THE ACTIVE TRANSPORT OF PHOSPHATE INTO THE YEAST CELL

Phosphate can distribute in the cell wall space, but is not bound to an appreciable extent at the cell surface in non-metabolizing yeast. During metabolism of sugars, phosphate is actively transported into the yeast cell by a mechanism specifically involving glycolysis reactions. The movement of phosphate is in the inward direction only (no appreciable efflux), and it can proceed against a concentration gradient of 100 to 1. It is dependent on external phosphate concentrations in an asymptotic relationship, but is independent of the cellular orthophosphate concentration. The pH optimum for the phosphate uptake of 6.5 is shifted to the acid side by potassium. At certain values of pH a stimulation of 700 per cent by potassium can be observed. The nature of the effects of K⁺ and H⁺ are discussed.