ion

(noun)

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.

Related Terms

Examples of ion in the following topics:

  • Ion Channels

    • Ion channels are membrane proteins that allow ions to travel into or out of a cell.
    • Most channels are specific (selective) for one ion.
    • When a channel is open, ions permeate through the channel pore down the transmembrane concentration gradient for that particular ion.
    • Ion pumps are not ion channels, but are critical membrane proteins that carry out active transport by using cellular energy (ATP) to "pump" the ions against their concentration gradient.
    • A schematic representation of an ion channel.

  • Body Fluid Composition

    • In contrast to extracellular fluid, cytosol has a high concentration of potassium ions and a low concentration of sodium ions.
    • The reason for these specific sodium and potassium ion concentrations are Na+/K ATPase pumps, which facilitate the active transport of these ions.
    • These pumps transport ions against their concentration gradients to maintain cytosol fluid composition of ions.
    • These ions are important for water transport throughout the body.
    • Some of the electrolytes present in the transcellular fluid are sodium ions, chloride ions and bicarbonate ions.

  • Membrane Potentials as Signals

    • The membrane serves as both an insulator and a diffusion barrier to the movement of ions.
    • Ion transporter/pump proteins actively push ions across the membrane to establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients, a process known as facilitated diffusion.
    • The opening and closing of ion channels can induce a departure from the resting potential.
    • The changes in membrane potential can be small or larger (graded potentials) depending on how many ion channels are activated and what type they are.
    • Action potentials are generated by the activation of certain voltage-gated ion channels.

  • Mechanism and Contraction Events of Cardiac Muscle Fibers

    • In cardiac muscle, ECC is dependent on a phenomenon called calcium-induced calcium release (CICR), which involves the influx of calcium ions into the cell, triggering further release of ions into the cytoplasm.
    • The mechanism for CIRC is receptors within the cardiomyocyte that bind to calcium ions when calcium ion channels open during depolarization, releasing more calcium ions into the cell.
    • Similarly to skeletal muscle, the influx of sodium ions causes an initial depolarization; however, in cardiac muscle, the influx of calcium ions sustains the depolarization so that it lasts longer.
    • CICR creates a "plateau phase" in which the cell's charge stays slightly positive (depolarized) briefly before it becomes more negative as it repolarizes due to potassium ion influx.
    • As the action potential travels between sarcomeres, it activates the calcium channels in the T-tubules, resulting in an influx of calcium ions into the cardiomyocyte.

  • Ionotropic and Metabotropic Receptors

    • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
    • Ionotropic receptors are a group of transmembrane ion channels that open or close in response to the binding of a chemical messenger (ligand) such as a neurotransmitter.The binding site of endogenous ligands on LGICs protein complexes are normally located on a different portion of the protein (an allosteric binding site) than the location of the ion conduction pore.The ion channel is regulated by a ligand and is usually very selective to one or more ions such as Na+, K+, Ca2+, or Cl-.
    • The prototypic ligand-gated ion channel is the nicotinic acetylcholine receptor .
    • This pore allows Na+ ions to flow down their electrochemical gradient into the cell.
    • With a sufficient number of channels opening at once, the inward flow of positive charges carried by Na+ ions depolarizes the postsynaptic membrane enough to initiate an action potential.

  • Carbon Dioxide Transport

    • The majority (85%) of carbon dioxide travels in the blood stream as bicarbonate ions.
    • The reaction that describes the formation of bicarbonate ions in the blood is:
    • This means that carbon dioxide reacts with water to form carbonic acid, which dissociates in solution to form hydrogen ions and bicarbonate ions.
    • Bicarbonate ions act as a buffer for the pH of blood so that blood pH will be neutral as long as bicarbonate and hydrogen ions are balanced.
    • Bicarbonate ions dissolved in the plasma enter the red blood cells by diffusing across a chloride ion gradient (replacing chloride inside the cell), and combining with hydrogen to form carbonic acid.

  • Tubular Reabsorption

    • Reabsorbed fluids, ions, and molecules are returned to the bloodstream through the peri-tubular capillaries, and are not excreted as urine.
    • Water can follow other molecules that are actively transported, particularly glucose and sodium ions in the nephron.
    • As filtrate passes through the nephron, its osmolarity (ion concentration) changes as ions and water are reabsorbed.
    • The filtrate osmolarity drops to 1200 mOsm/L as water leaves through the descending loop of henle, which is impermeable to ions.
    • In the ascending loop of henle, which is permeable to ions, but not water, osmolarity falls to 100-200 mOsm/L. finally in the distal convoluted tubule and collecting duct, a variable amount of ions and water are reabsorbed depending on hormonal stimulus.

  • Principles of Electricity

    • In the body, electrical currents reflect the flow of ions across cell membranes.
    • Since there is a slight difference in the number of positive and negative ions on the two sides of the cellular plasma membrane, there is a potential difference across the membranes.
    • Differences in concentration of ions on opposite sides of a cellular membrane lead to a voltage called the membrane potential.
    • Many ions have a concentration gradient across the membrane, including potassium (K+), which is at a high inside and a low concentration outside the membrane.
    • Sodium (Na+) and chloride (Cl–) ions are at high concentrations in the extracellular region and low concentrations in the intracellular regions.

  • Resting Membrane Potentials

    • Most of the time, the difference in ionic composition of the intracellular and extracellular fluids and difference in ion permeability generates the resting membrane potential difference.
    • It is based on the charges of the ions in question, as well as the difference between their inside and outside concentrations and the relative permeability of the plasma membrane to each ion where:
    • The three ions that appear in this equation are potassium (K+), sodium (Na+), and chloride (Cl−).
    • The Goldman formula essentially expresses the membrane potential as an average of the reversal potentials for the individual ion types, weighted by permeability.

  • Chemoreceptor Regulation of Breathing

    • Chemoreceptors detect the levels of carbon dioxide in the blood by monitoring the concentrations of hydrogen ions in the blood.
    • The respiratory chemoreceptors work by sensing the pH of their environment through the concentration of hydrogen ions.
    • As bicarbonate levels decrease while hydrogen ion concentrations stays the same, blood pH will decrease (as bicarbonate is a buffer) and become more acidic.
    • In cases of acidosis, feedback will increase ventilation to remove more carbon dioxide to reduce the hydrogen ion concentration.
    • Conversely, vomiting removes hydrogen ions from the body (as the stomach contents are acidic), which will cause decreased ventilation to correct alkalosis.