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#cell physiology

2 public questions tagged with this topic.

Membrane Transportation

Which of the following is a voltage-gated sodium channel subunit responsible for sensing membrane potential?

Voltage-gated sodium channels that drive rapid upstroke of action potentials are built from one polypeptide folded into four homologous repeats, each repeat having six transmembrane helices S1 to S6. S1 through S4 cluster peripherally as the voltage-sensing domain, while S5, S6 and the intervening loop dip back into the membrane as P-loop to create the ion conducting pore. The hallmark of voltage sensing resides in S4, which exhibits a repeating pattern Arg-X-X where positively charged arginine or lysine appears every third residue, flanked by hydrophobic side chains. These gating charges sit inside a water-filled crevice stabilized by acidic countercharges in S1 to S3 and a hydrophobic plug. At resting negative interior potential electrostatic attraction holds S4 inward. Depolarization weakens this force, permitting S4 to move outward about 10 to 12 angstroms via a helical screw or sliding helix mechanism, carrying three to four elementary charges across the field. This movement drags the short S4-S5 linker, levering S6 helices apart and opening the intracellular activation gate, allowing sodium influx. S1-S3 and P-loop contribute stabilization and selectivity respectively, but charge translocation and voltage detection originate from S4.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 11: Voltage-Gated Ion Channels and Electrical Properties of Membranes.

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Membrane Transportation

Which ion has the highest intracellular concentration in most animal cells?

Intracellular ion composition is highly asymmetric relative to extracellular fluid actively maintained using ATP to support excitability volume and enzyme function. In most mammalian cells at rest dominant intracellular cation is potassium one thirty to one forty millimolar inside versus four to five outside ten to thirtyfold gradient while sodium inverse ten to fifteen inside versus one forty five outside chloride four to twenty inside versus one ten outside bicarbonate ten inside versus twenty five outside free calcium extremely low one hundred nanomolar inside versus one point two millimolar outside twelve thousandfold plus magnesium half millimolar free. Asymmetry established primarily by continuous Na+/K+ ATPase exporting three sodium importing two potassium generating chemical and electrical gradients plus NKCC importing K+, chloride and sodium channels like Kir Kir4.1 leaking potassium to set resting potential close to potassium Nernst minus ninety millivolts. High potassium required for sixty S ribosomal peptidyl transferase pyruvate kinase maintaining negative potential counterbalancing anionic proteins buffering pH via K+/H+ exchange preventing apoptosis. Reduction triggers caspase activation and cell shrinkage during apoptosis.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 11: Intracellular Ion Composition – High K+.

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