Basic Components of Life

(d) Glycine is a sulphur containing amino acid.

This is the wrong statement. Glycine is the simplest amino acid, with only an H as its side chain, and contains no sulphur. The sulphur-containing amino acids are cysteine and methionine. Sucrose is a disaccharide, cellulose is a polysaccharide, and uracil is a pyrimidine, so those statements are true.

(b) One glycerol and three fatty acid molecules

A typical fat (triglyceride) forms when one glycerol, a trihydroxy alcohol, is esterified with three fatty acid molecules, releasing three molecules of water. The other options give the wrong ratio of glycerol to fatty acid.

(a) Nucleic acids in a nucleotide

Nucleic acids have a phosphodiester bond, where a phosphate links the 3' carbon of one sugar to the 5' carbon of the next, joining the nucleotides. Fatty acids in glycerides have ester bonds, polysaccharides have glycosidic bonds, and polypeptides have peptide bonds.

(a) Addition of lot of succinate does not reverse the inhibition of succinic dehydrogenase by malonate.

Malonate is a competitive inhibitor of succinic dehydrogenase, since it resembles succinate. Competitive inhibition is reversed by adding more substrate, so statement (a) is false. Statements (b), (c) and (d) are all correct.

(a) Sucrose

Sucrose is formed by an $\alpha$-1,2 glycosidic bond between the anomeric carbons of glucose and fructose, so no free aldehyde or ketone group is left to reduce Benedict's or Fehling's reagent. Lactose and maltose each keep one free anomeric carbon (hemiacetal), and ribose 5-phosphate is a monosaccharide with a free aldehyde, so all three are reducing.

(c) Transient and unstable

The transition state is the high-energy intermediate between substrate and product on the reaction path. It exists only for a moment and is unstable, and enzymes work by lowering the energy needed to reach it.

(c) A saturated or unsaturated fatty acid esterified to a glycerol molecule to which a phosphate group is also attached

Phosphoglycerides (such as lecithin) are made of glycerol with fatty acids esterified to it and a phosphate group on the third carbon. The fatty acid may be saturated or unsaturated, so (a) and (b) are too restrictive. In (d) the fatty acid is wrongly attached to phosphate.

(b) Holoenzyme = Apoenzyme + Coenzyme

The protein part is the apoenzyme and the non-protein cofactor (here a coenzyme) is the prosthetic or accessory part. Together they form the active holoenzyme. The other options reverse or confuse these relationships.

(d) Lipids

Nucleic acids, proteins and polysaccharides are polymers built from repeating nucleotide, amino acid and monosaccharide units. Lipids are not true polymers of repeating units; they are assemblies of glycerol and fatty acids that are held together by ester bonds, not chains of identical monomers.

(d) Carbonyl and hydroxyl

Sugars are polyhydroxy aldehydes or ketones, so they have a carbonyl group (aldehyde in aldoses, ketone in ketoses) and several hydroxyl groups. Methyl and phosphate groups are not characteristic of sugars in general.

(d) (A) is true but (B) is false.

A cofactor (coenzyme or metal ion) tightly bound to the enzyme protein is a prosthetic group, so (A) is true. The protein part alone is the apoenzyme, and apoenzyme plus cofactor is the holoenzyme. Statement (B) calls the complete active enzyme an apoenzyme, so it is false.

(a) a lectin

Concanavalin A is a lectin, a carbohydrate-binding protein, from jack bean (Canavalia). Lectins are included among the secondary metabolites in NCERT. It is not a pigment (anthocyanin, carotenoid), an alkaloid (morphine, codeine) or an essential oil.

(a) Lysine

Lysine has an extra amino group in its side chain, so it is a basic amino acid (like arginine and histidine). Valine is neutral, tyrosine is aromatic and neutral, and glutamic acid is acidic.

(d) Collagen

Collagen is the most abundant protein in animals. It is the main protein in connective tissue, skin, tendons and bones. RuBisCO is the most abundant protein in the whole biosphere. Haemoglobin, insulin and lectin are far less abundant.

(c) (a)-(ii), (b)-(iv), (c)-(iii), (d)-(i)

Malonate is a competitive inhibitor of succinic dehydrogenase. Collagen is a protein with peptide bonds, and chitin is the fungal cell wall polymer. Ricin is a toxic secondary metabolite, so the pairing (a)-(iii) or (c)-(i) in other options is wrong.

(a) Defence action

Nicotine, strychnine and caffeine are alkaloids, which are secondary metabolites. They have no direct role in growth or nutrition, but protect the plant against herbivores and pathogens, which is their ecological importance.

(b) Inulin, insulin

Inulin is a polysaccharide of fructose units linked by glycosidic bonds, and insulin is a protein (polypeptide) with peptide bonds. In the other options lecithin and cholesterol are lipids, and glycerol is not a glycosidic compound.

(b) It does not contain complex helices and hence cannot hold iodine molecules

Starch forms helical structures that trap iodine to give a blue colour. Cellulose is an unbranched linear polymer of $\beta$-glucose without such helices, so no colour forms. It is a polysaccharide, not a disaccharide.

(d) Succinic dehydrogenase

Malonate is a competitive inhibitor of succinic dehydrogenase because it resembles the substrate succinate. It blocks the enzyme in the Krebs cycle, inhibiting bacterial growth.