The physical characteristics of rubber particles from the four rubber (cis-1,4-polyisoprene) producing species Euphorbia lactiflua Phil., Ficus elastica Roxb., Hevea brasiliensis Mull. Arg., and Parthenium argentatum Gray, were investigated using transmission electron microscopy (TEM) and electron-paramagnetic-resonance (EPR) spin labeling spectroscopy. Transmission electron microscopy showed the rubber particles to be composed of a spherical, homogeneous, core of rubber enclosed by a contiguous, electron-dense, single-track surface layer. The biochemical composition of the surface layer and its single-track TEM suggested that a monolayer biomembrane was the surface structure most compatible with the hydrophobic rubber core. The EPR spectra for a series of positional isomers of doxyl stearic acid, used to label the surface layer of the rubber particles, exhibited flexibility gradients and evidence for lipid-protein interactions for all four rubber particle types that is consistent with a biomembrane-like surface. The EPR spectra confirmed that the surface biomembrane is a monolayer. Thus, rubber particles appear similar to oil bodies in their basic architecture. The EPR spectra also provided information on protein location and degree of biomembrane penetration that correlated with the known properties of the rubber-particle-bound proteins. The monolayer biomembrane serves as an interface between the hydrophobic rubber interior and the aqueous cytosol and prevents aggregation of the particles. An unexpected observation for the probes in pure polyisoprene was evidence of an intrinsic flexibility gradient associated with the stearic acid molecule itself.

Electron paramagnetic resonance (EPR) spectra have been obtained on iron and copper protein complexes of human blood serum transferrin, human lactotransferrin, and conalbumins obtained from chicken, turkey, and Japanese quail egg whites. The EPR results indicate that Fe+3 and Cu+2 both bind to similar sites and that there is no magnetic interaction between the bound metal atoms. No difference between the sites binding the first and second moles of metal was detected by EPR. It is proposed that Cu+2 is bound to two tyrosyl oxygen atoms and two nitrogen atoms of the protein with square planar coordination, and that Fe+S is octahedrally coordinated to three tyrosyl oxygen atoms, two nitrogen atoms, and one bicarbonate ion.

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(3,11,19,22,31,32) while the Ca2+-ATPases are associated with the ER and plasma membrane (6,7,9,11,17,28,31). It has been suggested that both types oftransporter may exist in a single membrane type (2, 30) although this has not been investigated in higher plants. We examined the Ca2+-transport activity of microsomes isolated from barley roots in order to determine which ofthe two types ofCa2+ transporters were associated with the major organelle fractions.The mechanisms by which Ca2+ is transported out of the cytoplasm against a large concentration gradient can be studied using isolated membrane vesicles (3,6,7,11,17,22,28,31,32 (13)(14)(15)(16). In the present paper the mechanisms and capacity for formation of Ca2`gradients by these vesicles are assessed. A prominent Ca2+/nH+ antiporter activity in tonoplast vesicles is characterized and evidence for a non-ER, non-plasma membrane Ca2+-ATPase is presented. A distinction is made between the activity of the Ca24/nH' antiporter and independent effects of Ca2+ on the formation and maintenance of ApH by the PM-and V-ATPases.Precise regulation ofcytoplasmic Ca2" may be an important element of signal transduction in plant cells (23,27), as it is in animal cells (12). Resting levels of cytoplasmic Ca2" are reported to be less than 0.5 AM in plants (8,18,26). These low levels ofcytoplasmic Ca2" are maintained by active uptake into organelles and export across the plasma membrane (7,12,26). To date, two types of Ca2" transport activity have been described in plant cells, one which is dependent on the MATERIALS AND METHODS Plant MaterialSeeds of barley (Hordeum vulgare L. cv CM72) were sown above an aerated solution containing full strength nutrients 2 Abbreviations: ApH, difference in pH across the vesicle membrane; ESR, electron spin resonance; PM-ATPase, plasma membrane H+-ATPase; PM, plasma membrane-enriched membrane fraction; 6,"5N,2,6,6, 1-'5N-1-oxyl; TN, tonoplast-enriched membrane fraction; Q, fluorescence quench; V-ATPase, vacuolar-type H+-ATPase; DCCD, dicyclohexylcarbodiimide; nH, number of H+ where n is unknown; BTP, bistris propane.

INTRODUCTIONThe fact that wool shows a considerable affinity for water when exposed to vapor pressures less than the vapor pressure for liquid water is well known and has received a considerable amount of study. One of the earliest attempts to explain the mechanism of sorption was made by Hedges,' who concluded from data on the heat of wetting of wool that the sorption proceeds in three loosely defined stages. A similar conclusion was arrived at by Speakman2 which was based upon a study of the effects of moisture upon the elastic properties of the fiber. The first application of the modern statistical treatment of multimolecular absorption to the sorption of water by wool was made by C a~s i e .~ According to Cassie, if A moles of vapor are sorbed per unit mass of dry wool a t a particular relative vapor pressure, p/po, and if there are B moles of localized sites distributed throughout the adsorber of which X moles are occupied by water molecules while the remaining A -X moles of water are sorbed on top of these, then the relations:are obtained. These can be combined to give the BET4 equation:with c = l/p. The factor j3 involves the binding energy of the localized molecules and the ratio of the partition functions for the localized and mobile water molecules. Before this theory can be applied to wool, however, the vapor pressure isotherm must be corrected for the swelling energy associated with the sorption of water. Cassie has applied such a correction to wool and determined the stress-free isotherm. It, is this isotherm to which Cassie has applied his analysis and it is this isotherm to which the present analysis is applied. Although Cassie's theory has been successful in predicting the vapor pressure-water uptake relationship and has given satisfactory agreement with the heat of wetting data, it has not been successful in its application to the microwave dielectric behavior of the wool-water ~y s t e m .~ In order to interpret the dielectric results, Cassie's theory was extended to include a third, intermediate species of sorbed water. In the present work the derivation of this new theory is given. 103 104 .I. 5. WINDLE THEORY The Statistics of SorptionThe derivation of the sorption isotherm follows a method given by Ballzs6 in his paper on the statistical theory of adsorption. I'n adsorption the molecules are visualized as forming mono-or multilayers upon an internal or external surface, but in the analysis of the sorption of vapors by high polymers the applicability of this view is questionable and, since there is nothing to be gained by retaining it, it will be abandoned. In fact, in the statistical derivation, such an assumption is not even required; instead, only a specification of the number of sorption sites (these sites, may be distributed throughout the absorber in any specified manner) and the potential energy of the sorbed molecules are all that is necessary. With this view, it is possible to have ideal sorption, i.e., negligible interaction between molecules sorbed on identical sites. The statistic...