Osmotic Responses of Chloroplasts

To illustrate the use of Equation 2.18 in interpreting osmotic data, we will consider osmotic responses of pea chloroplasts suspended in external solutions of various osmotic pressures. It is customary to plot the volume V versus the reciprocal of the external osmotic pressure, l/n°, so certain algebraic manipulations are needed to express Equation 2.18 in a more convenient form. After transferring r1 — P1 to the left-hand side of Equation 2.18 and then multiplying both sides by VwrCwf II0 — r1 + / ), can be shown to equal RT -n -/(Jl° — r1 +/ 1). The measured chloroplast volume V can be 

The relatively simple measurement of the volumes of pea chloroplasts for various external osmotic pressures can yield a considerable amount of information about the organelles. If we measure the volume of the isolated chloroplasts at the same osmotic pressure as in the cytosol, we can determine the chloroplast volume that occurs in the plant cell. Cell sap expressed from young pea leaves can have an osmotic pressure of 0.70 MPa such sap comes mainly from the central vacuole, but because we expect n05 10801 to be essentially equal to nvacuole (Eq. 2.14), nce11 8ap is about the same as n05 10801 (some uncertainty exists because during extraction the cell sap can come into contact with water in the cell walls). At an external osmotic pressure of 0.70 MPa (indicated by an arrow and dashed vertical line in Fig. 2-11), pea chloroplasts have a volume of 29 pm3 when isolated from illuminated plants and 35 pm3 when isolated from plants in the dark (Fig. 2-11). Because these volumes occur at approximately the same osmotic pressure as found in the cell, they are presumably reliable estimates of pea chloroplast volumes in vivo. 

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