Facilitated Diffusion

Minerals can be absorbed by physical processes called Passive absorption, which does not directly require metabolic activity. The concept of passive absorption is based on “outer space,” also known as “free space” or “diffusion space.” Ions will freely enter or exit the tissue if just a fraction of the tissue volume is accessible to free diffusion. After some time, the portion of the tissue that is experiencing free diffusion will achieve equilibrium with the external solution, resulting in the same ion concentration within the tissue as that of the exterior solution. The term “free space” or “outside space” refers to the region of a cell or tissue that permits ion diffusion freely. Usually, only 5% of the entire root volume is accessible for free space in the root tissue. For a particular volume of open space, the amount of solute flux depends on several variables, including the rate of transpiration, the solute concentration, and the production of root hair. Cortex cells can, however, directly take up solutes from the external solution to the existence of such a free area. Due to the presence of polygalacturonic acid [carboxylic groups (R. CHOO)] in the middle lamella, the cell walls have a negative charge. The apoplasm’s negative charges function as cation exchangers, causing cations to accumulate while repelling anions. As a result, charged solute access into the free space is limited. To define “free space,” Hope and Stevens (1952) used the term apparent free space (AFS). Water-free space (WFS), a component of AFS, is open to ions and both charged and uncharged molecules. Another term for the region where anion repulsion and cation exchange occur is Donnan free space (DFS).