Biomicromolecules

Biomicromolecules are the small carbon compounds of living tissue, with molecular weights below about one thousand daltons. They include amino acids, sugars, fatty acids, glycerol, nitrogen bases, nucleosides and nucleotides. This page explains how a tissue is analysed, what the acid-soluble pool contains, the structures of the main biomicromolecules, and primary versus secondary metabolites, as in the NCERT Class 11 chapter Biomolecules. NEET asks these points as match-the-list and statement questions on examples, numbers and structures.

1. Living and Non-living Matter: The Elements

• Living organisms show a wide diversity. Yet the question remains: are they all made of the same chemicals?
• Despite this diversity, the chemical composition and metabolic reactions of living organisms are remarkably similar.
• Elemental analysis of a plant tissue, an animal tissue or a microbial paste lists elements such as carbon, hydrogen and oxygen.
• It also gives the content of each element per unit mass of living tissue.
• The same analysis of a piece of the earth's crust (non-living matter) gives a similar list.
• ★ Exam imp In absolute terms there is no difference: every element of the earth's crust is also present in living tissue.
• ★ Exam imp A closer look shows the difference: the relative abundance of carbon and hydrogen is higher in every living organism.

2. How to Analyse Chemical Composition

2.1 Organic compounds: the acid-soluble pool

To find which organic compounds a tissue contains, a chemical analysis is done:

1. Take any living tissue, such as a vegetable or a piece of liver.
2. Grind it in trichloroacetic acid ($\mathrm{C}{\mathrm{l}}_3\mathrm{CCOOH}$) using a mortar and pestle. A thick slurry forms.
3. Strain the slurry through cheesecloth or cotton. Two fractions are obtained.
4. The filtrate, technically the acid-soluble pool, passes through. The retentate, the acid-insoluble fraction, stays behind.

• ★ Exam imp Scientists have found thousands of organic compounds in the acid-soluble pool.
• To identify one compound, it is extracted and then put through separation techniques until it is separated from all others.
• In other words, the compound is isolated and purified.
• Analytical techniques applied to the pure compound give its molecular formula and probable structure.

2.2 Inorganic compounds: the ash

Living tissues also contain inorganic elements and compounds. A slightly different, destructive experiment shows this:

1. Weigh a small amount of living tissue, such as a leaf or liver. This is the wet weight.
2. Dry it. All the water evaporates, and the remaining material gives the dry weight.
3. Burn the tissue fully. All the carbon compounds are oxidised to gases ($\mathrm{C}{\mathrm{O}}_2$ and water vapour) and removed.
4. The material that remains is the ash. It contains inorganic elements such as calcium and magnesium.

• Inorganic compounds such as sulphate and phosphate are also seen in the acid-soluble fraction.
• ★ Exam imp Elemental analysis gives the elemental composition of tissue: hydrogen, oxygen, chlorine, carbon and so on.
• Analysis for compounds shows the kinds of organic (Figures 1 to 3) and inorganic (table below) constituents.

• From a chemistry point of view, compounds are grouped by functional groups: aldehydes, ketones, aromatic compounds and so on.
• From a biological point of view, they are grouped as amino acids, nucleotide bases, fatty acids and so on.

3. Sugars and Amino Acids

3.1 Sugars

• Glucose (${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$) and ribose (${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_5$) are sugars (carbohydrates) found in living tissue.
• Both are monosaccharides. Monosaccharide and disaccharide sugars are among the small biomolecules.

3.2 Amino acids

• ★ Exam imp Amino acids: organic compounds with an amino group and an acidic (carboxyl) group on the same carbon, the $\alpha$-carbon.
• Hence they are called $\alpha$-amino acids.
• They are substituted methanes: four groups occupy the four valencies of the $\alpha$-carbon.
• The four groups are hydrogen, carboxyl group, amino group and a variable R group.
• Many amino acids exist, depending on the R group. Those found in proteins are only of 20 types.
• R group = hydrogen in glycine, methyl group in alanine, hydroxy methyl in serine (Figure 1).
• The chemical and physical properties of amino acids come from their amino, carboxyl and R groups.

3.3 Ionisable groups and the zwitterion

• ★ Exam imp A particular property of amino acids is the ionisable nature of their $-\mathrm{N}{\mathrm{H}}_2$ and $-\mathrm{COOH}$ groups.
• Hence the structure of an amino acid changes in solutions of different pH: forms A, B and C below.

4. Lipids

4.1 Fatty acids and glycerol

• ★ Exam imp Lipids are generally water insoluble. They could be simple fatty acids.
• A fatty acid has a carboxyl group attached to an R group.
• The R group could be a methyl ($-\mathrm{C}{\mathrm{H}}_3$), an ethyl ($-{\mathrm{C}}_2{\mathrm{H}}_5$) or a higher number of $-\mathrm{C}{\mathrm{H}}_2$ groups (1 carbon to 19 carbons).
• ★ Exam imp Palmitic acid has 16 carbons, including the carboxyl carbon: $\mathrm{C}{\mathrm{H}}_3(\mathrm{C}{\mathrm{H}}_2{)}_{14}\mathrm{COOH}$.
• Arachidonic acid has 20 carbon atoms, including the carboxyl carbon.
• Glycerol, another simple lipid, is trihydroxy propane.

4.2 Glycerides, fats and oils

• Many lipids have both glycerol and fatty acids. Here the fatty acids are esterified with glycerol.
• ★ Exam imp They can be monoglycerides, diglycerides or triglycerides, with one, two or three fatty acids.
• These are also called fats and oils, based on their melting point.
• Oils have a lower melting point (for example, gingelly oil), so they stay liquid in winter.

4.3 Phospholipids and complex lipids

• Phospholipids: lipids containing phosphorus and a phosphorylated organic compound.
• ★ Exam imp Phospholipids are found in the cell membrane. Lecithin is one example.
• A phospholipid carries a phosphorylated nitrogenous compound in addition to glycerol and fatty acids.
• Some tissues, especially neural tissues, have lipids with more complex structures.
• Cholesterol, with four fused rings, is another lipid shown among the small molecules (Figure 2).

5. Nitrogen Bases, Nucleosides and Nucleotides

• Many carbon compounds of living organisms contain heterocyclic rings.
• ★ Exam imp Some of these are the nitrogen bases: adenine, guanine, cytosine, uracil and thymine.
• A nitrogen base attached to a sugar is a nucleoside.
• If a phosphate group is also esterified to the sugar, it is a nucleotide.
• Nucleosides: adenosine, guanosine, thymidine, uridine and cytidine.
• Nucleotides: adenylic acid, thymidylic acid, guanylic acid, uridylic acid and cytidylic acid.
• ★ Exam imp Nucleic acids such as DNA and RNA consist of nucleotides only. DNA and RNA function as genetic material.
• In Figure 3, adenine is a purine and uracil is a pyrimidine.

6. Primary and Secondary Metabolites

• A major task of chemistry is to isolate thousands of compounds from living organisms, determine their structure and, if possible, synthesise them.
• A list of biomolecules would have thousands of organic compounds, including amino acids and sugars.
• ★ Exam imp These biomolecules can be called metabolites.
• Animal tissues contain all the categories of compounds shown in Figures 1 to 3. These are primary metabolites.
• Plant, fungal and microbial cells show thousands of other compounds as well. These are secondary metabolites.
• ★ Exam imp Examples: alkaloids, flavonoids, rubber, essential oils, antibiotics, coloured pigments, scents, gums and spices.

• Secondary metabolites useful to human welfare include rubber, drugs, spices, scents and pigments.

7. Exam Essentials

Pairs to Match

Numbers to Remember

• Human body: O 65.0%, C 18.5%, H 9.5%, N 3.3%, Ca 1.5%, S 0.3%, Na 0.2%, Mg 0.1%.
• Earth's crust: O 46.6%, Si 27.7%, Ca 3.6%, Na 2.8%, Mg 2.1%, H 0.14%, C and S 0.03% each.
• 20 types of amino acids in proteins; 5 nitrogen bases, 5 nucleosides and 5 nucleotides named.
• Fatty acid R group: 1 to 19 carbons; palmitic acid 16 C; arachidonic acid 20 C.
• Small biomolecules: molecular weight less than 1000 Da.

Examples to Remember

8. Quick Revision

• Living tissue and the earth's crust contain the same elements; C and H are relatively richer in life.
• Grinding tissue in trichloroacetic acid and straining gives the acid-soluble pool and the acid-insoluble fraction.
• Thousands of organic compounds occur in the acid-soluble pool.
• All carbon compounds from living tissue are called biomolecules.
• Wet weight, then dry weight, then ash: the ash holds inorganic elements such as Ca and Mg.
• $\alpha$-amino acids: H, COOH, $\mathrm{N}{\mathrm{H}}_2$ and R on the $\alpha$-carbon; 20 types in proteins.
• Glycine (H), alanine (methyl), serine (hydroxy methyl); glutamic acid, lysine, valine; aromatic tyrosine, phenylalanine, tryptophan.
• The amino acid structure changes with pH; form B is the zwitterion.
• Fatty acid = COOH + R (1 to 19 C); palmitic 16 C; arachidonic 20 C; saturated or unsaturated.
• Glycerol is trihydroxy propane; mono-, di- and triglycerides are fats or oils.
• Phospholipids (lecithin) occur in the cell membrane; neural tissue has complex lipids.
• Nucleoside = base + sugar; nucleotide = nucleoside + phosphate; DNA and RNA are made of nucleotides.
• Primary metabolites have known roles; secondary metabolites occur in plants, fungi and microbes.
• Secondary metabolites: pigments, alkaloids, terpenoids, essential oils, toxins, lectins, drugs, polymers.

9. Solved Examples

10. Practice Questions

Common Mistakes to Avoid