2.1: ion channels

summary

LO:

  • membrane transport proteins regulating flux

  • ion channels

  • channels vs transporters

  • passive vs active transport

  • ion channel measurement

pre-lec

the resting membrane potential

  • ion channels: on membrane, only allows one type of ion

  • negative membrane potential = cell is negative on the inside

  • mostly dictated by movement of K

    • K has the largest membrane permeability

    • some K channels are open at rest when others are closed


membrane potential: a general term for the electric potential difference across the membrane at any time

resting potential: the membrane potential when the cell is at rest


  1. In the example presented in the video, the net movement of K+out of the cell will stop when

Ā Ā Ā Ā the driving force from the concentration gradient is equal in magnitude and opposite in direction to the driving force from the membrane potential

  • Kāŗ starts high inside ā†’ low outside, so its chemical (concentration) gradient pushes it out of the cell.

  • As Kāŗ leaves, the inside becomes more negative.

  • This negative charge creates an electrical gradient that pulls Kāŗ back in.

āš– When do they balance?

Net movement stops when:

Chemical driving force (outward) = Electrical driving force (inward)

Equal magnitude, opposite direction ā†’ no net flux

  1. Does water feel a driving force to cross the membrane created by the resting membrane potential?

Ā Ā Ā Ā no, water is neutral - no charge

  1. Imagine a scenario where the starting ion concentrations were the same as in real life (K+ concentration high inside the cell, Na+ and Cl- high outside the cell), but no potassium channels were open at rest. Instead, a small number of Cl- channels were open in the resting state, giving this ion the highest membrane permeability. The resting potential of the cell would be:

Ā Ā Ā Ā negative

channels and transporters

  • used when solutes canā€™t easily cross membrane on their own

  • ion channels vs transporters

  • channels (passive)

    • sometimes contains a continuous pathway across membrane (like a non stop river)

    • voltage-gated: voltage signals to open or close

    • ligand-gated: extracellular and intracellular

    • mechanically gated

  • transporter (usually active where an energy source is required to activate)

    • no continuous pathway

    • bind first, change conformationally

    • slower transport

    • transported mediated (passive)

    • coupled transported

    • atp-driven pump

    • light-driven pump


  1. Membrane transporters sometimes contain a continous water filled pore from one side of the membrane to the other

true

  1. Which of the following membrane protein families are capable of facilitating active transport of solutes?

light driven pumps, secondary active transporters, ATP driven pumps

  1. Ion channels are grouped into families according to:

what stimulates the channel to open or close, what kind of ion they transport


myotonia congenita and channelopathy

  • congenital: present from birth

  • affects 1 in 100,000 people worldwide

  • over 80 different mutations that can cause the disorder

  • severity of symptoms can vary greatly between individuals and throughout their life

  • symptoms can often be treated


symptoms

  • delayed relaxation of skeletal muscle after contraction

  • spontaneous paralysis or muscle contracction

  • myotonia means ā€˜muscle stiffnessā€™


causes

  • membrane potential changes

  • positive ion influx ā†’ membrane depolarisation (action potential) ā†’ muscle contraction


channelopathy

  • disease caused by defective ion channels

  • ion channels regulate electrical activity of cells

  • usually altering membrane potentials

  • e.g.myatonia (faulty Cl or Na channels)

    • epilepsy

transport proteins

  • passive

    • energy not required

    • solutes move down the gradient

  • active

    • energy required to activate

    • solutes move against the gradient


channels vs transporters

measuring the action potential

sodium ions contribute to action potential

  • action potential propagation has a dependency for Na

  • varying Na concentration in solution

measuring electrical properties of a cell

  1. membrane potential measurement

    1. apply a stimulus/shock/current

    2. measure the membrane potential (voltage of the cell)

  2. voltage clamp (current measurement)

    1. fix the membrane potential

    2. measure the current flowing across the membrane (i.e. through channels)