question_answer 17)

  Many elements are found in living organisms either free or in the form of compounds. One of the following is not found in living organisms. (a) Silicon                           (b) Magnesium   (c) Iron                             (d) Sodium

Answer:

                  (a) Silicon is not found freely in nature, but it does occur as oxides and silicates, whereasmagnesium, iron and sodium are present in living organisms as ions. Silicon isessential to plant life but is often found in minute quantities in human body and itsfunction is still unknown. Magnesium is an abundant element. It is essential for a number of enzymes and theiraction, particularly those utilising ATP. Iron is an important constituent of. hemoglobin and plays a vital role by taking part in  transport, and electron transport chain. Sodium plays a vital role in animals by regulating nerve impulse transmission and altering the membrane permeability. It also has indispensable role in osmoregulation.