Intracellular pH

thumb|pH gradient across a membrane, with protons traveling through a transporter embedded in the membrane.|alt=|200x200px

Intracellular pH (pH_i) is the measure of the acidity or basicity (i.e., pH) of intracellular fluid. The pH_i plays a critical role in membrane transport and other intracellular processes. In an environment with the improper pH_i, biological cells may have compromised function. Therefore, pH_i is closely regulated in order to ensure proper cellular function, controlled cell growth, and normal cellular processes. There is also pH variation across different organelles, which can span from around 4.5 to 8.0. pH_i can be measured in a number of different ways.

Homeostasis

Intracellular pH is typically lower than extracellular pH due to lower concentrations of HCO₃⁻. A rise of extracellular (e.g., serum) partial pressure of carbon dioxide (pCO₂) above 45 mmHg leads to formation of carbonic acid, which causes a decrease of pH_i as it dissociates:

: H₂O + CO₂ H₂CO₃ H⁺ + HCO₃^–

Since biological cells contain fluid that can act as a buffer, pH_i can be maintained fairly well within a certain range. Cells adjust their pH_i accordingly upon an increase in acidity or basicity, usually with the help of CO₂ or HCO₃^– sensors present in the membrane of the cell.

Major intracellular buffer systems include those involving proteins or phosphates. Since the proteins have acidic and basic regions, they can serve as both proton donors or acceptors in order to maintain a relatively stable intracellular pH. In the case of a phosphate buffer, substantial quantities of weak acid and conjugate weak base (H₂PO₄^– and HPO₄^2–) can accept or donate protons accordingly in order to conserve intracellular pH:

:OH^– + H₂PO₄^– H₂O + HPO₄^2–

:H⁺ + HPO₄^2– H₂PO₄^–

In organelles

alt=|thumb|207x207px|Approximate pHs of various organelles within a cell. Since these are both degradative organelles that engulf and break down other substances, they require high internal acidity in order to successfully perform their intended function. Since [[oxidative phosphorylation must occur inside the mitochondria, this pH discrepancy is necessary to create a gradient across the membrane. This membrane potential is ultimately what allows for the mitochondria to generate large quantities of ATP.

thumb|Protons being pumped from the mitochondrial matrix into the intermembrane space as the electron transport chain runs, lowering the pH of the intermembrane space.

Measurement

There are several common ways in which intracellular pH (pH_i) can be measured including with a microelectrode, dye that is sensitive to pH, or with nuclear magnetic resonance techniques.

Overall, all three methods have their own advantages and disadvantages. Using dyes is perhaps the easiest and fairly precise, while NMR presents the challenge of being relatively less precise.

Fluorescence spectroscopy

Another way to measure Intracellular pH (pH_i) is with dyes that are sensitive to pH, and fluoresce differently at various pH values. This technique, which makes use of fluorescence spectroscopy, consists of adding this special dye to the cytosol of a cell.

Using NMR Spectroscopy, it has been determined that lymphocytes maintain a constant internal pH of 7.17± 0.06, though, like all cells, the intracellular pH changes in the same direction as extracellular pH.

pH-sensitive GFPs

To determine the pH inside organelles, pH-sensitive GFPs are often used as part of a noninvasive and effective technique. If certain GFP mutants that are highly sensitive to pH in intracellular environments are used in these experiments, the relative amount of resulting fluorescence can reveal the approximate surrounding pH.

References