Sagot :
Answer:
Complex Biomolecules
Explanation:
eavy metals dissolute to release metal ions. When present in the ionic state, they induce toxicity because of their affinity for cellular components and biomolecules and the subsequent formation of metal–biomolecule complexes. Metal ions can bind or block the functional groups in a biological molecule. It can also displace the essential metals in enzymes and bind to the cellular thiol pool or participate in deleterious chemical reactions within the cell. Ultimately, metal ions damage proteins, DNA, and biological membranes and interfere in enzyme function and cellular processes.39 In addition, toxicity of metal ions is normally accompanied by ROS-mediated toxicity. Bondarenko and co-authors showed that Cu ions released from CuO nanoparticles were the key factor in triggering the ROS and DNA damage.40 Ag nanoparticles can easily destroy the membrane, pass through the microbial body, and then convert to silver ions in cytoplasm to damage the intracellular structure.41 In one study, 19 different types of nanoparticles were examined and it was determined that ions released from nanoparticles were cytotoxic.42 In another study, Zheng et al. observed similar inhibitory effects on microbial denitrification by either Zn ions or ZnO nanoparticles, hence indicating that the toxicity of ZnO nanoparticles was because of Zn ions release.43
However, discerning the mode by which dissolved ions and nanoparticles lead to microbial cell death is difficult to prove experimentally and only few studies focus on this. To illustrate, Neal et al. compared the effects of exposing sublethal concentrations of ZnO nanoparticles and Zn ions to Cupriavidus necator, and results indicated that nanoparticles mainly affected membrane-associated proteins. In contrast, zinc ions exposure affected metabolic processes, likely because Zn ions can act as important cofactor for a number of enzymes including DNA and RNA polymerases.44 Similar result was observed when Daphnia magna was exposed to Ag nanoparticles and ions.45 Ag nanoparticles could disrupt the major biological processes including protein metabolism and signal transduction, while the Ag ions disrupted developmental processes. It is likely that both metal ions and nanoparticles affect different metabolic pathways because of varying affinities toward certain biomolecules but synergistically complement each other to result in cell death.