63.24.191.46 Comments Individual cells vary widely in size and shape, reflecting their unique functions in their body. All cells have some functions in common, but only certain cell types form the transparent lens of the eyes, secrete mucus, or conduct nerve impulses. The simplest living creatures are composed of single cells, but in complex organisms such as human beings, the hierarchy continues on to the tissue level.
Tissues are groups of similar cells that have a common function.
An organ is a structure composed of at least two tissues types that performs a specific function for the body. Organs that work closely with one another to accomplish a common purpose are said to be part of a particular organ system. The highest level of organization is the organism, the living human being, which represents the sum total of all structural levels working in unison to promote life.