The Effect of pH on the Availability of Acid Substrates for Polytomella caeca with Unexpected Activity by Succinate*

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Abstract
SYNOPSIS. Membrane permeability of Polytomella caeca for acid substrates was examined by determining inhibition of growth by high concentrations of sole carbon sources in a pH range of 2.5–7.5, growth stimulation by compounds which are inadequate as sole carbon sources, and respirometry of these compounds at various pH levels. Fatty acid penetration appeared to be based on lipid solubility. Succinate, however, was better absorbed when ionized and therefore less lipid soluble. There was an indication that pyruvate and α‐ketoglutarate may be more easily absorbed as ions also. Ethanol, glyceraldehyde, pyruvate, lactate, α‐ketoglutarate, succinate and malate stimulated oxygen uptake at different pH values. Fumarate was inactive in both growth and respiration experiments.

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