**5.1. Metabolism dependent biosorption**

Metabolism dependent biosorption is exhibited by living biological material. It involves various mechanisms like chelation; a specific way in which ions and molecules bind to metal ions and it involves the formation or presence of two or more separate coordinate bonds between a polydentate ligand and a single central atom, physical adsorption; adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It is a surface phenomenon. Generally the adsorption process is classified as physisorption, characteristic of weak van der Waals forces, or chemisorption, characteristic of covalent bonding. It may also occur due to electrostatic attraction, precipitation; it is the creation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid. When this reaction occurs in a liquid solution, the solid formed is called the "precipitate" and the chemical that causes the solid to form is called the "precipitant") or complexation (it consists of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are known as ligands or complexing agents. Many metal-containing compounds, especially those of transition metals, are coordination complexes). There may involve a single process or combination of these processes [50, 51]. If the metal binding to cell wall is metabolism dependent then it involves energy from ATP. The ligands present on the cell wall of biological material such as phosphoryl, carboxyl, carbonyl, sulfhydryl and hydroxyl groups immobilizes the metal ion [32] and then uptake occurs [5]. Other factors that effect the metal uptake by living biomass includes nature of heavy metals ions, conditions of the medium, cell wall composition, etc. [5]. The uptake process by living biomass involves adsorption to cell wall and entering into the cytoplasm [29, 31, 52, 53].
