ion channel

Examples of ion channel in the following topics:

• Nerve Impulse Transmission within a Neuron: Resting Potential

  • To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane.
  • Ion channels have different configurations: open, closed, and inactive .
  • Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell.
  • Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels.
  • Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell.

• Transduction and Perception

  • Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential .
  • (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane.
  • When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell.
  • (b) Stereocilia in the human ear are connected to mechanosensitive ion channels.
  • When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.

• Types of Receptors

  • There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
  • Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through.
  • Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions.
  • The activated G-protein then interacts with either an ion channel or an enzyme in the membrane.
  • Gated ion channels form a pore through the plasma membrane that opens when the signaling molecule binds.

• Synaptic Transmission

  • Na+ ions enter the cell, further depolarizing the presynaptic membrane.
  • This depolarization causes voltage-gated Ca2+ channels to open.
  • Calcium ions entering the cell initiate a signaling cascade.
  • The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open.
  • The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron.

• Nerve Impulse Transmission within a Neuron: Action Potential

  • When neurotransmitter molecules bind to receptors located on a neuron's dendrites, voltage-gated ion channels open.
  • As K+ ions leave the cell, the membrane potential once again becomes negative.
  • Eventually, the extra K+ ions diffuse out of the cell through the potassium leakage channels, bringing the cell from its hyperpolarized state back to its resting membrane potential.
  • The flow of ions through these channels, particularly the Na+ channels, regenerates the action potential over and over again along the axon.
  • At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell.

• Chemiosmosis and Oxidative Phosphorylation

  • Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient.
  • The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient) owing to the hydrogen ions' positive charge and their aggregation on one side of the membrane.
  • If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient.
  • However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
  • At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions.

• Facilitated transport

  • Similarly, a gated channel protein often remains closed, not allowing substances into the cell until it receives a signal (like the binding of an ion) to open.
  • However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane.
  • The attachment of a particular ion to the channel protein may control the opening or other mechanisms or substances may be involved.
  • In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues, a gate must be opened to allow passage.
  • Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney.

• Secondary Active Transport

  • Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
  • As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created.
  • If a channel protein exists and is open, the sodium ions will be pulled through the membrane.
  • This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.
  • The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.

• Transport of Electrolytes across Cell Membranes

  • Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
  • Salt and other compounds that dissociate into their component ions are called electrolytes.
  • In water, sodium chloride (NaCl) dissociates into the sodium ion (Na+) and the chloride ion (Cl–).
  • The mechanisms that transport ions across membranes are facilitated diffusion and active transport.
  • Facilitated diffusion of solutes occurs through protein-based channels.

• Regulatory Proteins

  • Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax.
  • A change in the receptor conformation causes an action potential, activating voltage-gated L-type calcium channels, which are present in the plasma membrane.
  • The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum.
  • It is not understood whether the physical opening of the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open.
  • Cross-bridge cling continues until the calcium ions and ATP are no longer available.