what are the biologically important compounds?

Answer 1

The four most important compounds are carbohydrates, lipids, proteins and nucleic acids.

Answer 2

At one level, clearly Carbon Dioxide for plants, Oxygen and water for ourselves are clearly essential for life, so biologically important.

At a more fundamental level amino acids are very important, proteins are made up of different amino acids. All cells contain some of the 20 amino acids, and DNA is made up of joined proteins, so I would say amino acids.
http://en.wikipedia.org/wiki/Amino_acid

however also biologically important is chlorphyll, which is essential for plants to photosynthesise. But this only becomes important once you have the amino acids, because then complex cellular organisms become possible.

Answer 3

carbon forms the basis of all known life forms

can't think of any others right now- besides i need to go t' pub
;)

Answer 4

Water
Lipids
Nucleic acids
Proteins
Carbohydrates

Answer 5

H2O?