There are two phases during the process of ion absorption. In the initial phase there is a rapid uptake of ions into outer or free space of the cells. Outer or free space comprises intercellular spaces and cell walls (i.e., apoplast). Ions absorbed in free space are freely exchangeable, e.g., replacement of unlabelled K+ ions with labelled K+ ions. In the metabolic phase, the ions pass into inner space. The latter comprises cytoplasm and vacuole (i.e., symplast). In the inner space the ions are not freely exchangeable with those of external medium. Entry of ions into outer space is passive absorption as no energy is required for it. Absorption of ions into inner space requires metabolic energy, which is, therefore, an active absorption.