Sentence:  The critical component of the photosynthetic system is the "water-oxidizing complex", made up of manganese atoms and a calcium atom. This system splits water <WSD>molecules</WSD> and delivers some of their electrons to other <WSD>molecules</WSD> that help build up carbohydrates. 
Meaning of the word marked with <WSD> tags:  The smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds.