[NewtoAtoms]

Hello Biochemists,

This my first venture into Biochemistry, and I must admit I AM EXCITED! 

We had a class discussion today and this was left on the chalkboard before we left:

Lipids, by definition, are

a. fats                                                                                 (this is just one type of lipid)
b. nonpolymeric biomolecules                                                    (false lipids are complex polymeric biomolecules)
c. water-insoluble but organic solvent-extractable biochemicles      (I think this is the best definition)
d. glyceride esters                                                                  (hum.... some lipids are)
e. biomolecules which form micelles                                             (yes some too!)
 
Would I be correct to say that c. is the BEST answer for this definition?

Thank you for your time.

[Borek]

Would I be correct to say that c. is the BEST answer for this definition?

It would be, assuming there are no other molecules sharing the same characteristic. But they do exist.

[Yggdrasil]

Lipids certainly are complex biomolecules, but I'm not aware of any lipids which are polymers (but I would agree that b probably is not the best description of lipids).

[aHerraez]

Defining lipids is always tricky. Here is my 2 cents:

(b) Certainly lipids are NOT polymeric. But that is not good as a definition, as there are many other non-polymeric biomolecules.

(c) is quite good, but I don't see it as perfect. Lipids are characterised by being hardly soluble in water, but they are not very soluble in organic solvents if these are very nonpolar. The characteristic of most lipids is to have both polar and nonpolar moieties (hydrophilic and hydrophobic parts). That's why they form micelles easily in water (e). But again (e) is not a perfect definition.

So I think the best answer must be chosen not by being the best definition of lipids, but by being the least erroneous sentence. And the important key is not which answer you choose, but the discussion you make of all choices.