Why does water need a protein channel to move across the cell membrane?

What is thought to have caused the mass extinction at the end of the cretaceous period?

(6.02 mc)what affect do marine protected areas have on environmental quality? they increase environmental quality by preventing resource overuse. they increase environmental quality by adding new species. they decrease environmental quality by removing all human activity. they decrease environmental quality by adding recycling facilities.

An uncomfortable feeling in the ears when descending in an airplane is caused by changes in air pressure on the: a. inner ear b. middle ear c. eardrum d. hammer/anvil