Osmotic pressure chloroplasts

Like T° (Eq. 2.16), the water potential inside the chloroplasts, T1, also depends on osmotic pressure. However, the internal hydrostatic pressure (P1) may be different from atmospheric pressure and should be included in the expression for T1. In addition, macromolecules and solid-liquid interfaces inside the chloroplasts can lower the activity coefficient of water, which we can represent by a matric pressure t1 (see Eq. 2.11). To allow for this possibility, the internal osmotic pressure (n1) is the sum of the solute and the inter facial contributions, in the manner expressed by Equation 2.11. Using Equation 2.13a, T1 is therefore... 

Figure 2-11. Volumes of pea chloroplasts at various external osmotic pressures, 11°. The chloroplasts were isolated from plants in the light or the dark, indicating that illumination decreases chloroplast volume. [Source Nobel (1969b) used by permission.]...

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... 

Figure 2-11 indicates that the volume of pea chloroplasts varies linearly with l/n° over a considerable range of external osmotic pressure. Therefore, -t1 + Pl in Equation 2.19 must be either negligibly small for pea chloroplasts... 

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. 

The intercept on the ordinate in Figure 2-11 is the chloroplast volume theoretically attained in an external solution of infinite osmotic pressure —a l/n° of zero is the same as a n° of infinity. For such an infinite 11°, all of the internal water would be removed = 0), and the volume, which is obtained by extrapolation, is that of the nonaqueous components of the chloroplasts. (Some water is tightly bound to proteins and other substances and presumably remains bound even at the hypothetical infinite osmotic pressure such water is not part of the internal water, Vwn v). Thus the intercept on the ordinate of a F-versus-l/n° plot corresponds to b in the conventional Boyle-Van t Hoff relation (Eq. 2.15). This intercept (indicated by an arrow in Fig. 2-11) equals 17 pm3 for chloroplasts both in the light and in the... 

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