Biomacromolecules

Biomacromolecules are the large molecules of living tissue that collect in the acid-insoluble fraction: proteins, nucleic acids and polysaccharides, with lipids separating alongside them. This page explains why lipids fall in this fraction, the average composition of a cell, proteins and their functions, polysaccharides such as cellulose, starch, glycogen, inulin and chitin, nucleic acids, and the four levels of protein structure, as in the NCERT Class 11 chapter Biomolecules. NEET asks biomacromolecules mostly as statement sets and match lists.

1. The Acid-insoluble Fraction

1.1 Micromolecules and macromolecules

• ★ Exam imp All compounds of the acid-soluble pool have molecular weights from 18 to around 800 daltons (Da).
• The acid-insoluble fraction has only four types of organic compounds: proteins, nucleic acids, polysaccharides and lipids.
• Except lipids, these classes have molecular weights of ten thousand daltons and above.
• For this reason, biomolecules (chemical compounds of living organisms) are of two types.
• ★ Exam imp Except lipids, the molecules of the insoluble fraction are polymeric substances.

1.2 Why lipids appear in the insoluble fraction

Lipids have molecular weights that do not exceed 800 Da, yet they come down with the macromolecules:

1. Lipids are small molecules. They occur as such and are also arranged into the cell membrane and other membranes.
2. Grinding a tissue disrupts the cell structure, and the membranes break into pieces.
3. The pieces form vesicles, which are not water soluble.
4. These vesicles separate along with the acid-insoluble pool, so lipids appear in the macromolecular fraction.

1.3 Average composition of a cell

• The acid-soluble pool represents roughly the cytoplasmic composition.
• The macromolecules from the cytoplasm and organelles form the acid-insoluble fraction.
• Together, the two fractions represent the entire chemical composition of living tissues or organisms.
• ★ Exam imp Arranged class-wise by abundance, water is the most abundant chemical in living organisms.

2. Proteins

• ★ Exam imp Proteins are polypeptides: linear chains of amino acids linked by peptide bonds (Figure 2).
• Each protein is a polymer of amino acids.
• There are 20 types of amino acids, for example alanine, cysteine, proline, tryptophan and lysine.
• ★ Exam imp So a protein is a heteropolymer, not a homopolymer.
• Homopolymer: a polymer in which only one type of monomer repeats 'n' number of times.
• Certain amino acids are essential for our health and must be supplied through the diet. Dietary proteins are the source of these essential amino acids.

• Proteins carry out many functions: some transport nutrients across the cell membrane, some fight infectious organisms, some are hormones and some are enzymes.

3. Polysaccharides

• The acid-insoluble pellet also has polysaccharides (carbohydrates), another class of macromolecules.
• ★ Exam imp Polysaccharides are long chains of sugars: threads (literally like a cotton thread) with different monosaccharides as building blocks.
• Cellulose is a polymeric polysaccharide made of only one type of monosaccharide, glucose. So cellulose is a homopolymer.
• Starch is a variant of this, present as a store house of energy in plant tissues.
• Animals have another variant called glycogen.
• ★ Exam imp Inulin is a polymer of fructose.
• In a polysaccharide chain (say glycogen), the right end is the reducing end and the left end the non-reducing end.
• Glycogen has branches, as shown in the cartoon (Figure 1).

• ★ Exam imp Starch forms helical secondary structures and can hold ${\mathrm{I}}_2$ molecules in the helical portion. The starch-${\mathrm{I}}_2$ is blue.
• Cellulose does not contain complex helices, so it cannot hold ${\mathrm{I}}_2$.
• Plant cell walls are made of cellulose. Paper made from plant pulp and cotton fibre are cellulosic.
• More complex polysaccharides have amino-sugars and chemically modified sugars as building blocks, for example glucosamine and N-acetyl galactosamine.
• ★ Exam imp The exoskeleton of arthropods has a complex polysaccharide called chitin. These complex polysaccharides are mostly homopolymers.
• Polysaccharides form the cell walls of plants and fungi and the exoskeleton of arthropods. Starch and glycogen are storage forms of energy.

4. Nucleic Acids

• ★ Exam imp Nucleic acids are polynucleotides, found in the acid-insoluble fraction of every living tissue.
• Together with polysaccharides and polypeptides, they form the true macromolecular fraction of any living tissue or cell.
• The building block of a nucleic acid is a nucleotide.
• A nucleotide has three chemically distinct components: a heterocyclic compound, a monosaccharide, and phosphoric acid or phosphate.
• The heterocyclic compounds are the nitrogenous bases: adenine, guanine, uracil, cytosine and thymine.
• ★ Exam imp Adenine and guanine are substituted purines; uracil, cytosine and thymine are substituted pyrimidines.
• The skeletal heterocyclic rings are called purine and pyrimidine respectively.
• The sugar is either ribose (a monosaccharide pentose) or 2′-deoxyribose.
• Nucleic acids serve as genetic material; they carry hereditary information from the parental generation to the progeny.

5. Structure of Proteins

• Proteins are heteropolymers containing strings of amino acids.
• 'Structure of a molecule' means different things in different contexts:

Biologists describe protein structure at four levels:

1. Primary structure: the sequence of amino acids, that is, the positional information (which amino acid is first, which second, and so on). The protein is imagined as a line: the left end is the first amino acid (N-terminal) and the right end the last amino acid (C-terminal).
2. Secondary structure: the thread is not an extended rigid rod. Some portions fold into a helix, like a revolving staircase; only right-handed helices occur in proteins. Other regions fold into other forms.
3. Tertiary structure: the long chain is folded upon itself like a hollow woollen ball. This gives a three-dimensional view of the protein.
4. Quaternary structure: some proteins are an assembly of more than one polypeptide or subunit. The arrangement of the folded subunits with respect to each other (a linear string of spheres, or spheres stacked as a cube or plate) is the protein's architecture.

• ★ Exam imp Adult human haemoglobin has 4 subunits: two identical $\alpha$ subunits and two identical $\beta$ subunits.

6. Exam Essentials

Pairs to Match

Numbers to Remember

• Acid-soluble pool: 18 to about 800 Da; lipids: not above 800 Da.
• Micromolecules: less than 1000 Da; macromolecules (except lipids): 10,000 Da and above.
• Cell mass: water 70-90%, proteins 10-15%, nucleic acids 5-7%, carbohydrates 3%, lipids 2%, ions 1%.
• 20 types of amino acids; 4 classes in the insoluble fraction; 3 true macromolecules.
• Protein structure: 4 levels. Adult human haemoglobin: 4 subunits (2 $\alpha$ + 2 $\beta$).
• Nucleotide: 3 components. Bases: 2 purines and 3 pyrimidines.

Examples to Remember

7. Quick Revision

• Acid-soluble pool: 18 to about 800 Da; acid-insoluble fraction: proteins, nucleic acids, polysaccharides, lipids.
• Micromolecules are below 1000 Da; macromolecules (except lipids) are 10,000 Da and above and are polymers.
• Lipids come down as membrane vesicles, so they are not strictly macromolecules.
• The soluble pool roughly equals the cytoplasm; both fractions together give the whole composition.
• Water is the most abundant chemical: 70-90% of cell mass.
• Proteins are heteropolymers of 20 amino acids linked by peptide bonds.
• Essential amino acids come from the diet; non-essential ones are made by the body.
• Collagen is most abundant in animals; RuBisCO in the whole biosphere.
• Cellulose (glucose homopolymer), starch (plants), glycogen (animals, branched), inulin (fructose), chitin (arthropods).
• Starch helices hold iodine and turn blue; cellulose cannot.
• In glycogen, the right end is reducing and the left end non-reducing.
• Nucleotide = heterocyclic base + monosaccharide + phosphate; A and G are purines; U, C and T are pyrimidines.
• DNA has deoxyribose; RNA has ribose; nucleic acids are genetic material.
• Primary (sequence, N- to C-terminal), secondary (right-handed helix), tertiary (woollen ball), quaternary (subunits).
• Adult human haemoglobin: 2 $\alpha$ + 2 $\beta$ subunits.

8. Solved Examples

9. Practice Questions

Common Mistakes to Avoid