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A cell is the smallest measureunitpartportion unit of structure and function. Thus, cells are the building blocks of all organisms. Cells vary in size. With few exceptions, individual cells are thereforethenquiteso so small that cannot be seen unaided. In 1665, a British scientist named Robert Hooke observed cells for the firstbeginninglastsame first sequencecircleconsequencetime time using a microscope. A microscope is an instrument that magnifies an object. Most images of cells are taken with a microscope and are called micrographs.