Fig 1. Homeostasis in a general uptake system.

Schematic of a general nutrient uptake system. The system is comprised of a transporter that can transport nutrient across a plasma membrane, i.e. it converts external nutrient (S_ext) to internal nutrient (S_int). Transporters are synthesized (thin green arrow) and downregulated (thin red arrow). α_T denotes transporter synthesis, γ_T transporter downregulation, and γ_s internal nutrient usage. All these terms represent fluxes. Regulation of synthesis can be achieved by a number of mechanisms, e.g. regulation of transcription or trafficking. Regulation of downregulation can be achieved by a number of mechanisms, e.g. regulation of transporter activity or transporter degradation/dilution. Nutrients are imported (thick green arrow) and used (think red arrow). S_int [3,4,11,27] and S_ext [19] can in principle be sensed and therefore could affect transporter synthesis and downregulation (dotted lines).