Observations have been reported in earlier communications [Hanes, 1940, 1, 2, 3] on the reversible transformation of glucose-l-phosphate into starch and inorganic orthophosphate, catalysed by an enzyme (phosphorylase) which is present in many higher plants. The reaction may be represented as follows:Starch + inorganic phosphate = glucose-I-phosphate.(free-P) (ester-P) Experiments with purified phosphorylase from potatoes showed that the equilibrium state, defined by the values of the ratio free-P/(free-P + ester-P), or by the ratio free-P/ ester-P, was not significantly affected by alterations within wide limits in the gross concentration of starch or in the concentration of total phosphate (free-P + ester-P). It was observed, however, that the equilibrium was markedly affected by alterations in the concentration of hydrogen ions, larger proportions of the reactants existing in the form of starch and inorganic phosphate under more acid conditions. Thus, as the pH value was varied from 5.0 to 7 0, the values of the ratio free-P/ester-P decreased progressively from about 10-8 to 3-1 [Hanes, 1940, 1, 2].It is ofinterest that a similar effect has been reported recentlybyCori & Cori [1940] in connexion with the action of the corresponding enzyme (glycogen-phosphorylase) from animal tissues (brain, liver, and muscle). The average values which they give for the equilibrium ratio free-P/ester-P are 5-7 at pH 60 and 2-9 at pH 7*55. Also they show that this ratio is unaffected by wide variations in the concentration of polysaccharide (glycogen) present.The purpose of the present note is to compare the effects of H+ concentration upon the equilibria attained in these reversible reactions catalysed by the plant and animal phosphorylases and at the same time to extend and. modify the interpretation of this effect which has already been advanced. It was concluded earlier [Hanes, 1940, 3] that alterations in the hydrogen-ion concentration affected the equilibrium owing to the differential effects upon the dissociation of orthophosphoric acid and glucose-l-phosphoric acid. Whereas the values of the ratio free-P/ester-P, i.e. total orthophosphate/total glucose-i-phosphate, decreased from about 10-8 to 3-1 over the range of pH from 5 0 to 7-0, calculations showed that the ratio of the divalent ions of the two acids remained approximately constant. This ratio [HOPO%]/[ROPO=] (R representing the glucose molecule substituted at carbon atom 1) had a value of about 2*2. It was thus clear that an equilibrium, independent of H+, existed between similarly charged ions of the opposing reactants. This preliminary analysis was, however, incomplete, and misleading to the extent that emphasis was placed. on the divalent ions to the exclusion of the other species of the two reactants. It should have been made clear that a constant ratio for the divalent ions implies a constant ratio also for the undissociated acids and for the monovalent ions. For, given the two acids HOPO3H2 (with dissociation constants k1 = 1i07 x 10-2 and k2= 1i51 x 1...
Vegetable Assimilation and Respiration. freely, and which is fastened securely with shellac to the back plate of the chamber over the hole. Contact with the leaf is made, as in Darwin's experi ments, by means of a perforated disc of 30 to 40 per cent, gelatine, the usual dimensions of which were : outer diameter, 2*3 cm. ; inner, 0*9 cm.The leaf is put into the leaf chamber, the front glass waxed on, and the porometer chamber pushed home so as just to make air-tight contact between leaf and gelatine and between gelatine and porometer chamber. This involves only a slight pressure on the leaf. The porometer chamber is held in this position while paraffin wax (60° m.p.) is melted into the space between it and the supporting tube, and when the wax sets the porometer chamber is firmly held and the leaf chamber is air-tight.To measure the rate at which air could be drawn through the leaf the air flow meter shown in fig. 1, b, was used. Air suction is produced by means of a column of mercury, about 15 mm. long, falling vertically in the straight limb of the flow meter, and the time taken for the mercury to pass a marked distance down the tube is observed with a stop-watch. One limb of this meter is attached by on May 10, 2018 http://rspb.royalsocietypublishing.org/ Downloaded from 490 E. J. Maskell. pressure tubing to the porometer chamber, the other to a " T " piece, which puts it into communication with the leaf chamber. The whole apparatus therefore forms a closed system. After making a reading the mercury is returned, by inverting the tube, to its proper end, and the tube is then set horizontal until the next reading. Readings were always taken in the direction in which air is drawn through the deaf.A great advantage of the gelatine-disc method of securing contact between the porometer chamber and the leaf is that, before setting up the assimilation experiment, readings can be rapidly taken with the same disc over the whole leaf surface. Usually three readings on each side of the main vein were thus taken. Unless all six readings were fairly close to one another, the leaf was not used.Similar readings at the end of the experiment enabled one to check the reading for the part of the leaf used during the experiment against the other parts of the leaf. Usually the average reading for the rest of the leaf agreed well (after the experiment as well as before it) with that for the area used in the experiment. There were cases, however, of marked divergence ; the stomata of the rest of the leaf might be either more widely open or more closely shut than those of the area within the porometer chamber. Several experiments had to be rejected on these grounds.In all the experiments to be described, unless a special note is made to the contrary, it may be taken that the average porometer rate for the leaf surface both before and after the assimilation experiment did not differ by more than 10 per cent, from the rate given by the area that was under the porometer chamber during the experiment.In calculating the assimilation per 5...
Summary. Under controlled conditions in water cultures soluble silicate was found to have little effect upon the growth of barley if phosphorus were also present, but if the latter were absent a significant increase in dry weight was induced by the silicate. The addition of silicate caused an appreciable increase in the height of the main shoot, which was most marked in phosphate‐free solutions, becoming less evident as the quantity of phosphate present was increased. Leaf development was retarded by phosphate deficiency and hastened by the addition of silicate. A close association exists between the amount of phosphate present, and the effect of silicate upon the rate of tillering and the number of tillers developed. Soil cultures with barley and mustard in pots with various forms of silicate showed that soluble silicates are more active than glass silicates, tending to cause increase in dry weight with deficient mineral manuring, and in some cases also with complete manuring. Further soil experiments revealed variations in the response of barley and mustard to silicate on different types of soil. A general improvement occurred with increasing doses of silicate together with various combinations of manures, notably when phosphorus or potash was deficient. The significance of the results obtained has been examined statistically, and an attempt made to formulate the effect of added silicate in terms of an increase in the efficiency of the superphosphate present.
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