Ascientist is comparing the outer layer of an onion cell to the outer layer of a human skin cell. what is unique about the outer layer of the onion cell compared to the skin cell? it contains proteins. it contains cellulose. it contains carbohydrates. it is selectively permeable.

The outer layer of the onion cell, which is a plant cell, contains cellulose, however, the human skin cell (which is an animal cell) does not contain cellulose in it. The plant cells have a rigid cell wall present around the cell membrane, which protects the cell from the outer environment and keeps it rigid. This cell wall of the plant cells is composed of the cellulose. The animal cells have no cell wall present around them, and thus, contain no cellulose.

Hence, the correct answer is 'it contains cellulose'.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.