A) water ava ability. B) temperature C) light intensity. D) atmospheric CO2 levels. E) atmospheric N2 levels. 79) Molecules assisted by carrier proteins may cross a selectively permeable membrane by: 79) simple diffusion. A) B) osmosis. C) active transport. D) endocytosis E) facilitated diffusion. 80) What is NOT a feature of a prokaryotic cel A) a plasma membrane B) a nuclear membrane C) enzymes D) DNA E) ribosomes 81) Phospholipids spontaneously form a bilayer in an aqueous solution. Why do the heads of the 81) phospholipids point out and the tails point toward another? on A) The tails are repelled by the aqueous environment B) The heads are repelled by the water inside and outside the cell C) The tails are nonpolar and form hydrogen bonds with another. one 82) Mutation is essential for: 82) A) growth and development. B) sperm and egg formation. C) natural selection D) survival E) sexual reproduction. 83) The carbohydrate in DNA is: 83 A) deoxyribose B) phosphate C) ribose D) glucose cellulose E) 84) The formation of sodium chloride (NaCl) is the result of 84) chemical unreactivity. A) covalent bonding. C) the lack of chemical attraction. Dy attraction between opposite charges