Study of Protein Facilitated Water and Nutrient Transport in Plant Phloem

Abstract

Biological systems use transporter proteins to create concentration gradients for a variety of purposes. In plant, sucrose transporter proteins play a vital role in driving fluid flow through the phloem by generating chemical potential. In this study, we investigate these nanoscale phenomena of protein directed active transport in a microscale biological system. We presented a mathematical model for protein facilitated sucrose loading considering six different states of the sucrose transporter protein. In addition, we developed a quasi-one dimensional transport model to study protein facilitated pumping mechanisms in plant phloem. Here we specifically study the influence of transporter protein reaction rates, apoplast proton concentration, membrane electrical potential, and cell membrane hydraulic permeability on flow through the phloem. This study reveals that increasing companion cell side deprotonation rate significantly enhances the sieve tube sugar concentrations, which results in much higher water transport. Lower apoplast pH increases the transport rate, but the flow control is less noticeable for a pH less than 5. A more negative membrane electrical potential difference will significantly accelerate the transporter proteins' ability to pump water and nutrients. Higher companion cell and sieve element membrane hydraulic permeability also promotes flows through the phloem; however, the flow difference is less noticeable at higher permeabilities when near typical plant cell membrane ranges.