One difference between human cheek cells and onion cells is: 1. the presence of cytoplasm 2. the absence of a nucleus in the plant cells 3. the absence of a cell wall in human cells 4.none of the above

3. the absence of a cell wall in human cells

Explanation:

Animal cells do not have cell walls. Cell membranes separate the cytoplasm of the animal cells from the surroundings and maintain their interior. Plant cells have cellulosic cell walls. A cell wall surrounds the cell membrane of a plant cell. Cell walls serve to provide structural support and protect plant cells from pathogens. Cell walls also help keep excess water out of cells so they do not burst. Therefore, human cheek cells would not have cell walls while the onion cells would have cell walls made up of cellulose.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."