L2 Membrane Transport I

define macrotransfer

large scale movement of molecules

define endocytosis

membrane wraps around stuff and brings it into the cell

define exocytosis

vesicle fuses with membrane to release substances

define microtransfer

smaller scale movement (cell membrane transport of small molecules and ions)

what is vesicular transport involved in

the movement of proteins into and out of the cell

what hydrophobic molecules have high plasma membrane permeability

O2, CO2, N2 and steroid hormones

what small uncharged polar molecules have some plasma membrane permeability

H2O, Urea, Glycerol

what large polar molecules have little plasma membrane permeability

glucose, sucrose

what ions have low plasma membrane permeability

H+, Na+, K+, Ca2+, Cl-, Mg2+

what is the relative plasma membrane permeability of hydrophobic molecules

high

what is the relative plasma membrane permeability of small uncharged polar molecules

relatively high

what is the relative plasma membrane permeability of large polar molecules

relatively low

what is the relative plasma membrane permeability of ions

low

what is the resting membrane potential determined by

the distribution of ions across the membrane

what is the resting membrane potential of ions mostly due to

Na+ and K+

what is the resting membrane potential related to

equilibrium potential (determined by the Nernst equation)

what do ions need for transport across the membrane

membrane proteins

what is the artificial semi-permeable membrane only permeable to

K+

what does the artificial semi-permeable membrane measure charge across

the membrane

what does 3+ve and 3-ve ions on each side of an artificial semi-permeable membrane suggest

no charge difference across the membrane (and no concentration difference)

what is the Nernst equation

what is Em

membrane potential

what is z

number of charges on ion

what is F

Faraday’s number (96485.3 C.mol-1)

what is [X]o

conc of ion outside

what is [X]i

conc of ion inside

what is the ICF of Na+ in the Skeletal muscle

12mM

what is the ICF of K+ in the Skeletal muscle

155mM

what is the ICF of Cl- in the Skeletal muscle

4.2mM

what is the ECF of Na+ in skeletal muscle

145mM

what is the ECF of K+ in skeletal muscle

4.5mM

what is the ECF of Cl- in skeletal muscle

116mM

what is the ENa in skeletal muscle

+67mV

what is the EK in the skeletal muscle

-95 mV

what is the ECl in the skeletal muscle

-89 mV

what does the Nernst equation predict

equilibrium membrane potential based on concentration of that ion across the membrane

what is concentration of that ion across the membrane

Nernst potential

what is the concentration gradient maintained by

Na+/K+ -ATPase

what is the membrane permeable to

multiple ions

how can the membrane potential of multiple ions be calculated

the GHK equation

what is the GHK equation

what does the GHK membrane take into account

all ions and their retrospective permeabilities

what is the resting membrane potential generated from

the asymmetrical distribution of ions

what ions is the resting membrane potential generated from

Na+ and K+

what if permeability of the membrane is higher for K+ than Na+?

membrane potential will be closer to the equilibrium potential for K+

what will the membrane potential be if the permeability of the membrane is higher for K+ than Na+

about -70mV

• EK= -95mV

what do non-excitable cells have a K:Na permeability ratio of

2:1

what do nerve/muscle cells have a K:Na permeability ratio of

25:1

what is the Nernst-Goldman calculator

what is the voltage clamp

most powerful electrophysiological method for basic research

what does the voltage clamp allow

detailed measurement and analysis of electrical activity across a tissue, cell or artificial membrane that is mediated by specialised ion channels and electrogenic carriers

what can the voltage clamp, combined with molecular biology provide

fundamental information on structure, function and regulation of transport proteins

what is voltage controlled by

an electronic feedback circuit

how is the voltage clamp set up

voltage is stepped (in a preset pattern) and current is required to hold voltage as each step is measured

• current is equivalent to the total ionic current flowing across the membrane

what does the current clamp circuit control

amplitude of injected current via a microelectrode and allows the voltage to vary

what does the amplifier in a current clamp record

voltage generated by the cell

what is the current clamp used to study

how a cell responds when electric current enters a cell

give an example of how a cell responds when electric current enters a cell

how a neuron responds to neurotransmitters that act by opening ion channels

what is the method of the voltage clamp

• investigator sets holding voltage (command potential)

• voltage clamp uses negative feedback to maintain the cell at this command potential

• ion channels open/close as normal, but apparatus compensates for changes in current to maintain a constant membrane potential

who was the voltage clamp first developed by

Cole and Marmont in 1930s/40s

who was the voltage clamp further developed by

Alan Hodgkin and Andrew Huxley in the 1950s

what was the voltage clamp first used in

a squid giant axon

how long is the squid giant axon

1mm

in the patch clamp, what does the recording pipette physically isolate

a patch of membrane on the cell surface

what can the patch clamp electrically ‘clamp’

the potential across the ‘patch’ to measure current flow through a single ion channel

who was the patch clamp invented by

Neher and Sakmann in 1976

who further refined the patch clamp

Hamill et al. in 1981

what can the patch clamp record the activity of

• single ion channels

• measure whole-cell currents

what does the patch clamp form

a high resistance seal (giga-seal > 10Gomega) between membrane and micropipette

what does the giga-seal of the patch clamp mean

there is current flowing through ion channel in the patch that can be recorded with minimal noise

why is it important for there not to be noise in the patch clamp

single-channel currents are tiny (pA or less) so can easily be swamped by background electrical noise

what is movement across the membrane directly through the lipid bilayer through

passive (simple) diffusion

what does passive (simple) diffusion obey

Fick’s law of diffusion

give an example of passive (simple) diffusion obeying Fick’s law of diffusion

O2 from alveoli to pulmonary capillaries

what is movement across the membrane via integral membrane proteins through

• facilitated diffusion

• active transport

• secondary active transport

describe facilitated diffusion (via integral membrane proteins)

through pores, channels and carriers (uniports)

describe active transport (via integral membrane proteins)

energy (ATP) is required to transport across the membrane

what are co-transporters (symporters) as an example of secondary active transport (via integral membrane proteins)

movement of a solute coupled to the movement of another down its concentration gradient

what are counter-transporters (antiporters) as an example of secondary active transport (via integral membrane proteins)

coupled movement of two or more solutes in opposite directions

what is another word for symporter

co-transporter

what is another word for antiporter

counter-transporter

what is passive (non-coupled) transport of

solute/gas passes down conc gradient

what happens at finish of passive (non-coupled) transport

inward flux = outward flux

• net flux = 0

if a substance can pass through a membrane, that membrane is said to be…

permeable

• the substance is ‘permeant’

what is the driving force in passive (simple) diffusion

the electrochemical gradient = sum of chemical energy differences as well as charge differences

what is J

flux

how do we calculate flux

permeability x concentration difference

how do we calculate permeability (P)

diffusion coefficient (D)_{K (partition coefficient)/X (distance)}

what is the diffusion coefficient (D) a measure of

the size of the diffusion area at a given timepoint

what is the partition coefficient (K)

how easily a substance crosses a membrane

what are the three types of transporters that facilitate diffusion

• channels (non-gated)

• gated channels

• uniporters

what do transporters all facilitate

uncoupled transport of a solute down a concentration gradient

what are non-gated channels

integral membrane proteins that allow direct access to the cell

give examples of non-gated channels

• porins in bacteria, mitochondria, nuclear pore complex

• aquaporins

what is ENaC

epithelial sodium channel

give examples of gated channels

• ENaC

• K+ channels

• Ca2+ channels

• almost all ion channels

what are the functional components of gated channels

• gate

• sensor (detects signal to open gates)

• selectivity filter (stops other substances)

what are gated channels modulated by

• voltage

• mechanical stimuli

• ligand binding

what does carrier-mediated (uniport) facilitated diffusion include

• GLUT1

• GLUT2

• GLUT4

where is GLUT 1 in carrier-mediated (uniport) facilitated diffusion

Red blood cells