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Cytoplasmic hydrogen ion diffusion coefficient.

N F al-Baldawi1, R F Abercrombie

  • 1Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322.

Biophysical Journal
|June 1, 1992
PubMed
Summary

This study measured how quickly protons (hydrogen ions) move in the cytoplasm of a marine invertebrate neuron. Using pH electrodes and cytoplasm samples, the researchers found that proton diffusion is much slower than in free solution. At acidic and neutral pH, the diffusion coefficient was about 1.4 × 10⁻⁶ cm²/s, which is five times lower than mobile pH buffers and 68 times lower than hydronium ions. At higher pH, the coefficient increased to about 4.1 × 10⁻⁶ cm²/s, likely due to more active buffering by amino acids. The study also found that buffering power is split into mobile (15%) and immobile (85%) components, which explain the pH-dependent changes in diffusion rates. These findings help clarify how pH and buffering influence proton movement in cells.

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Area of Science:

  • Cellular physiology
  • Acid-base regulation
  • Biophysics of ion transport

Background:

Understanding proton movement in cytoplasm is central to cellular function. Prior research has shown that proton diffusion is influenced by pH buffers and ion mobility. However, the exact rate of proton diffusion in cytoplasm remains unclear. This gap motivated the need for direct measurements in a controlled system. The marine invertebrate giant neuron offers a unique model for such studies. Known knowledge suggests that proton diffusion is slower than in free solution due to buffering. But the extent of this slowdown had not been quantified in acidic to neutral pH ranges. No prior work had resolved how buffering power changes with pH. This uncertainty drove the current investigation into cytoplasmic proton diffusion.

Purpose Of The Study:

The aim of this study was to measure the apparent cytoplasmic proton diffusion coefficient in a marine invertebrate neuron. The specific problem addressed was the lack of direct measurements of proton diffusion in cytoplasm under varying pH conditions. The motivation was to determine how pH affects proton diffusion and buffering. The study sought to clarify whether mobile and immobile buffers influence diffusion rates. The researchers focused on comparing proton diffusion with that of hydronium and mobile buffers. The goal was to quantify the role of buffering power in altering diffusion coefficients. This work aimed to provide a baseline for future studies on cytoplasmic ion transport. The findings could help refine models of intracellular acid-base regulation.

Keywords:
proton diffusion in cytoplasmpH buffering in cellsion transport in neuronsmarine invertebrate physiology

Frequently Asked Questions

The study found that the apparent cytoplasmic proton diffusion coefficient is 1.4 ± 0.5 × 10⁻⁶ cm²/s at acidic and neutral pH, significantly lower than hydronium ion diffusion.

The researchers used pH electrodes and cytoplasm samples from a marine invertebrate neuron, applying sudden pH changes and measuring pH relaxation.

The coefficient increases at alkaline pH due to enhanced buffering power of mobile amino acids, which facilitates proton movement.

Mobile buffers (15% buffering power) and immobile buffers (85%) influence diffusion rates, with mobile buffers becoming more active at higher pH.

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Main Methods:

The study used pH electrodes and cytoplasm samples from a marine invertebrate giant neuron. The researchers applied sudden pH changes at one surface of the sample. They recorded the pH relaxation within the sample over time. This method allowed them to calculate the apparent diffusion coefficient. The experiments were conducted at acidic, neutral, and alkaline pH ranges. The mobile and immobile buffering power was estimated from the data. The diffusion coefficient of hydronium ions was used as a reference. The results were compared to known values for mobile pH buffers. This approach enabled the researchers to isolate the effects of buffering on proton diffusion. The setup minimized interference from hydroxide movement in alkaline conditions.

Main Results:

The apparent proton diffusion coefficient was measured at 1.4 ± 0.5 × 10⁻⁶ cm²/s in acidic and neutral pH (6.0–7.2). This value is about five times lower than that of mobile pH buffers (8 × 10⁻⁶ cm²/s). It is also about 68 times lower than the hydronium ion diffusion coefficient (93 × 10⁻⁶ cm²/s). At alkaline pH (8.2–8.6), the coefficient increased to 4.1 ± 0.8 × 10⁻⁶ cm²/s. This increase was attributed to enhanced buffering by mobile amino acids. The buffering power was split into 15% mobile and 85% immobile components. These proportions accounted for the observed pH-dependent changes in diffusion. The data suggest that buffering power is a key factor in proton diffusion rates.

Conclusions:

The study found that cytoplasmic proton diffusion is significantly slower than in free solution. The diffusion coefficient at acidic and neutral pH is about five times lower than mobile buffers. At alkaline pH, the coefficient increases due to enhanced buffering by amino acids. The researchers propose that mobile and immobile buffers account for the observed pH dependence. The immobile buffer contributes most to buffering power in acidic and neutral pH. The mobile buffer becomes more active at higher pH, increasing diffusion rates. These findings align with the hypothesis that buffering power influences proton movement. The results suggest that hydroxide movement does not significantly affect the measured coefficients.

The cytoplasmic coefficient is about 68 times lower than that of hydronium ions (93 × 10⁻⁶ cm²/s).

The findings suggest that buffering power is a key determinant of proton diffusion rates, which may influence intracellular pH regulation models.