Enclosed by a membrane and regulate passage of material in and out of cell; use dna for genetic infor. Come from preexisting cells (via cell growth and division) Unity of life and diversity of life. Because all cells come from preexisting cells, they must have a common ancestor. Bacteria, archaea, eukarya: archae and eukarya are more closely related. Proks are small, organisms without a nucleus, lack organelles. The structure and higher org. of euks is what allows them to achieve larger sizes. The new cells will differentiate into specific cell types. During embryonic development, stem cells are essential to the formation of differentiated cells in multicellular organisms. Stem cells are also found in adults (iunder skin, gut, bone marrow) Form hypothesis: null or alternative hypothesis, hypothesis can be falsifiable. Reject or revise hypothesis based on results. Covalent bond: atoms share electrons: non polar: electrons shared equally, polar: electrons not shared equally; partial charges are created.