Influx of labelled D‐glucose into isolated spinach (Spinacia oleracea L. cv. Melody hybrid) chloroplasts was initially rapid followed by a period ofslower influx. The stroma glucose concentration attained equilibrium rapidly with low external glucose concentrations and the two were linearly proportional. The period of slower influx resulted from conversion of glucose to acidic products that remained trapped in thechloroplast. As the external glucose concentration increased, the stroma glucose concentration increased less and less, attaining a maximal concentration of 72 mol m–3. The maintenance of an equilibrium stroma glucose concentration lower than that in the external medium is evidence that plastid glucose efflux involves secondary activetransport. The equilibrium stroma glucose concentration increased in response to light and protonophoric uncouplers. It is proposed thatglucose efflux is coupled with a proton and the stroma glucoseconcentration equilibrates in response to the proton gradient across the membrane. To determine if glucose is a significant product of starch mobilization, chloroplasts were isolated from spinach leaves labelled with 14CO2 during the preceding light period. Chloroplasts degraded starch at the same rate as the intact leaf. Glucose, maltose, and isomaltose were the principal labelled products that appeared in the medium during starch mobilization. The glucose concentration in thechloroplast was 2 mol m–3, which is similar to the measured Km for zerotrans efflux. The data support the role of the glucose translocator as an important component in the pathway for sucrose synthesis at night.
- Glucose translocator,
- maltose translocator,
- starch mobilization,
- stroma space
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