Practice Questions

Non-essential amino acids are those the human body can synthesize de novo. Alanine can be produced from pyruvate. Lysine, phenylalanine, and valine are essential amino acids that cannot be synthesized and must be obtained from the diet.

The bulk of a lipid molecule, like a fatty acid or triglyceride, consists of long hydrocarbon chains (C-H bonds). These bonds are non-polar and hydrophobic, repelling interaction with polar water molecules and leading to insolubility.

A triglyceride has glycerol esterified to three fatty acids. A phospholipid is a modified triglyceride where one fatty acid chain is replaced by a highly polar phosphate group, which is often further linked to a nitrogenous compound, creating an amphipathic molecule.

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.