Reversible Swelling and Contraction of Isolated Spinach Chloroplasts

By use of a micro technique for producing extracts of spinach mesophyll cells, chloroplasts were isolated in a state wherein they displayed microscopically visible, reversible osmotic properties. Swollen spherical chloroplasts treated with hypertonic sucrose or mannitol media, but not NaCl, could be shrunken to a state resembling their disk appearance in living cells. Reversible osmotic behavior was more easily demonstrated when the chloroplasts were initially isolated from cells in a relatively low osmolar concentration in contrast to using 0.25 m sucrose or more concentrated media. Individual chloroplasts could be swollen and contracted repeatedly through as many as 4 cycles. The relationship between the capacity for osmotic behavior and chloroplast appearance in cell extracts is discussed.