● The most exciting aspect of chemistry deals with `color{violet}("isolating thousands of compounds")`, small and big, from `color{violet}("living organisms")`, determining their structure and if possible synthesising them.

● If one were to make a list of `color{violet}("biomolecules,")` such a list would have `color{violet}("thousands of organic compounds")` including `color{violet}("amino acids")`, sugars, etc.

● We can call these `color{violet}("biomolecules")` as `color{brown}("‘metabolites.")`

● In `color{violet}("animal tissues,")` one notices the presence of all such `color{violet}("categories of compounds.")`

● These are called `color{brown}("primary metabolites.")`

● However, when one analyses `color{violet}("plant, fungal")` and `color{violet}("microbial cells")`, one would see thousands of compounds other than these called `color{violet}("primary metabolites,")` e.g. alkaloides, flavonoides, rubber, essential oils, antibiotics, coloured pigments, scents, gums, spices.

● These are called `color{brown}("secondary metabolites.")`

● While `color{violet}("primary metabolites")` have identifiable functions and play known roles in `color{violet}("normal physiologial")` processes, we do not at the moment, understand the role or functions of all the `color{violet}("‘secondary metabolites’")` in host organisms.

● However, many of them are useful to `color{violet}("‘human welfare’")` (e.g., rubber, drugs, spices, scents and pigments).

● Some `color{violet}("secondary metabolites")` have `color{violet}("ecological importance.")`

● There is one feature common to all those compounds found in the `color{brown}("acid soluble pool.")`

● They have molecular weights ranging from `color{brown}("18")` to around `color{brown}("800 daltons (Da) ")` approximately.

● The `color{brown}("acid insoluble fraction,")` has only four types of `color{violet}("organic compounds ")`i.e., proteins, nucleic acids, polysaccharides and lipids.

● These classes of compounds with the exception of `color{violet}("lipids")`, have `color{violet}("molecular weights")` in the range of ten `color{violet}("thousand daltons")` and above.

● For this very reason, `color{violet}("biomolecules,")` i.e., chemical compounds found in `color{violet}("living organisms")` are of two types.

● One, those which have molecular weights less than `color{violet}("one thousand dalton")` and are usually referred to as `color{violet}("micromolecules")` or `color{violet}("simply biomolecules")` while those which are found in the acid insoluble fraction are called macromolecules or`color{brown}(" biomacromolecules.")`

● The molecules in the `color{violet}("insoluble fraction")` with the exception of lipids are`color{brown}(" polymeric substances.")`

● There is a reason that `color{violet}("lipids,")` whose molecular weights do not exceed `color{brown}("800 Da,")` come under acid insoluble fraction, i.e., `color{brown}("macromolecular fraction.")`



● `color{violet}("Lipids")` are indeed small molecular weight compounds and are present not only as such but also arranged into structures like `color{violet}("cell membrane")` and `color{violet}("other membranes.")`

● When we `color{violet}("grind a tissue,")` we are disrupting the `color{violet}("cell structure.")`

● `color{violet}("Cell membrane")` and other membranes are broken into pieces, and form vesicles which are not water soluble.

● Therefore, these membrane fragments in the form of vesicles get separated along with the `color{violet}("acid insoluble pool")` and hence in the `color{violet}("macromolecular fraction.")`

● Lipids are not strictly `color{violet}("macromolecules.")`

● The `color{brown}("acid soluble pool")` represents roughly the `color{violet}("cytoplasmic composition.")`

● The `color{violet}("macromolecules")` from `color{violet}("cytoplasm and organelles")` become the `color{violet}("acid insoluble fraction.")`

● Together they represent the entire chemical composition of `color{violet}("living tissues")` or `color{violet}("organisms.")`

● In summary if we represent the chemical composition of `color{violet}("living tissue")` from abundance point of view and arrange them class-wise, we observe that `color{brown}("water")` is the most abundant chemical in `color{violet}("living organisms.")`