What is the term for the electrical charge difference between the inside and outside of the cell?

What is the term for the electrical charge difference between the inside and outside of the cell?
This page describes how neurons work. I hope this explanation does not get too complicated, but it is important to understand how neurons do what they do. There are many details, but go slow and look at the figures.

What is the term for the electrical charge difference between the inside and outside of the cell?
Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to help the squid move. How giant is this axon? It can be up to 1 mm in diameter - easy to see with the naked eye.

Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable.

Resting Membrane Potential

What is the term for the electrical charge difference between the inside and outside of the cell?
A neuron is at rest when it is not sending an an electrical signal. During this time, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-) and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane.
What is the term for the electrical charge difference between the inside and outside of the cell?
In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

Action Potential

What is the term for the electrical charge difference between the inside and outside of the cell?

The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired - this is the "ALL OR NONE" principle.

What is the term for the electrical charge difference between the inside and outside of the cell?
Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.

And there you have it...the Action Potential

Did you know?
What is the term for the electrical charge difference between the inside and outside of the cell?
The giant axon of the squid can be 100 to 1000 times larger than a mammalian axon. The giant axon innervates the squid's mantle muscle. These muscles are used to propel the squid through the water.

Copyright © 1996-2020, Eric H. Chudler All Rights Reserved.

Membrane potential is what we use to describe the difference in voltage (or electrical potential) between the inside and outside of a cell.

Without membrane potentials human life would not be possible. All living cells maintain a potential difference across their membrane. Simply stated, membrane potential is due to disparities in concentration and permeability of important ions across a membrane. Because of the unequal concentrations of ions across a membrane, the membrane has an electrical charge. Changes in membrane potential elicit action potentials and give cells the ability to send messages around the body. More specifically, the action potentials are electrical signals; these signals carry efferent messages to the central nervous system for processing and afferent messages away from the brain to elicit a specific reaction or movement. Numerous active transports embedded within the cellular membrane contribute to the creation of membrane potentials, as well as the universal cellular structure of the lipid bilayer. The chemistry involved in membrane potentials reaches to many scientific disciplines. Chemically it involves molarity, concentration, electrochemistry and the Nernst equation. From a physiological standpoint, membrane potential is responsible for sending messages to and from the central nervous system. It is also very important in cellular biology and shows how cell biology is fundamentally connected with electrochemistry and physiology. The bottom line is that membrane potentials are at work in your body right now and always will be as long as you live.

The subject of membrane potential stretches across multiple scientific disciplines; Membrane Potential plays a role in the studies of Chemistry, Physiology and Biology. The culmination of the study of membrane potential came in the 19th and early 20th centuries. Early in the 20th century, a man named professor Bernstein hypothesized that there were three contributing factors to membrane potential; the permeability of the membrane and the fact that [K+] was higher inside and lower on the outside of the cell. He was very close to being correct, but his proposal had some flaws. Walther H. Nernst, notable for the development of the Nernst equation and winner of 1920 Nobel Prize in chemistry, was a major contributor to the study of membrane potential. He developed the Nernst equation to solve for the equilibrium potential for a specific ion. Goldman, Hodgkin and Katz furthered the study of membrane potential by developing the Goldman-Hodgkin-Katz equation to account for any ion that might permeate the membrane and affect its potential. The study of membrane potential utilizes electrochemistry and physiology to formulate a conclusive idea of how charges are separated across a membrane.

What is the term for the electrical charge difference between the inside and outside of the cell?

Figure 1. Differences in concentration of ions on opposite sides of a cellular membrane produce a voltage difference called the membrane potential. The largest contributions usually come from sodium (Na+) and chloride (Cl–) ions which have high concentrations in the extracellular region, and potassium (K+) ions, which along with large protein anions have high concentrations in the intracellular region. Calcium ions, which sometimes play an important role, are not shown.

In discussing the concept of membrane potentials and how they function, the creation of a membrane potential is essential. The lipid bilayer structure of the cellular membrane, with its lipid-phosphorous head and fatty acid tail, provides a perfect building material that creates both a hydrophobic and hydrophilic side to the cellular membrane. The membrane is often referred to as a mosaic model because of its semi-permeability and its ability to keep certain substances from entering the cell. Molecules such as water can diffuse through the cell based on concentration gradients; however, larger molecules such as glucose or nucleotides require channels. The lipid bilayer also houses the Na+/K+ pump, ATPase pump, ion transporters, and voltage gated channels, and it is the site of vesicular transport. The structure regulates which ions enter and exit to determine the concentration of specific ions inside of the cell.

Why is membrane potential essential to the survival of all living creatures?

Animals and plants require the breakdown of organic substances through cellular respiration to generate energy. This process, which produces ATP, is dependent on the electron transport chain. Electrons travel down this path to be accepted by oxygen or other electron acceptors. The initial electrons are obtained from the breakdown of water molecules. The hydrogen build up in the extracellular fluid leaving a gradient. As per membrane potentials, when there a gradient, the molecules flow in the opposite direction. In this case, hydrogen flows back into the cell through a protein known as ATP synthase which creates ATP in the process. This action is essential to life because the number of ATP created from each glucose increases drastically. Chemical disequilibrium and membrane potentials allow bodily functions to take place.

What is the term for the electrical charge difference between the inside and outside of the cell?

Figure 2

Transport proteins, more specifically the 'active' transport proteins, can pump ions and molecules against their concentration gradient. This is the main source of charge difference across the cellular membrane.

The following points should help you to understand how membrane potential works

  • The difference between the electrical and chemical gradient is important.
    • Electrical Gradient
      • Opposes the chemical gradient.
      • Represents the difference in electrical charge across the membrane
    • Chemical Gradient
      • Opposes the electrical gradient
      • Represents the difference in the concentration of a specific ion across the membrane.
    • A good example is K+. The membrane is very permeable to K+ and the [K+] inside the cell is great, therefore a positive charge is flowing out of the cell along with K+. The [K+] inside the cell decreases causing the concentration gradient to flow towards the outside of the cell. This also causes the inside of the cell to become more electronegative increasing its electrical gradient.
  • The Nernst equation can help us relate the numerical values of concentration to the electrical gradient.
  • Leak Channels
    • Channels that are always open
    • Permit unregulated flow of ions down an electrochemical gradient.
  • Na+/K+ ATPase Pump
    • Actively transports Na+ out of the cell and K+ into the cell.
    • Helps to maintain the concentration gradient and to counteract the leak channels.

Human nerve cells work mainly on the concept of membrane potentials. They transmit chemicals known as serotonin or dopamine through gradients. The brain receives these neurotransmitters and uses it to perform functions.

  • Na+ has a much higher concentration outside of the cell and the cell membrane is very impermeable to Na+
  • K+ has a high concentration inside the cell due to the fact that the cell membrane is very permeable to K+
  • A- is used to refer to large ions that are found completely inside of the cell and cannot penetrate the cell membrane.

Concentration (in Millimoles/ Liter) and permeability of Ions Responsible for Membrane Potential in a Resting Nerve Cell

ION Extracellular Intracellular Relative Permeability
Na+ 150 15 1
K+ 5 150 25-30
A- 0 65 0

Check out this YouTube video if you want to know more about how the Na+/K+ pump and how the membrane potential works. www.youtube.com/watch?v=iA-Gdkje6pg

The calculation for the charge of an ion across a membrane, The Nernst Potential, is relatively easy to calculate. The equation is as follows: (RT/zF) log([X]out/[X]in). RT/F is approximately 61, therefore the equation can be written as

(61/z) ln([X]out/[X]in)

  • R is the universal gas constant (8.314 J.K-1.mol-1).
  • T is the temperature in Kelvin (°K = °C + 273.15).
  • z is the ionic charge for an ion. For example, z is +1 for K+, +2 for Mg2+, -1 for F-, -1 for Cl-, etc. Remember, z does not have a unit.
  • F is the Faraday's constant (96485 C.mol-1).
  • [X]out is the concentration of the ion outside of the species. For example the molarity outside of a neuron.
  • [X]in is the concentration of the ion inside of the species. For example, the molarity inside of a neuron.

The only difference in the Goldman-Hodgkin-Katz equation is that is adds together the concentrations of all permeable ions as follows

(RT/zF) log([K+]o+[Na+]o+[Cl-]o /[K+]i+[Na+]i+[Cl-]i)

What is the term for the electrical charge difference between the inside and outside of the cell?

Figure 3. (Clockwise From Upper Left) 1) The charges are equal on both sides; therefore the membrane has no potential. 2)There is an unbalance of charges, giving the membrane a potential. 3) The charges line up on opposite sides of the membrane to give the membrane its potential. 4) A hypothetical neuron in the human body; a large concentration of potassium on the inside and sodium on the outside.

References

  1. Kaiser, Chris A., et al. Molecular Cell Biology. 6th ed. New York. W. H. Freeman, 2007.
  2. Orians, Gordon H., et al. Life: The Science of Biology. 8th ed. Gordonville, VA. Sinaver Associates, Inc., 2008.,
  3. Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. 9th ed. New Jersey. Pearson Education International, 2007.Fuel Cells#
  4. Sherwood, Lauralee. Human Physiology: From Cells to Systems (International Edition). International ed ed. New York: Brooks Cole, 2009. Print.
  5. Hietler, W.J.. "Membrane Potential Tutorial." St. Andrews Biology Dept.. St. Andrews University., 13 Aug. 2007. Web. 24 May 2010. <http:/http://www.st-andrews.ac.uk/~wjh/neurotut/

1. List the following in order from highest to lowest permeability. A-, K+, Na+

2. Which of the following statement is NOT true?

  1. The membrane potential usually requires a minimal difference of electrocharges across the membrane
  2. Membrane impermeability plays a role in membrane potentials.
  3. Membrane potential exists in all cellular structures, except for neurons.
  4. The active transports play a vital role in membrane potentials.

3. What would be the equilibrium potential for the ion K+ be if [K+]in= 5mM and [K+]in=150mM?

4. True or false: At resting membrane potential, the inside of the membrane is slightly negatively charged while the outside is slightly positively charged.

1. K+ > Na+ > A-

2. Answer c.)is not true; membrane potential exists in neurons and is responsible for action potential propagation in neurons.

3. Ek+ = (61/z) log([K+]out/[K+]in) = (61/1) log([5mM]/[150mM]) = -90mV

z=1

4. True. The resting membrane potential is negative as a result of this disparity in concentration of charges.

  • Dan Chong, Matt Klingler (UCD)