Many freshwater one celled organisms, like paramecium, have contractile vacuoles. there's structures collect and pump out excess water that accumulates in the cell. explain why these organisms needs such a structure.

This is because their cells are usually hypertonic to the freshwater they swim in. Their cells will always have dissolved osmotically-active molecules that are critical for cell function.  Therefore, osmotically, water will move from the freshwater to the inside of the cells. If not regulated, the cells would swell and lyse. Therefore excess water has to be actively pumped out to maintain cell homeostasis.    

hydroxyl -oh

alcohols, always polar, can form hydrogen bonds with h2o molecules

carboxyl -cooh  

cargoxylic acids, has acidic properties which is a source of hydrogen ions, found in cells in ionized for with a charge of oh-1

carbonyl > co

ketone if within skeleton, aldehyde if group is at the end of skeleton, k & a may be structural isomers, it determines 2 sugars (ketose & aldose)

amino -nh2

amines, acts as a base, it can pick up h+ ions from surrounding solution, ionized with charge of 1+

methyl ch2

addition to rna/dna affects and gene expression

phosphate -opo3 ²⁻

organic phosphates, contributes to negative charge to, molecule that it is a part of, and potential to react with h20

sulfhydryl -sh

thiols, cross-linking stabilize the protein, and it is structure