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The transport and metabolism of urea in Chara australis: I. Passive diffusion, specific transport and metabolism of urea, thiourea and methylurea

Journal Article


Abstract


  • Most of the urea entering Chara australis cells is rapidly metabolized to produce CO2, which diffuses out of the cells into the surrounding medium. A simple and convenient apparatus to measure both the 14C-urea retained by cells and the 14CO2 released into the medium was developed and used in a study of urea transport in Chara. The permeability coefficient for urea in the Chara plasmalemma was estimated from the slope of an uptake versus concentration function as 85 nm s-1. Computer modelling of urea uptake and metabolism suggests that this could be a 20% underestimate of the true value.The corresponding permeability coefficients for thiourea and N-methyl-urea were estimated in the same way as 34 and 35 nm s-1, respectively. These permeabilities are much greater than expected on the basis of either/water partition coefficients for the solutes and are consistent with the diffusion of urea and its similarly-sized analogues through aqueous pores in the plasmalemma.At external concentrations of urea less than 20 mmol m-3, the bulk of the uptake is effected by a specific transport mechanism with an apparent Km for urea of less than 1.0 mmol m-3. This transport system operates most rapidly with external pH in the range 6.5-7.5 and is influenced by the nitrogen status of the cell.Evidence is produced here suggesting that the specific transport of urea may be an active process. © 1988 Oxford University Press.

Publication Date


  • 1988

Citation


  • Wilson, M. R., & Walker, N. A. (1988). The transport and metabolism of urea in Chara australis: I. Passive diffusion, specific transport and metabolism of urea, thiourea and methylurea. Journal of Experimental Botany, 39(6), 739-751. doi:10.1093/jxb/39.6.739

Scopus Eid


  • 2-s2.0-0001510186

Start Page


  • 739

End Page


  • 751

Volume


  • 39

Issue


  • 6

Abstract


  • Most of the urea entering Chara australis cells is rapidly metabolized to produce CO2, which diffuses out of the cells into the surrounding medium. A simple and convenient apparatus to measure both the 14C-urea retained by cells and the 14CO2 released into the medium was developed and used in a study of urea transport in Chara. The permeability coefficient for urea in the Chara plasmalemma was estimated from the slope of an uptake versus concentration function as 85 nm s-1. Computer modelling of urea uptake and metabolism suggests that this could be a 20% underestimate of the true value.The corresponding permeability coefficients for thiourea and N-methyl-urea were estimated in the same way as 34 and 35 nm s-1, respectively. These permeabilities are much greater than expected on the basis of either/water partition coefficients for the solutes and are consistent with the diffusion of urea and its similarly-sized analogues through aqueous pores in the plasmalemma.At external concentrations of urea less than 20 mmol m-3, the bulk of the uptake is effected by a specific transport mechanism with an apparent Km for urea of less than 1.0 mmol m-3. This transport system operates most rapidly with external pH in the range 6.5-7.5 and is influenced by the nitrogen status of the cell.Evidence is produced here suggesting that the specific transport of urea may be an active process. © 1988 Oxford University Press.

Publication Date


  • 1988

Citation


  • Wilson, M. R., & Walker, N. A. (1988). The transport and metabolism of urea in Chara australis: I. Passive diffusion, specific transport and metabolism of urea, thiourea and methylurea. Journal of Experimental Botany, 39(6), 739-751. doi:10.1093/jxb/39.6.739

Scopus Eid


  • 2-s2.0-0001510186

Start Page


  • 739

End Page


  • 751

Volume


  • 39

Issue


  • 6