Relative Volumes of Cytoplasm and Vacuole in Nitella

Abstract

IN order to understand the mechanism by which various organic molecules are accumulated by the vacuoles of the green alga Nitella, it was necessary to determine accurately the relative volumes of cytoplasm and vacuole in the cell. The unusually large size of these cells has made the genus popular as a tool for the study of biological transport mechanisms^1–5. Single cells can be obtained up to nearly 2 mm. in diameter and more than 10 cm. in length. The contents of the large vacuole can be obtained relatively free of cytoplasm by cutting off one end and gently squeezing the solution on to a microscope slide or into a capillary para tube. There seems to be little agreement in the literature concerning the relative volumes of cytoplasm and vacuole. Thus Brooks¹ arrived at a value based on intracellular distribution of radioactive potassium and estimated 33 per cent cytoplasm, 67 per cent vacuole. Holm-Jensen et al.² measured the depth of the immobile chloroplast layer plus the streaming cytoplasm by differential focusing with a water-immersion objective, and the diameter of the cell with a micrometer eyepiece in a dissecting microscope. Calculations from their results give values of 10 per cent cytoplasm, 90 per cent vacuole. Recently, MacRobbie and Dainty⁴, measuring the rate of loss of sodium-22 from Nitellopsis cells previously loaded with the isotope, calculated that the cytoplasmic compartment consisted of about 3 per cent of the cell. In a similar fashion, using potassium-42, Diamond and Solomon⁵ arrived at values of 1.8 per cent cytoplasm.