Why is the cell membrane described as being selectively permeable?
a. it allows anything to enter and exit the cell.
b. it allows nothing to enter or exit the cell.
c. it allows only water to enter the cell.
d. it allows needed materials to enter the cell and wastes to leave the cell.

Which of the following statements is true regarding a basic amino acid? a.) the hydrophilic r group of a basic amino acid will be located on the interior of a protein b.) the positively charged r group of a basic amino acid could bind dna c.) the r group of a basic amino acid would only be able to form covalent bonds with other molecules d.) a basic amino acid would be considered both polar and hydrophobic e.) all of the above