Chloroplasts osmotic responses

In this chapter we will derive the Boyle-Van t Hoff relation using the chemical potential of water, and in Chapter 3 (Section 3.6B) we will extend the treatment to penetrating solutes by using irreversible thermodynamics. Although the Boyle-Van t Hoff expression will be used to interpret the osmotic responses only of chloroplasts, the equations that will be developed are general and can be applied equally well to mitochondria, whole cells, or other membrane-surrounded bodies. 

To appreciate the refinements that this thermodynamic treatment introduces into the customary expression describing the osmotic responses of cells and organelles, we compare Equation 2.18 with Equation 2.15, the conventional Boyle-Van t Hoff relation. The volume of water inside the chloroplast is VM,n because n v is the number of moles of internal water and Vw is the volume per mole of water. This factor in Equation 2.18 can be identified with V — b in Equation 2.15. Instead of being designated the nonosmotic volume, b is more appropriately called the nonwater volume, as it includes the volume of the internal solutes, colloids, and membranes. In other words, the total volume (V) minus the nonwater volume (b) equals the volume of internal water (Ew ). We also note that the possible hydrostatic and matric contributions included in Equation 2.18 are neglected in the usual Boyle-Van t Hoff relation. In summary, although certain approximations and assumptions are incorporated into Equation 2.18 (e.g., that solutes do not cross the limiting membranes and that the... 

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... 

B. Suppose that chloroplasts isolated from such a cell have the same osmotic responses as in Problem 2.5. What is the volume of such chloroplasts in vivo Assume that activity coefficients are 1 and that the temperature is 20°C. 

Potter, J.R. Boyer, J.S. (1973). Chloroplast response to low leaf water potentials. II. Role of osmotic potential. Plant Physiology, 51, 993-7. 

Swelling Responses. Effects of the herbicides on the "fluidity" and permeability properties of membranes were measured. Alterations to the fluidity and permeability of organelle membranes can alter the osmotic properties of the membranes. Chloroplast, thylakoid, and mitochondrial membranes are known to be relatively impermeable to cations such as and IT , but freely permeable to lipophilic anions such as SCN (23,... 

Some authors(1) concluded that photosynthesis at low Fw was more limited by the loss of chloroplast activity than by increased difussive resistance. RBPC activity decreased in bean and cotton plants at water stress(2)(3) (4). PEPC and RBPC activities decreased at water stress in barley plants(5). In mesophyll cells from bean and tomato plants there was decreased in CO2 fixation at fairly low osmotic potentials which simultaneous with stomatal closure(6). Recently it was reported(7) that in soybean leaves a non stomatal limitations of leaf photosynthesis under drought stress conditions appears to be due in part to a reduction in the in vivo activity of RBPC. On the other hand it has been reported that chlorophyll content (8) (9) (10) shows alterations due to water stress. In the present work we pretend to compare the responses of carboxylase activities and chlorophyl content to water deficit in two maize hybrids (Ci ) (CPB2 and CPB8), two tomato cultivars (C3) (Pera Quibor, PQ and Rio Grande, RG) and two bean cultivars CC3)(Tacarigua,T and VUL-73-401,V). 

The plants were grown as described previously (R.L.A. Peters et al. 1983). Intact chloroplasts routinely were isolated according to a modified method of Walker (W. Cockburn, D.A. Walker 1968) as described by Schapendonk (A.H.C.M. Schapendonk 1980). Broken chloroplasts were obtained by a 60 sec osmotic shock on ice, as described previously (R.L.A. Peters et al. 1983). Determination of a) Hill reaction rate, b) chlorophyll content, c) absorbance changes at 518 nm and d) ATP hydrolysis in chloroplasts was performed as described previously (R.L.A. Peters et al. 1983). For the determination of the flash-induced P515 response in chloroplasts at low ion concentrations, chloroplasts were isolated in a medium containing Tris/Mes 5 mM pH 7.8, Mg(Ac)2 1 mM and Sorbitol 330 mM. Broken chloroplasts in this case were obtained by osmotic shock in a medium containing 5 mM Tris/Mes pH 7.8 and 1 mM Mg(Ac)2 and subsequent addition of an equal volume of a medium containing Tris/Mes 5 mM pH 7.8, Mg(Ac)2 1 niM and sorbitol 660 mM. 

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