ON THE MECHANISM OF SUGAR UPTAKE BY FLOATING LEAF DISKS

Abstract

SummarySugar uptake by leaf disks of Atropa belladonna floating on sugar solution was measured as increase in dry weight of the disks.Evidence of the importance of metabolism in sugar uptake was established as follows:(a) Uptake was reduced by as much as 75 per cent under anaerobic conditions. Uptake equal to that in air took place with a partial pressure of oxygen as low as 2 per cent. At this oxygen concentration 65 per cent of the full rate of aerobic respiration occurred.(b) Uptake was considerably reduced by cyanide, azide and 2–4‐di‐nitro‐phenol.(c) The relationship between rate of uptake and external concentration was not linear, the increment in rate getting progressively less with increasing concentration.(d) Competition occurred between sugars of similar structure.(e) Chemical structure of the absorbed sugar played an important part in defining penetrability.The uptake persisting under anaerobic conditions was designated ‘passive’, and this subtracted from the uptake in air gave the ‘active’ component of the uptake in air.Chromatographic carbohydrate analyses of the disks revealed that ‘passive’ uptake resulted solely in an increase in sucrose content which reached a constant level within 2 hours. This is interpreted as equilibration of the apparent free space with the external solution.Under aerobic conditions a higher level of sucrose content was achieved. This is thought to be due to transfer of sucrose into an ‘inner‐space’, possibly the vacuole. In addition synthesis of polysaccharide occurred and it was this continued synthesis which led to persistent sucrose uptake from the medium.When disks which had been floating on sugar solution were transferred to water, they lost sugar. The quality and quantity of this leakage is consistent's with the hypothesis outlined above.It is concluded that the processes involved in sugar uptake were (a) diffusion into the outer space, (b) facilitated diffusion into an inner space, (c) synthesis of polysaccharide. There was no evidence of any ‘uphill’ transport being involved.