Nutrition Basics — Set 5

Correct Answer: A. Enzymes

• **Enzymes** = biological catalysts made predominantly of protein; they lower the activation energy of reactions and can be reused thousands of times without being consumed. • **Speed of action** — enzymes accelerate reaction rates by 10⁶ to 10¹⁷ fold compared to uncatalyzed reactions, making life processes feasible at body temperature. • The question specifies 'when in the form of proteins' — this is the defining description of enzymes (as distinct from ribozymes, which are RNA catalysts). • 💡 Option B (Hormones) is wrong because hormones are chemical messenger molecules that signal cells to change their activity, not catalysts that speed up reactions; Option C (Vitamins) is wrong because vitamins act as cofactors that assist enzymes but are not themselves catalytic proteins; Option D (Antibodies) is wrong because antibodies are immune system proteins that recognize and bind antigens, not catalytic molecules.

Correct Answer: A. Vitamin B9

• **Vitamin B9 (Folic Acid / Folate)** = donates one-carbon units essential for synthesizing purines and thymidylate, the building blocks of DNA and RNA. • **Pregnancy importance** — adequate folate in the first four weeks of pregnancy prevents neural tube defects (spina bifida, anencephaly); supplementation of 400 µg/day is universally recommended before conception. • Folate also works with B12 to convert homocysteine to methionine — deficiency in either raises homocysteine, a cardiovascular risk factor. • 💡 Option B (Vitamin B1) is wrong because thiamine is a coenzyme for pyruvate dehydrogenase in carbohydrate metabolism and is linked to beriberi and Wernicke's encephalopathy, not DNA synthesis; Option C (Vitamin B12) is wrong because cobalamin is known as cyanocobalamin and is essential for nerve myelination and methionine synthesis, not folic acid; Option D (Vitamin B2) is wrong because riboflavin forms FAD and FMN for the electron transport chain, not for DNA building block synthesis.

Correct Answer: B. Magnesium

• **Magnesium (Mg²⁺)** = essential cofactor for over 300 enzymes including ATPases; the ATP molecule active in muscle must be bound to Mg²⁺ (Mg-ATP complex) to trigger the myosin cross-bridge cycle. • **Blood pressure link** — magnesium acts as a natural calcium channel blocker in vascular smooth muscle, promoting vasodilation; low magnesium is associated with hypertension. • After a muscle contraction, the SR (sarcoplasmic reticulum) Ca²⁺-ATPase, which is Mg-dependent, pumps calcium back to allow relaxation. • 💡 Option A (Iodine) is wrong because iodine is incorporated into thyroid hormones T3/T4 that regulate basal metabolic rate, not direct muscle contraction; Option C (Copper) is wrong because copper functions in cytochrome c oxidase for electron transport and in collagen cross-linking, not muscle mechanics; Option D (Iron) is wrong because iron is the oxygen-binding component of hemoglobin and myoglobin, not an electrolyte for contraction signaling.

Correct Answer: A. Vitamin D

• **Calciferol = Vitamin D** — the name derives from Latin 'calx' (calcium) + 'ferre' (to carry/bring), reflecting its role in increasing intestinal calcium absorption. • **Two main forms** — ergocalciferol (D2) from UV-irradiated plant sterols, and cholecalciferol (D3) synthesized in human skin from 7-dehydrocholesterol under sunlight and also obtained from oily fish. • Vitamin D is activated in two steps: liver converts it to 25-hydroxyvitamin D, then kidneys produce active 1,25-dihydroxyvitamin D (calcitriol), which acts as a hormone. • 💡 Option B (Vitamin E) is wrong because vitamin E is called tocopherol and functions as a fat-soluble antioxidant protecting polyunsaturated fatty acids in cell membranes; Option C (Vitamin A) is wrong because vitamin A is called retinol and is essential for rhodopsin synthesis and epithelial integrity; Option D (Vitamin K) is wrong because vitamin K is called phylloquinone (K1) or menaquinone (K2) and activates clotting factors II, VII, IX, and X.

Correct Answer: C. Carbohydrates

• **Carbohydrates** = preferred fuel for all tissues at rest; glucose from dietary carbohydrates is oxidized via glycolysis and the citric acid cycle to produce ATP for basal metabolic functions. • **Brain dependency** — the brain (which is highly active even at rest) consumes about 120 g of glucose per day and cannot use fatty acids directly, making carbohydrates indispensable even for inactive people. • Sedentary individuals need around 45–65% of total calories from carbohydrates per standard dietary guidelines. • 💡 Option A (Proteins) is wrong because proteins are primarily used for tissue repair, enzyme production, and immune functions; the body uses amino acids for energy only as a last resort during starvation; Option B (Fats) is wrong because while fats are the secondary energy reserve, fat mobilization via lipolysis is slower and less preferred than carbohydrate oxidation for everyday activity; Option D (Water) is wrong because water has zero caloric value and functions purely as a solvent, transport medium, and temperature regulator.

Correct Answer: B. Emulsifying fats

• **Bile emulsification** = bile salts (sodium taurocholate, sodium glycocholate) are amphipathic molecules that coat large fat globules, breaking them into tiny micelles (~5 nm) and dramatically increasing surface area for pancreatic lipase to act. • **Mechanical vs. chemical** — bile does not chemically digest fat; it physically disperses it, allowing lipase (the actual enzyme) to hydrolyze triglycerides into fatty acids and monoglycerides. • Bile is produced by liver hepatocytes, stored in the gallbladder, and released into the duodenum when fatty food arrives. • 💡 Option A (Breaking down proteins) is wrong because protein digestion is carried out by pepsin in the stomach and trypsin/chymotrypsin from the pancreas, not bile; Option C (Digesting starch) is wrong because starch hydrolysis is performed by salivary amylase and pancreatic amylase; Option D (Storing vitamins) is wrong because fat-soluble vitamin storage occurs in the liver and adipose tissue, a function separate from bile secretion.

Correct Answer: D. Vitamin A

• **Vitamin A (Retinol)** = required for two distinct functions: (1) synthesis of rhodopsin and cone pigments in the retina for both dim-light and colour vision; (2) maintenance of mucus-secreting epithelial cells throughout the body, including skin. • **Night blindness** — the earliest sign of vitamin A deficiency is nyctalopia (inability to see in dim light) because rhodopsin cannot be regenerated without retinal (the aldehyde form of vitamin A). • Xerophthalmia (dry eyes, corneal ulceration) and dry, keratinized skin follow prolonged deficiency — both directly reflect vitamin A's role in epithelial maintenance. • 💡 Option A (Vitamin C) is wrong because ascorbic acid is needed for collagen synthesis and immune function, but it does not produce visual pigments or maintain squamous epithelium specifically; Option B (Vitamin B) is wrong because B-complex vitamins are metabolic coenzymes with no direct role in skin epithelial integrity or vision pigments; Option C (Vitamin D) is wrong because calciferol regulates calcium and phosphate homeostasis for bone health, not skin or eye tissue.

Correct Answer: C. Anemia and fatigue

• **Iron-deficiency anemia** = insufficient iron means hemoglobin synthesis is impaired; red blood cells become small and pale (microcytic hypochromic), reducing oxygen delivery to muscles and organs. • **Fatigue mechanism** — with less oxygen reaching cells, ATP production slows, causing persistent tiredness, exertional dyspnea, and pallor of skin and mucous membranes. • Other iron-deficiency signs include pica (craving non-food items), koilonychia (spoon-shaped nails), and angular stomatitis. • 💡 Option A (Brittle bones) is wrong because fragile bones result from calcium and Vitamin D deficiency (osteoporosis/rickets), not iron deficiency; Option B (Night blindness) is wrong because nyctalopia is specifically caused by Vitamin A deficiency impairing rhodopsin regeneration; Option D (Bleeding gums) is wrong because gingival bleeding is the hallmark of Vitamin C deficiency (scurvy), which weakens collagen in blood vessel walls.

Correct Answer: D. Vitamin C

• **Vitamin C (Ascorbic Acid)** = a required cofactor for prolyl and lysyl hydroxylase enzymes that form stable collagen cross-links; without it, collagen fibres are weak and gum tissue breaks down. • **Wound healing role** — ascorbic acid supports fibroblast proliferation, collagen deposition in new tissue, and neutrophil function essential for fighting infection in healing wounds. • Deficiency causes scurvy: swollen, spongy, bleeding gums; slow-healing wounds; and sub-periosteal hemorrhages — all directly reflecting collagen failure. • 💡 Option A (Vitamin A) is wrong because retinol maintains epithelial differentiation and vision but does not directly catalyze collagen synthesis needed for gum integrity; Option B (Vitamin D) is wrong because calciferol regulates calcium absorption and bone mineralization, not gum collagen or wound repair; Option C (Vitamin B) is wrong because B-complex vitamins serve as metabolic coenzymes in energy pathways but do not participate in the collagen cross-linking required for healthy gums.

Correct Answer: A. Eggs

• **Complete Protein** = a food that contains all nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine) in adequate proportions that the human body cannot synthesize. • **Eggs as the gold standard** — the egg's amino acid profile is so well-balanced that it is used as the reference protein (biological value ≈ 100) against which all other dietary proteins are measured. • Animal proteins (eggs, meat, fish, dairy) are generally complete; most plant proteins are incomplete because they lack or have insufficient amounts of one or more essential amino acids. • 💡 Option B (Beans) is wrong because legumes are deficient in methionine (a sulfur-containing essential amino acid), making them incomplete — though combining beans with rice compensates; Option C (Rice) is wrong because rice is low in lysine, an essential amino acid critical for growth and immune function; Option D (Bread) is wrong because wheat-based bread is also deficient in lysine and does not provide all nine essential amino acids in sufficient amounts.