Ion Pumps and Ion Channels

CHAPTER 48 SECTION 2

Overview

All cells have membrane potential across their plasma membrane Membrane potential is the difference in electrical charge between the

inside and the outside of a membrane

The membrane potential of a resting neuron—one that is not sending signals—is its resting potential

Formation of the Resting Potential

Potassium ions and sodium ions play critical roles in the formation of the resting potential. For each there is a concentration gradient across the plasma membrane of a neuron.

In mammalian neurons there is more potassium inside the cell and more sodium outside the cell.

Sodium-potassium pumps maintain these gradients These pumps are used for ATP hydrolysis

These pumps use active transport

These pumps are ion channels

Ion Channels

Ion Channels- pores formed by clusters of specialized proteins that span the membrane

The concentration gradients represent a chemical form of potential energy for the neuron

In order to change the chemical potential into electrical potential ion channels are needed

As the ions diffuse through the channels they carry with them an electrical charge

The net movement of positive or negative charge will generate a voltage, or potential, across the membrane

Ion channels continued

Ion channels have selective permeability, meaning that they allow only certain ions to pass

A resting neuron has a lot of open potassium channels but few open sodium channels

The K+ ions can flow out of the cell but the ion channels do not allow anions such as Cl- into the cell, therefore there is a negative buildup inside the cell causing membrane potential

The negative charge keeps the positive ions inside the cell

Modeling of Resting Potential

When the electrical gradient exactly balances the chemical gradient the model neuron has reached equilibrium

An ion’s equilibrium potential is the magnitude of the membrane voltage at equilibrium for that particular ion

To calculate equilibrium potential we use the Nernst equation

Eion = 62mV(log([ionoutside]/[ioninside]))