Structural proteins like collagen and keratin provide support. Transport proteins like haemoglobin carry oxygen. Protective proteins like antibodies defend the body. Contractile proteins like actin and myosin facilitate muscle movement. The mapping a-2, b-1, c-4, d-3 correctly identifies these biological roles.

Vitamins are indeed complex organic compounds essential for metabolic processes (Statement 1) and are required in trace amounts (Statement 2). Statement 3 is incorrect because water-soluble vitamins (like B-complex and C) are generally not stored in the body in significant amounts and must be replenished regularly through diet, unlike fat-soluble vitamins which are stored in adipose tissue.

Proteins often feature secondary structures such as the alpha-helix, which is a coiled conformation stabilized by hydrogen bonding between amino acid residues. This helical structure is fundamental to the functional shape and stability of many proteins, including fibrous proteins like keratin and collagen.

During digestion, proteins are broken down into amino acids (a-4). Carbohydrates are hydrolyzed into monosaccharides (b-3). Nucleic acids are broken down into nitrogenous bases, pentose sugars, and phosphates (c-1). Lipids (fats) are broken down into fatty acids and glycerol (d-2). This matches the sequence a-4, b-3, c-1, d-2.

Most enzymes are globular proteins, making statement 2 correct. However, not all proteins function as enzymes (e.g., structural proteins like collagen or transport proteins like hemoglobin), making statement 4 correct. Statement 1 is false because many proteins are not enzymes, and statement 3 is generally considered false in standard biochemistry as enzymes are defined as proteinaceous catalysts, though ribozymes are a known exception.

Oxytocin is a classic example of a peptide hormone, consisting of a chain of nine amino acids. While insulin and corticotropin are also peptide/protein hormones, oxytocin is frequently cited in introductory biochemistry as a primary example of a small cyclic nonapeptide. Cortisone is a steroid hormone, not a peptide.

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Human sweat is primarily composed of water, but it also contains various dissolved solutes. The most significant solute is sodium chloride, commonly known as salt. Other components include trace amounts of urea, proteins, and minerals, but salt is the defining electrolyte that gives sweat its characteristic saline taste.

Fats (lipids) are highly reduced molecules compared to proteins and carbohydrates like glucose. Upon oxidation, fats yield approximately 9 kcal/g of energy, which is more than double the energy density of proteins or carbohydrates (approximately 4 kcal/g), making them the most efficient energy storage form.

Vitamin C is chemically known as L-ascorbic acid. It is a water-soluble vitamin that acts as a potent antioxidant in the human body, protecting cells from oxidative stress and playing a critical role in collagen synthesis and immune system support.