MCQs

The covalent linkage in the sugar-phosphate backbone of nucleic acids is a phosphodiester bond. It is formed between the 3' carbon of one sugar and the 5' phosphate group of the adjacent sugar. Glycosidic bonds link sugar to base, and peptide bonds link amino acids.

A reducing sugar has a free aldehyde or ketone group that can reduce (donate electrons to) another compound, such as Cu²⁺ to Cu⁺ in Benedict's test. Glycosidic bond formation masks this group. Polymerization is a separate property, and glucose is lipid-insoluble.

While organic molecules are the focus of discussion, water is the most abundant molecule constituting 70-90% of the cell's mass. Among organic biological molecules, carbohydrates like cellulose are the most abundant, but the question specifies "in the living world," making water the correct overarching answer.

Chitin is a linear polysaccharide of N-acetylglucosamine monomers, linked by β-glycosidic bonds, providing structural support in arthropod exoskeletons and fungal cell walls. Cellulose is the structural polymer in plant cell walls. Starch and glycogen are energy storage molecules.

A competitive inhibitor structurally resembles the substrate and competes for binding at the enzyme's active site. This effect can be overcome by increasing substrate concentration. It does not bind to the allosteric site or permanently alter the enzyme.

Unlike the rigid lock-and-key model, the induced fit model proposes that the active site is flexible. The initial substrate binding induces a conformational change in the enzyme, molding the active site into a precise complementary shape around the substrate.

Waxes are hydrophobic lipids that form impermeable coatings. In plants (e.g., cutin on leaves) and animals (e.g., sebum on skin/fur), their main function is to prevent water loss and provide protection, not energy storage, which is the role of fats and oils.

The active site is a 3D pocket formed by amino acid residues brought together via the protein's tertiary folding. It is complementary to the substrate's shape and chemistry, and models like "induced fit" show it is flexible, not rigid.

Unsaturated fatty acids contain kinks due to double bonds, preventing tight packing of the hydrocarbon tails. This increased space between lipids makes the membrane more fluid and permeable compared to a membrane rich in straight-chained saturated fatty acids.

Denaturation unfolds a protein by disrupting the non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) that stabilize secondary, tertiary, and quaternary structures. The primary structure's covalent peptide bonds usually remain intact.

The model proposes a dynamic, fluid phospholipid bilayer where individual lipid molecules can move laterally. Proteins are not just on the surface but are integral or peripheral, creating a "mosaic" pattern that floats within the fluid lipid sea.

A conjugated protein (holoprotein) consists of a protein part (apoprotein) and a non-protein part (prosthetic group). If the prosthetic group is a cofactor and the protein is an enzyme, its removal yields an inactive apoenzyme. The term specifically relates to the loss of the non-protein component.

The fundamental function of an enzyme is to act as a biological catalyst, lowering the activation energy and speeding up a reaction while remaining unchanged at the end. Specificity is about substrate choice, sensitivity relates to environmental factors, and regulation refers to control of its activity.

Secondary structure refers to the regular, repeated local spatial conformations of the polypeptide backbone, stabilized by hydrogen bonds between backbone atoms. The primary structure is the sequence, tertiary is the overall 3D fold of one chain, and quaternary is multi-subunit assembly.

Triglycerides are composed of a single glycerol backbone esterified to three fatty acid chains. Saponification or enzymatic hydrolysis breaks these ester bonds, yielding the original components.

A peptide bond is a covalent bond formed via a dehydration reaction between the α-carboxyl group (-COOH) of one amino acid and the α-amino group (-NH2) of another, releasing a water molecule. R-groups are involved in tertiary structure interactions, not the primary backbone linkage.

The most fundamental structural difference is that DNA is a stable, double-stranded helix, whereas RNA is usually single-stranded. While sugar differences (deoxyribose vs. ribose) are also key, the overall strandedness is a major distinguishing feature. DNA contains thymine, RNA contains uracil.

Lipids are defined by their hydrophobic nature, being soluble in non-polar organic solvents (like ether, chloroform) but insoluble in water. This contrasts with carbohydrates and proteins which are generally hydrophilic.

The formation of a disaccharide involves the removal of a water molecule (dehydration synthesis) to form a glycosidic linkage between two monosaccharides. A peptide bond links amino acids, and an ester bond links fatty acids to glycerol.