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NCERT Biology Quiz

Biomolecules (NCERT Biology)

Biomolecules are the chemical constituents of living tissue. The NCERT treatment moves from how we find out what a cell is made of, through the four great classes of macromolecule (carbohydrates, proteins, lipids and nucleic acids), to enzymes and the metabolic pool. These notes cover chemical analysis and the acid-soluble and acid-insoluble pools, primary and secondary metabolites, the structure and classification of carbohydrates, amino acids and the four levels of protein structure, lipids, nucleotides and nucleic acid structure, the dynamic state of body constituents, and enzymes with their kinetics, cofactors and inhibition. A 20-question practice set follows.

Chemical Analysis of Living Tissue

To find what a tissue is made of, a piece of living tissue is ground in trichloroacetic acid and filtered. What passes through the filter is the acid-soluble pool of small molecules; what stays on the filter is the acid-insoluble pool of macromolecules. This one experiment organises the whole chapter.

  • Acid-soluble pool: molecules with molecular weight generally between 18 and about 800 daltons, including amino acids, nucleotides, sugars, vitamins and organic acids. This is roughly the cytoplasmic pool.
  • Acid-insoluble pool: macromolecules with molecular weight above about 10000 daltons, namely proteins, nucleic acids, polysaccharides and lipids.
  • Lipids are an anomaly: their individual molecular weight is not high, but because they form membrane fragments and vesicles during grinding they are trapped in the filtrate residue and so appear in the acid-insoluble fraction.
  • Approximate composition of a living cell by weight: water about 70 to 90 percent, proteins about 10 to 15 percent, carbohydrates about 3 percent, lipids about 2 percent, nucleic acids 5 to 7 percent, ions about 1 percent.
  • The most abundant chemical in living organisms is water; the most abundant organic compound (macromolecule) in the cell is protein; the most abundant organic compound in the whole biosphere is cellulose.
  • Primary metabolites have identifiable roles in normal physiological processes: amino acids, sugars, nucleotides and lipids. Secondary metabolites, seen mainly in plants, fungi and microbes, include alkaloids (morphine, codeine), flavonoids, rubber, essential oils, antibiotics, gums, spices, scents, pigments, drugs and toxins such as ricin and abrin. Their role in the producing organism is often unclear, but they have great ecological and human use.

Carbohydrates and Lipids

Carbohydrates are polyhydroxy aldehydes or ketones, and are the main source of energy and an important structural material. Lipids are defined operationally as substances that dissolve in organic solvents rather than by a single chemical structure.

  • Monosaccharides: glucose, fructose and galactose (six carbon, hexoses), ribose and deoxyribose (five carbon, pentoses). Glucose is an aldose; fructose is a ketose.
  • Disaccharides: sucrose (glucose plus fructose), maltose (glucose plus glucose), lactose (glucose plus galactose). They are joined by a glycosidic bond.
  • Polysaccharides: starch (plant storage, forms a helix and gives a blue colour with iodine), glycogen (animal storage, highly branched), cellulose (plant cell wall, unbranched, no helix, not digestible by humans), chitin (arthropod exoskeleton and fungal wall, a polymer of N-acetylglucosamine), inulin (a polymer of fructose).
  • In a polysaccharide chain the end that carries a free aldehyde or ketone group is the reducing end; the other is the non-reducing end.
  • Fatty acids may be saturated (no double bond, for example palmitic acid with 16 carbons) or unsaturated (one or more double bonds, for example oleic acid with 18 carbons and one double bond).
  • Glycerol is trihydroxy propane. One glycerol plus three fatty acids linked by ester bonds gives a triglyceride, which is a neutral fat.
  • Phospholipids such as lecithin contain glycerol, two fatty acids and a phosphate group; they are amphipathic, with a hydrophilic head and hydrophobic tails, which is the basis of the cell membrane bilayer.
  • Fats with a melting point above room temperature are solid (for example ghee); those with a lower melting point are oils (for example gingelly oil).

Amino Acids, Proteins and Nucleic Acids

Proteins are heteropolymers of amino acids, not homopolymers. There are twenty standard amino acids, and the sequence in which they are joined by peptide bonds determines the shape and therefore the function of every protein.

  • An amino acid has a central carbon bearing an amino group, a carboxyl group, a hydrogen and a variable R group. Glycine has hydrogen as its R group, alanine a methyl group, serine a hydroxymethyl group.
  • Amino acids are classified by their R group as acidic (glutamic acid), basic (lysine, arginine), neutral (valine), aromatic (tyrosine, tryptophan, phenylalanine) and sulphur-containing (cysteine, methionine).
  • The ionisable nature of the amino and carboxyl groups means the charge of an amino acid changes with pH; at a particular pH it exists as a zwitterion.
  • The bond joining two amino acids is a peptide bond, formed with the loss of a water molecule (a dehydration or condensation reaction).
  • The most abundant protein in the whole biosphere is RuBisCO (ribulose bisphosphate carboxylase-oxygenase); the most abundant protein in the animal world is collagen.
  • Levels of protein structure: primary is the sequence and position of amino acids; secondary is the regular local folding into a right-handed helix or sheet; tertiary is the overall three-dimensional folding needed for biological activity; quaternary is the arrangement of two or more polypeptide subunits, as in haemoglobin with two alpha and two beta chains.
  • Nucleotide equals nitrogenous base plus pentose sugar plus phosphate. Without the phosphate it is a nucleoside (adenosine, guanosine, thymidine, uridine, cytidine).
  • Purines are adenine and guanine (two rings); pyrimidines are cytosine, thymine and uracil (one ring). Thymine occurs in DNA and uracil in RNA.
  • The base is linked to the sugar by an N-glycosidic bond, and nucleotides are joined to one another by 3 prime to 5 prime phosphodiester bonds.
  • The Watson and Crick double helix: two antiparallel strands, right-handed, base pairs held by hydrogen bonds with adenine pairing to thymine by two bonds and guanine pairing to cytosine by three bonds. The pitch is 3.4 nanometres with about 10 base pairs per turn, so adjacent base pairs are about 0.34 nanometres apart.

Metabolism and Enzymes

Every biomolecule in a living organism is constantly being made and broken down; this turnover is called the dynamic state of body constituents. Metabolic reactions do not occur in isolation but form linked pathways, and almost every one of them is catalysed by an enzyme. Living organisms exist in a steady state, not at equilibrium, and this non-equilibrium state is maintained by energy input.

  • Anabolic pathways consume energy and build molecules (for example amino acids to protein); catabolic pathways release energy by breaking molecules down (for example glucose to lactic acid).
  • Enzymes are almost all proteins; the exceptions are ribozymes, which are nucleic acids with catalytic activity.
  • An enzyme lowers the activation energy of a reaction and so speeds it up, without being consumed and without changing the equilibrium point.
  • Enzymes are far more efficient than inorganic catalysts. Carbonic anhydrase, for example, can hydrate about 600000 molecules of carbon dioxide per second, roughly ten million times faster than the uncatalysed reaction.
  • The catalytic cycle: the substrate binds the active site to form an enzyme-substrate complex, the enzyme changes shape to bring the substrate closer, the transition state forms, products are released and the free enzyme is regenerated.
  • Enzyme activity is affected by temperature and pH, each with an optimum, and by substrate concentration. As substrate concentration rises the rate rises and then plateaus at Vmax because all active sites are saturated.
  • Cofactors: prosthetic groups are tightly bound organic cofactors (haem in peroxidase and catalase), coenzymes are loosely bound organic cofactors often derived from vitamins (NAD and NADP contain niacin), and metal ions act as cofactors (zinc in carboxypeptidase). The protein part without its cofactor is the apoenzyme.
  • Inhibition: a competitive inhibitor resembles the substrate and binds the active site, for example malonate competing with succinate for succinic dehydrogenase. Its effect can be overcome by raising the substrate concentration.
  • The six classes of enzymes are oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases.

Key Terms

Acid-insoluble pool
The fraction of a tissue left on the filter after grinding in trichloroacetic acid, containing the macromolecules: proteins, nucleic acids, polysaccharides and lipids.
Secondary metabolite
A compound such as an alkaloid, rubber, essential oil or antibiotic whose role in the producing organism is often unclear but which has ecological or human value.
Zwitterion
The dipolar ionic form of an amino acid in which the amino group is protonated and the carboxyl group deprotonated at a particular pH.
Ribozyme
A nucleic acid that acts as a biological catalyst, the exception to the rule that all enzymes are proteins.

Practice Quiz — 20 Questions

correct out of 20

  1. Tissue ground in trichloroacetic acid and filtered leaves proteins and nucleic acids in the:

    • A.Acid-soluble pool
    • B.Acid-insoluble pool
    • C.Supernatant only
    • D.Gaseous phase
    B. Acid-insoluble pool — Macromolecules are retained on the filter and form the acid-insoluble pool; small molecules pass through into the acid-soluble pool.

  2. Lipids appear in the acid-insoluble fraction even though their molecular weight is low because:

    • A.They are true macromolecules
    • B.They form membrane fragments and vesicles that are retained
    • C.They react with trichloroacetic acid
    • D.They dissolve in water
    B. They form membrane fragments and vesicles that are retained — Grinding breaks membranes into vesicles that are too large to pass the filter, so lipids are trapped with the macromolecules.

  3. The most abundant organic compound in the biosphere is:

    • A.Protein
    • B.Cellulose
    • C.Starch
    • D.Chitin
    B. Cellulose — Cellulose in plant cell walls is the most abundant organic compound on Earth; protein is the most abundant organic compound within a cell.

  4. Morphine, codeine and rubber are examples of:

    • A.Primary metabolites
    • B.Secondary metabolites
    • C.Coenzymes
    • D.Prosthetic groups
    B. Secondary metabolites — These are secondary metabolites, produced mainly by plants and microbes and often of great human use.

  5. Which of the following is a ketose sugar?

    • A.Glucose
    • B.Galactose
    • C.Fructose
    • D.Ribose
    C. Fructose — Fructose carries a ketone group, whereas glucose, galactose and ribose are aldoses.

  6. Lactose is composed of:

    • A.Glucose and glucose
    • B.Glucose and fructose
    • C.Glucose and galactose
    • D.Fructose and galactose
    C. Glucose and galactose — Lactose, the sugar of milk, is a disaccharide of glucose and galactose.

  7. Chitin is a polymer of:

    • A.Glucose
    • B.N-acetylglucosamine
    • C.Fructose
    • D.Galactose
    B. N-acetylglucosamine — Chitin, found in arthropod exoskeletons and fungal cell walls, is built from N-acetylglucosamine units.

  8. A triglyceride is formed from glycerol and three fatty acids joined by:

    • A.Peptide bonds
    • B.Ester bonds
    • C.Glycosidic bonds
    • D.Phosphodiester bonds
    B. Ester bonds — Each fatty acid carboxyl group esterifies one hydroxyl group of glycerol.

  9. The property of phospholipids that allows them to form the cell membrane bilayer is that they are:

    • A.Fully hydrophobic
    • B.Fully hydrophilic
    • C.Amphipathic
    • D.Charged only at low pH
    C. Amphipathic — A hydrophilic phosphate head and hydrophobic fatty acid tails cause spontaneous bilayer formation in water.

  10. The R group of glycine is:

    • A.A hydrogen atom
    • B.A methyl group
    • C.A hydroxymethyl group
    • D.A carboxyl group
    A. A hydrogen atom — Glycine is the simplest amino acid, with hydrogen as its side chain, making it achiral.

  11. Two amino acids are joined by a:

    • A.Glycosidic bond
    • B.Peptide bond
    • C.Ester bond
    • D.Hydrogen bond
    B. Peptide bond — The peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water.

  12. The most abundant protein in the whole biosphere is:

    • A.Collagen
    • B.Haemoglobin
    • C.RuBisCO
    • D.Insulin
    C. RuBisCO — RuBisCO, the carbon-fixing enzyme of photosynthesis, is the most abundant protein on Earth; collagen is the most abundant animal protein.

  13. The sequence of amino acids in a polypeptide represents its:

    • A.Primary structure
    • B.Secondary structure
    • C.Tertiary structure
    • D.Quaternary structure
    A. Primary structure — Primary structure is the linear order and positional identity of the amino acids in the chain.

  14. Haemoglobin, with two alpha and two beta chains, illustrates:

    • A.Primary structure
    • B.Secondary structure
    • C.Tertiary structure
    • D.Quaternary structure
    D. Quaternary structure — Quaternary structure describes the spatial arrangement of more than one polypeptide subunit in a functional protein.

  15. A nucleoside differs from a nucleotide in that a nucleoside lacks:

    • A.The nitrogenous base
    • B.The pentose sugar
    • C.The phosphate group
    • D.The hydroxyl group
    C. The phosphate group — Nucleoside equals base plus sugar; adding phosphate makes it a nucleotide.

  16. Which pair of bases is joined by three hydrogen bonds in DNA?

    • A.Adenine and thymine
    • B.Guanine and cytosine
    • C.Adenine and uracil
    • D.Thymine and cytosine
    B. Guanine and cytosine — Guanine pairs with cytosine through three hydrogen bonds; adenine pairs with thymine through two.

  17. The distance between two adjacent base pairs in the B-form DNA double helix is about:

    • A.0.34 nanometres
    • B.3.4 nanometres
    • C.34 nanometres
    • D.0.034 nanometres
    A. 0.34 nanometres — The pitch is 3.4 nanometres and contains about 10 base pairs, so consecutive pairs lie about 0.34 nanometres apart.

  18. Enzymes increase the rate of a reaction by:

    • A.Raising the activation energy
    • B.Lowering the activation energy
    • C.Changing the equilibrium constant
    • D.Supplying energy to the substrate
    B. Lowering the activation energy — Catalysts provide an alternative route with a lower activation energy; they do not shift the equilibrium position.

  19. Malonate inhibits succinic dehydrogenase by acting as a:

    • A.Competitive inhibitor
    • B.Non-competitive inhibitor
    • C.Coenzyme
    • D.Prosthetic group
    A. Competitive inhibitor — Malonate resembles succinate closely enough to occupy the active site, and its effect is reversed by more substrate.

  20. Niacin is a component of which coenzyme?

    • A.FAD
    • B.NAD
    • C.Coenzyme A
    • D.Biotin
    B. NAD — Nicotinamide adenine dinucleotide is derived from the vitamin niacin and acts as a loosely bound organic cofactor.

References