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H⁺ Uptake and Extrusion by Nitella clavata¹

Department of Biological Sciences, State University College, Brockport, New York 14420

Very high rates of H⁺ extrusion by internodal cells of Nitella clavata Kutz were measured after acid loading at pH 4.6. The highest rate observed, 160 picomoles per square centimeter per second, was more than twice the rate of photosynthetic bicarbonate utilization under saturating light. These results are consistent with the recently proposed hypothesis that bicarbonate is not taken in directly but is protonated at the exterior surface; the CO₂ thereby formed diffuses preferentially into the cell because of the asymmetric concentration gradient.

The H⁺ taken up, about 150 nanomoles per square centimeter in 2 hours, was distributed in three fractions: 30% in the cell wall, 40% in the cytoplasm, and 30% in the vacuole. This was concluded from the kinetics of the H⁺ release by intact cells and isolated walls, and from the pH decrease of the vacuolar sap.

The cytoplasmic H⁺ was extruded rapidly, with a half-time of about 2 minutes when the external pH was 5.7 or higher. The extrusion of the vacuolar H⁺ only proceeded at a measurable rate when the [K⁺] in the medium was raised to 20 millimolar; the half-time was about 100 minutes. There was little H⁺ extruded when the external pH was 5.0.

¹ Supported by grant PCM 75-10106 from the National Science Foundation.