(Cells are the basic units of life.) Cells can exist by themselves, like bacteria, or as part of a larger organism, like our cells. As opposed to Viruses which are non-living infectious particles, much smaller than a cell, and need a living host to reproduce.
Because our eyes see what's reflected not absorbed. Leaves contain chlorophyll which contains a pigment of green that absorbs red and blue wavelengths, while reflecting green.
