Dr Smith is distinguished for his work on membrane transport in plant cells. A pioneer in the application of chemiosmotic theory to eukaryotic cell membranes, his work has continually emphasised the role of proton extrusion in powering transport, in maintaining electric potential differences and in contributing to internal pH regulation. He has made major and decisive contributions to the resolution of mechanisms for, and control of, uptake of ammonium, chloride and bicarbonate ions, and recently to the discovery of a new transporter, the electrogenic potassium-sodium symporter of Chara.
He has worked with algal and charophyte cells, with the leaves and roots of higher plants, and recently with mycorrhizal associations of plant and fungus. The implications of his work range from membrane biophysics to plant nutrition and aquatic botany.