Biomolecules (NEET Biology Class 11): Proteins, Enzymes, Nucleic Acids & Cofactors
Chemical Composition: Primary & Secondary Metabolites
🎯 NEET priority: High-yield. 4–5 questions every year. Enzymes alone has produced more NEET questions over the years than any other single sub-topic in this chapter — give it your sharpest attention.
Grind any living tissue in trichloroacetic acid and strain it: you get an acid-soluble pool (the filtrate — thousands of small organic compounds) and an acid-insoluble fraction (the retentate — the macromolecules). Elemental analysis shows living tissue is NOT chemically different from non-living matter in which elements are present — only in their relative abundance: carbon and hydrogen are far more abundant in living tissue (18.5% and 9.5% of the human body by weight) than in the earth's crust (0.03% and 0.14%).
Primary vs secondary metabolites
Primary metabolites (amino acids, sugars, etc.) have identifiable functions and known roles in normal physiology — found across all organisms.
Secondary metabolites are found in plants, fungi and microbes and include pigments (carotenoids, anthocyanins), alkaloids (morphine, codeine), terpenoids, essential oils, toxins (abrin, ricin), lectins and drugs (vinblastine, curcumin) — their role in the host organism is often not fully understood, but many are valuable to human welfare (rubber, spices, drugs).
Biomacromolecules: Proteins & Polysaccharides
Micromolecules vs macromolecules
Compounds in the acid-soluble pool have molecular weights 18 to ~800 daltons — micromolecules. The acid-insoluble fraction has only four classes: proteins, nucleic acids, polysaccharides and lipids — macromolecules, with weights ≥10,000 Da (lipids are the exception: they're small, but end up in the insoluble fraction because grinding shatters cell membranes into insoluble vesicles).
By average cellular mass: water is the most abundant (70–90%), then proteins (10–15%), nucleic acids (5–7%), carbohydrates (~3%), lipids (~2%).
Proteins — four levels of structure
Proteins are heteropolymers of amino acids (not homopolymers — 20 types of amino acids occur in proteins, joined by peptide bonds). The primary structure is the amino acid sequence (N-terminal → C-terminal). Portions fold into a right-handed helix or other shapes — the secondary structure. The chain then folds on itself like a hollow woollen ball — the tertiary structure, essential for biological activity. Multiple polypeptide subunits arranged together give the quaternary structure — e.g. adult human haemoglobin has 4 subunits (2 α-type + 2 β-type).
Collagen is the most abundant protein in the animal world; RuBisCO is the most abundant protein in the entire biosphere.
Polysaccharides
Cellulose (plant cell walls; homopolymer of glucose only), starch (plant energy store, forms a helix that binds Iâ‚‚ giving a blue-black colour) and glycogen (the animal equivalent of starch) are all glucose polymers. Cellulose has no helix, so it does NOT bind iodine. Chitin forms arthropod exoskeletons, built from amino-sugars.
Nucleic Acids & Enzyme Mechanism/Classification
Nucleic acids — nucleoside vs nucleotide
A nucleotide has three components: a nitrogenous base + a pentose sugar + a phosphate group. Base + sugar alone (no phosphate) = a nucleoside (e.g. adenosine); base + sugar + phosphate = nucleotide (e.g. adenylic acid).
Adenine and guanine are purines; cytosine, uracil and thymine are pyrimidines. A nucleic acid with deoxyribose is DNA; one with ribose is RNA.
Enzymes — mechanism & classification
Almost all enzymes are proteins (a few catalytic RNAs are called ribozymes). The active site is the crevice/pocket where the substrate binds. Enzymes accelerate reactions enormously — carbonic anhydrase speeds up CO₂ + H₂O ⇌ H₂CO₃ by about 10 million times (200 molecules/hour uncatalysed vs 600,000/second catalysed).
Enzymes lower the activation energy — the extra energy the substrate needs to reach the unstable transition state before becoming product. The catalytic cycle: (1) substrate binds the active site, (2) the enzyme changes shape to fit more tightly (induced fit), (3) bonds break and the enzyme-product (EP) complex forms, (4) product is released and the free enzyme repeats the cycle.
Enzymes are sorted into 6 classes by reaction type, numbered in this exact order (a common "match the class to its reaction" NEET question):
Class | Reaction catalysed | Example |
Oxidoreductases | Oxidation-reduction between two substrates | Dehydrogenases |
Transferases | Transfer of a group (other than hydrogen) between substrates | Transaminases |
Hydrolases | Hydrolysis of ester, ether, peptide, glycosidic bonds | Amylase, lipase, protease |
Lyases | Non-hydrolytic removal of groups, leaving a double bond | Decarboxylase |
Isomerases | Inter-conversion of optical, geometric or positional isomers | Isomerase |
Ligases | Joining two compounds (e.g. C-O, C-S, C-N, P-O bonds) | Synthetase |
Enzymes: Factors, Inhibition & Cofactors
Factors affecting enzyme activity
Each enzyme has an optimum temperature and pH; activity falls on either side. Low temperature only makes an enzyme temporarily inactive, but high temperature denatures it (proteins unfold) — this is why most enzymes fail above ~40°C, though enzymes from thermophilic organisms (hot springs, sulphur vents) stay stable up to 80–90°C.
As substrate concentration rises, reaction velocity rises too, then plateaus at Vmax — beyond this, all enzyme molecules are saturated, so adding more substrate does nothing.
A competitive inhibitor structurally resembles the substrate and competes for the active site, blocking the true substrate — e.g. malonate inhibits succinic dehydrogenase because it resembles the real substrate, succinate. This principle is used to control bacterial pathogens with drugs.
Cofactors — three kinds
The protein part of an enzyme needing a cofactor is called the apoenzyme. Prosthetic groups are organic and tightly, permanently bound (e.g. haem in peroxidase/catalase). Coenzymes are organic but bind only transiently during catalysis — many are vitamin-derived (e.g. NAD/NADP contain the vitamin niacin). Metal ions form coordination bonds with both enzyme and substrate — e.g. zinc is a cofactor for carboxypeptidase. Remove the cofactor and catalytic activity is lost.
Why this matters for NEET
High-value one-liners: 20 amino acid types; haemoglobin's 4 subunits (2α+2β); starch binds iodine (blue-black), cellulose does NOT; DNA=deoxyribose, RNA=ribose; 6 enzyme classes; 3 cofactor types.
Trap: a nucleoside (base+sugar) is NOT the same as a nucleotide (base+sugar+phosphate) — don't swap them. Cellulose and starch are BOTH glucose homopolymers, but only starch's helical shape binds iodine. Prosthetic groups are permanently bound; coenzymes are only transiently bound — this exact distinction is a repeat NEET question.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Compounds found in the acid-soluble pool of a ground tissue are generally called:
(a) Macromolecules
(b) Nucleic acids only
(c) Polysaccharides only
(d) Micromolecules
Show answer
Answer: (d) — The acid-soluble pool has small organic compounds (18–800 Da), called micromolecules.
Q2. Which of these is NOT one of the four classes of macromolecules in the acid-insoluble fraction?
(a) Amino acids
(b) Nucleic acids
(c) Proteins
(d) Polysaccharides
Show answer
Answer: (a) — The four macromolecule classes are proteins, nucleic acids, polysaccharides and lipids; free amino acids are micromolecules.
Q3. The amino acid sequence of a protein is called its:
(a) Quaternary structure
(b) Primary structure
(c) Tertiary structure
(d) Secondary structure
Show answer
Answer: (b) — Primary structure is simply the linear sequence of amino acids from N-terminal to C-terminal.
Q4. Adult human haemoglobin is made up of:
(a) 1 single polypeptide
(b) 6 identical subunits
(c) 4 subunits: 2 alpha and 2 beta
(d) 2 subunits, both alpha
Show answer
Answer: (c) — Human haemoglobin has 4 subunits — 2 of the alpha type and 2 of the beta type — its quaternary structure.
Q5. Which polysaccharide forms a helix and gives a blue-black colour with iodine?
(a) Chitin
(b) Starch
(c) Glycogen only
(d) Cellulose
Show answer
Answer: (b) — Starch's helical secondary structure can trap I2 molecules, producing the blue-black starch-iodine colour; cellulose has no helix.
Q6. A nucleoside differs from a nucleotide in that a nucleoside lacks:
(a) Both base and sugar
(b) A phosphate group
(c) A pentose sugar
(d) A nitrogenous base
Show answer
Answer: (b) — A nucleoside is base + sugar only; adding a phosphate group makes it a nucleotide.
Q7. Which of the following is a pyrimidine?
(a) Guanine
(b) Both adenine and guanine
(c) Adenine
(d) Cytosine
Show answer
Answer: (d) — Cytosine, uracil and thymine are pyrimidines; adenine and guanine are purines.
Q8. The crevice or pocket on an enzyme into which the substrate fits is the:
(a) Transition state
(b) Apoenzyme
(c) Prosthetic group
(d) Active site
Show answer
Answer: (d) — The active site is the pocket formed by the enzyme's folded tertiary structure where the substrate binds.
Q9. The extra energy needed to bring the substrate to its transition state is called the:
(a) Kinetic energy
(b) Activation energy
(c) Potential energy of product
(d) Free energy
Show answer
Answer: (b) — Activation energy is the energy gap between the substrate's stable state and the unstable transition state; enzymes lower this barrier.
Q10. Enzymes are classified into how many major classes based on reaction type?
(a) 6
(b) 5
(c) 8
(d) 4
Show answer
Answer: (a) — Enzymes are divided into 6 classes: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases.
Q11. Malonate inhibits succinic dehydrogenase by acting as a:
(a) Coenzyme
(b) Prosthetic group
(c) Metal ion cofactor
(d) Competitive inhibitor
Show answer
Answer: (d) — Malonate structurally resembles succinate (the true substrate) and competes for the active site — a competitive inhibitor.
Q12. Haem, in enzymes like catalase and peroxidase, is an example of a:
(a) Prosthetic group
(b) Metal ion cofactor
(c) Substrate
(d) Coenzyme
Show answer
Answer: (a) — Haem is tightly, permanently bound to the apoenzyme, making it a prosthetic group, not a transiently-bound coenzyme.
Q13. Beyond the substrate concentration that produces Vmax, the reaction rate:
(a) Keeps increasing linearly
(b) Does not increase further
(c) Becomes negative
(d) Decreases sharply
Show answer
Answer: (b) — At Vmax all enzyme molecules are saturated with substrate, so further substrate increase does not raise the rate.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q14. The most abundant protein in the biosphere is: (NEET PYQ)
(a) RuBisCO
(b) Haemoglobin
(c) Collagen
(d) Insulin
Show answer
Answer: (a) — RuBisCO is the most abundant protein in the entire biosphere; collagen is the most abundant in the animal world specifically.
Q15. Which statement about coenzymes is correct? (NEET PYQ)
(a) They are inorganic and permanently bound
(b) They are organic and bind only transiently during catalysis
(c) They are always metal ions
(d) They are identical to prosthetic groups
Show answer
Answer: (b) — Coenzymes are organic cofactors that associate with the apoenzyme only transiently, often during catalysis; many are vitamin-derived.
Q16. A nucleic acid containing deoxyribose sugar is called: (NEET PYQ)
(a) RNA
(b) A ribozyme
(c) DNA
(d) A nucleoside only
Show answer
Answer: (c) — A nucleic acid built on deoxyribose is DNA; one built on ribose is RNA.
Q17. The reaction catalysed by carbonic anhydrase is accelerated by approximately how many times compared to the uncatalysed reaction? (NEET PYQ)
(a) 2 times
(b) 10 million times
(c) 100 times
(d) 10,000 times
Show answer
Answer: (b) — Carbonic anhydrase speeds up CO2 + H2O to carbonic acid by roughly 10 million times.
Q18. Proteins are described as heteropolymers because: (NEET PYQ)
(a) They contain only sugars
(b) They contain only one type of amino acid
(c) They are not polymers at all
(d) They contain 20 different types of amino acids
Show answer
Answer: (d) — Proteins are built from 20 different amino acid types, unlike a homopolymer which repeats a single monomer.
Q19. Zinc acts as a cofactor for the enzyme: (NEET PYQ)
(a) Carbonic anhydrase
(b) Carboxypeptidase
(c) Catalase
(d) Succinic dehydrogenase
Show answer
Answer: (b) — Zinc is a metal ion cofactor required by the proteolytic enzyme carboxypeptidase.
Q20. Which polysaccharide is a homopolymer of glucose found in the cell wall of plants? (NEET PYQ)
(a) Inulin
(b) Cellulose
(c) Chitin
(d) Glycogen
Show answer
Answer: (b) — Cellulose, a homopolymer of glucose only, forms the plant cell wall.
Q21. The tertiary structure of a protein is essential for its: (NEET PYQ)
(a) Biological activity
(b) Solubility in acid only
(c) Colour
(d) Molecular weight
Show answer
Answer: (a) — The 3-dimensional tertiary structure, formed when the protein chain folds on itself, is absolutely necessary for a protein's biological activity.
Active Recall Prompt
Write everything you can recall about Biomolecules, naming each part first: chemical composition (acid-soluble vs acid-insoluble, primary vs secondary metabolites); micromolecules vs macromolecules and average cell composition; protein structure (primary through quaternary, haemoglobin, collagen, RuBisCO); polysaccharides (cellulose, starch, glycogen, chitin, the iodine test); nucleic acids (nucleoside vs nucleotide, purines vs pyrimidines, DNA vs RNA); and enzymes (active site, activation energy, the catalytic cycle, the 6 classes, factors affecting activity, competitive inhibition, and the 3 types of cofactors). Begin each fact with its topic and end it with a full stop.