Abstract
Sucrose uptake into cotyledons of castor beans ( Ricinus communis) proceeds partly by an active transport system and partly by passive permeation; α-methylglucoside (α-MG) is almost exclusively transported by the passive route. The Km for active sucrose uptake is 18 mM and the Vmax 4–5 μmoles/100 mg fresh weight per 1 h. The tissue is able to accumulate sucrose more than 100-fold. Sucrose but not α-MG induces a respiratory increase, which shows a similar sucrose concentration dependence as active sucrose transport. The addition of saturating amounts of sucrose to the incubation medium leads to a transient alkalinization of the medium. A second addition does not show this effect, neither does α-MG at any time. In the presence of 100 mM K +, which depolarizes the membrane potential, the uptake of sucrose is strongly inhibited. The results suggest that active sucrose transport in this tissue is mediated by an electrogenic proton cotransport system.
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