Phosphorylation by Barley Root Mitochondria & Phosphate Absorption by Barley Roots.

The absorption by roots of ions in general, and phosphate absorption in particular, can occur against concentration gradients so great as to require free energy changes of the order of 8,000 calories per nmole. This implies coupling in the absorption process with reactions Jeroceecling wdith considerable free energy decrease. Few reacticius in plants other than those for production or destructionn of nucleotide triphosphates can lead to the necessary free energy changes. The kinetics of phosphate uptake by barley roots (10) indicate that the rate-limiting step is the incorporation of phosphate in an energetically active form accompanying oxidative phosphorylation associated with oxidation of components of the respiratory chain. Mitochondria effective for oxidative phosphorylation can be separated from animal and plant tissues. The kinetics of phosphate uptake by roots can, accordingly, be compared with oxidative phosphorylation by mitochondria isolated from roots. The comparison is made in this work with the finding that the rate-limiting steps of phosphate uptake 1w barley roots and esterification by barley root mitochondria are closely similar in their enzymatic constants and responses to inhibitors and substrates for oxidative phosphorylation. The conclusion is that mitochondria are the particulate site of the rate-limiting step for absorption.