated diffusion

down the the electrochemical gradient

• large nonpolar molecules= glucose

• amino acids

• ions

primary active transport

uses ATP hydrolysis against the electrochemical gradient

• Na+/K+ ATPase

• SERCA

secondary active transport

against electrochemical gradient driven ion moving against gradient

• lac permease

• Na+/glucose transporter

channel proteins

show fast diffusion because of continuous pores

carrier proteins/transport proteins

slow diffusion because a conformational change must happen when a ligand binds to the protein and behave like Michaels- Menten like saturation

glucose uptake in intestinal cells

• Na+glucose symport into cell (secondary active)

• GLUT2-mediated exit (uniport) (facilitated diffusion)

• Na+/K+ ATPase resets Na+ gradient (primary active)

glucose transporter structure

• GLUT proteins contain 12 alpha helicies forming a central cavity for glucose to bind to

• the rocker switch mechanism alternates between outward-open- and inward open states exposing the binding site one side at a time

• Trp and Tyr at interface stabilize helix orientation and anchor the transporter in the membrane.