Sentence:  Many of the <WSD>cells</WSD> and microorganisms which are transparent to visible light , absorb or reflect the much shorter wavelengths of the ultraviolet spectrum . 
Meaning of the word marked with <WSD> tags:  (biology) the basic structural and functional unit of all organisms; they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals