Q.What are macromolecules? Give examples.

Macromolecules are the large biomolecules that make up the acid-insoluble fraction of a living tissue. They have very high molecular weights, generally about ten thousand daltons and above, and (except for lipids) they are polymers — long chains built by joining many small repeating units.

There are only four kinds of organic compound in this fraction:

• Proteins — heteropolymers of amino acids.
• Nucleic acids (DNA and RNA) — polymers of nucleotides.
• Polysaccharides — polymers of sugars, such as cellulose, starch and glycogen.
• Lipids — which are not truly macromolecules by size (their weight does not exceed about 800 daltons) but travel with this fraction because they form membrane fragments and vesicles that are insoluble in water.

When the compounds of a living tissue are sorted by their size, a clear pattern appears. Grinding tissue in acid and straining it separates the material into two pools: an acid-soluble pool of small molecules and an acid-insoluble fraction of large ones. The molecules in the acid-soluble pool are all fairly small, with molecular weights roughly in the range of eighteen to about eight hundred daltons. The acid-insoluble fraction is quite different: it contains only a few kinds of organic compound, and these run to molecular weights of about ten thousand daltons and above. This size difference is exactly the basis for calling one group micromolecules (or simply biomolecules) and the other group macromolecules, or biomacromolecules.

What sits in the acid-insoluble, macromolecular fraction is a short and important list:

• Proteins — polymers of amino acids joined by peptide bonds. Because twenty different amino acids can be used, a protein is a heteropolymer (a polymer built from more than one kind of building block).
• Nucleic acids — DNA and RNA, which are polynucleotides, long chains built from repeating nucleotide units.
• Polysaccharides — long chains of sugars, such as cellulose (a homopolymer of glucose), starch, glycogen and inulin.
• Lipids — grouped here with a special explanation (below).

A key feature of these compounds is that, with the single exception of lipids, they are polymers — long molecules assembled by stringing together many smaller units, much like a thread made from many small beads.

Method: Apply the Acid-Solubility Test to Define Macromolecules

The cleanest way into this definition is to recall the actual laboratory operation that separates macromolecules from everything else in a cell: grinding tissue in a suitable acid (like trichloroacetic acid) and filtering it produces two fractions — a small-molecule, acid-soluble fraction, and a large-molecule, acid-insoluble fraction. Macromolecules are, by definition, whatever ends up in that acid-insoluble fraction, and the reason to lead with this operational definition rather than jumping straight to "proteins, nucleic acids, polysaccharides" is that it also explains WHY these particular four categories are grouped together, including the odd one out.

Once you have the operational definition, add the size criterion that goes with it: molecules in the acid-insoluble fraction run to roughly ten thousand daltons and above, versus roughly eighteen to eight hundred daltons for the small, acid-soluble molecules — this size gap is what earns the group the name "macro"-molecules.

The most commonly missed point in this answer is explaining why lipids are included in the list despite not being true macromolecules by weight. Treat this as its own short paragraph: lipids are chemically small (well under a thousand daltons) but end up in the acid-insoluble fraction anyway, because in the intact cell many lipids are organised into membranes, and when the tissue is homogenised those membrane fragments reseal into tiny water-insoluble vesicles that get carried along with the acid-insoluble pool for a purely physical, not chemical, reason. Naming this exception explicitly is what separates a strong answer from a merely adequate list of the four categories.