托福听力的背景知识很重要，出国留学网托福栏目小编整理了相关的资料，希望可以帮助考生度过考试的难关，下面是小编搜集的托福听力生物学背景知识的相关资讯，欢迎参考!
托福听力生物学背景知识：痕量金属
Introduction：
Trace Metal
痕量金属,是指只含量极少的金属。一般是指水中、气体中的微量金属。
Background information：
Trace metals are metals in extremely smallquantities that are present in animal and plant cells and tissue. They are anecessary part of nutrition and physiology. Ingestion of, or exposure to,excess quantities is often toxic. However, insufficient plasma or tissue levelsof certain trace metals can cause pathology as well; as is the case with iron.
Trace metals are depleted through theexpenditure of energy by various metabolic processes living organisms. They arereplenished in animals through diet as well as environmental exposure, and inplants through the uptake of nutrients from the soil in which the plant grows.
Human vitamin pills and plant fertilizersboth contain trace metals as additional sources for trace metals. The harderyou work to expend energy, the more trace metals you will lose. These metals needto...