Question 1

Membrane lipids — What characteristic structural features does a typical biological phospholipid contain in its amphipathic architecture?

Accepted Answer

Introduction: Phospholipids are the principal building blocks of cellular membranes. Their amphipathic nature drives spontaneous bilayer formation and underlies membrane fluidity, curvature, and protein association. This question tests recognition of the core structural motif of phospholipids. Given Data / Assumptions: Glycerophospholipids contain a glycerol backbone, two fatty acyl chains, and a phosphate-containing headgroup. Sphingophospholipids have a sphingosine backbone but retain amphipathic design. Amphipathic molecules possess both polar and nonpolar regions. Concept / Approach: The hydrophilic (polar) headgroup, often carrying a charge (e.g., choline, ethanolamine, serine, inositol), interfaces with water, while hydrophobic fatty acyl tails cluster away from water. This duality drives bilayer assembly with heads facing the aqueous phases and tails forming the hydrophobic core. Step-by-Step Solution: 1) Identify polarity: phosphate + headgroup = polar, water-interacting region.2) Identify hydrophobicity: long hydrocarbon chains = nonpolar tails.3) Infer arrangement: heads outward to water; tails inward forming bilayer core.4) Conclude: phospholipids have hydrophilic heads and hydrophobic tails. Verification / Alternative check: Liposome formation in vitro and the classic fluid mosaic model of membranes demonstrate amphipathic self-assembly. Cryo-EM and X-ray scattering confirm bilayer thickness and tail packing consistent with this architecture. Why Other Options Are Wrong: (b) Water-soluble long carbon chains: pure hydrocarbon chains are hydrophobic, not water-soluble. (c) Exclusively positive groups: real phospholipids carry polar/charged headgroups plus hydrophobic tails. (d) Combination of incorrect statements remains incorrect. (e) Purely nonpolar molecules cannot form stable bilayers in water. Common Pitfalls: Assuming all headgroups are positively charged; many are zwitterionic or negatively charged, but all contribute to hydrophilicity. Final Answer: hydrophilic heads and hydrophobic tails.

Question 2

Lipid catabolism energetics — Using the classical P/O assumptions, how many ATP are generated by the complete oxidation of palmitate (C16:0) to CO2 and H2O, net of activation?

Accepted Answer

Introduction: Quantifying ATP yield from fatty acid oxidation is a cornerstone of bioenergetics. Palmitate (C16:0) is a standard example used to illustrate β-oxidation cycles, acetyl-CoA production, and coupling to the TCA cycle and oxidative phosphorylation. This question adopts the classical textbook P/O ratios (NADH ≈ 3 ATP; FADH2 ≈ 2 ATP) and subtracts activation cost. Given Data / Assumptions: Palmitate undergoes 7 β-oxidation cycles to yield 8 acetyl-CoA. β-Oxidation directly produces 7 NADH and 7 FADH2. Each acetyl-CoA in the TCA cycle yields 3 NADH, 1 FADH2, and 1 GTP (≈ 1 ATP). Activation of palmitate to palmitoyl-CoA costs the equivalent of 2 ATP (ATP → AMP + PPi). Concept / Approach: Compute ATP from β-oxidation reducing equivalents plus ATP from acetyl-CoA oxidation, then subtract the activation cost. Classical P/O: NADH → 3 ATP; FADH2 → 2 ATP; GTP → 1 ATP. Modern estimates (NADH ≈ 2.5; FADH2 ≈ 1.5) give ~106 ATP, but this problem’s canonical answer uses the older convention. Step-by-Step Solution: β-Oxidation reducing equivalents: 7 NADH * 3 = 21 ATP; 7 FADH2 * 2 = 14 ATP → subtotal 35.Acetyl-CoA oxidation (8 rounds of TCA): per acetyl-CoA = (3 NADH * 3) + (1 FADH2 * 2) + (1 GTP * 1) = 9 + 2 + 1 = 12 ATP.Total from 8 acetyl-CoA: 8 * 12 = 96 ATP.Grand total before activation: 35 + 96 = 131 ATP.Subtract activation cost: 131 − 2 = 129 ATP (net). Verification / Alternative check: Using modern P/O (2.5 and 1.5) gives ≈ 10 ATP per acetyl-CoA cycle and ≈ 106 net ATP, but many exam keys still reference the classical 129 number for palmitate under “old P/O” assumptions. Why Other Options Are Wrong: 35: accounts only for β-oxidation NADH/FADH2, ignores TCA. 96: counts only TCA yield from acetyl-CoA, omitting β-oxidation and activation. 131: gross yield before subtracting activation cost. 106: close to modern estimates but not under the stated classical assumptions. Common Pitfalls: Forgetting the 2-ATP activation penalty or mixing classical and modern P/O ratios in one calculation. Choose one framework and apply it consistently. Final Answer: 129.

Question 3

Endocrinology overview — Into how many broad classes are human steroid hormones commonly grouped (for example, glucocorticoids vs mineralocorticoids)?

Accepted Answer

Introduction: Steroid hormones are cholesterol-derived signaling molecules that regulate metabolism, salt balance, inflammation, and reproduction. This question asks for the standard number of broad classes used in human physiology and biochemistry texts. Given Data / Assumptions: Steroid hormone classes are defined by biosynthetic pathways and receptor targets. Categories group hormones with related structures and functions. Examples include adrenal and gonadal steroids. Concept / Approach: The commonly cited five classes are: glucocorticoids (e.g., cortisol), mineralocorticoids (e.g., aldosterone), androgens (e.g., testosterone), estrogens (e.g., estradiol), and progestogens (e.g., progesterone). Some texts also discuss secosteroids (vitamin D) separately, but the classic endocrine classification lists five groups. Step-by-Step Solution: 1) Adrenal cortex: glucocorticoids regulate glucose metabolism and stress.2) Adrenal cortex: mineralocorticoids control sodium/potassium and blood pressure.3) Gonads: androgens drive male secondary sex characteristics.4) Gonads/placenta: estrogens drive female reproductive physiology.5) Gonads/placenta: progestogens support pregnancy and cycle regulation. Verification / Alternative check: Endocrinology references uniformly present these five categories; clinical assays and receptor families (GR, MR, AR, ER, PR) map onto them. Why Other Options Are Wrong: 2–4 classes: omit established categories. 6: overcounts beyond the standard five (unless including secosteroids as a separate class, which is not the typical exam convention). Common Pitfalls: Confusing structural classes with tissue sources or receptor subtypes. The accepted introductory answer is five classes. Final Answer: 5.

Question 4

Cholesterol derivatives — Cholesterol serves as a biochemical precursor for which of the following molecules in human physiology?

Accepted Answer

Introduction: Cholesterol is more than a membrane component; it is a versatile precursor for several vital molecules. This question checks your knowledge of which products are biosynthesized from cholesterol in humans. Given Data / Assumptions: Cholesterol is synthesized from acetyl-CoA via the mevalonate pathway. Downstream pathways convert cholesterol into hormones and detergents for lipid digestion. Vitamin A originates from carotenoids, not from cholesterol. Concept / Approach: In endocrine tissues, cholesterol is converted into steroid hormones (glucocorticoids, mineralocorticoids, and sex steroids). In the liver, cholesterol is oxidized to bile acids/salts (e.g., cholic acid) that emulsify dietary lipids. Vitamin A derives from dietary carotenoids (beta-carotene), not from cholesterol, distinguishing option (d) as correct. Step-by-Step Solution: 1) Endocrine conversion: cholesterol → pregnenolone → steroid hormones.2) Hepatic conversion: cholesterol → bile acids → bile salts for digestion.3) Exclude vitamin A: derived from retinoids/carotenoids, unrelated to cholesterol backbone.4) Therefore, both steroid hormones and bile salts are cholesterol-derived. Verification / Alternative check: Clinical pharmacology (e.g., statins affecting cholesterol availability) can influence steroidogenesis and bile acid pools, underscoring cholesterol’s precursor role. Why Other Options Are Wrong: Vitamin A: not cholesterol-derived. Single selections (a) or (c) omit one correct pathway; (e) wrongly pairs steroid hormones with vitamin A. Common Pitfalls: Confusing fat-soluble vitamins: vitamin D is secosteroidal and can be produced from 7-dehydrocholesterol in skin, but vitamin A is not derived from cholesterol. Final Answer: both (a) and (c).

Question 5

Lactation physiology — Which pituitary or hypothalamic hormone is classically termed the “milk ejection” (let-down) hormone in humans?

Accepted Answer

Introduction: Lactation involves both milk production and milk ejection. These processes are hormonally distinct. This question tests whether you can differentiate the hormone that drives the let-down reflex from the one that stimulates milk synthesis. Given Data / Assumptions: Nipple stimulation activates neuroendocrine reflex arcs. Posterior pituitary releases neurohypophyseal hormones. Anterior pituitary secretes prolactin for milk synthesis. Concept / Approach: Oxytocin, released from the posterior pituitary, triggers contraction of myoepithelial cells around alveoli and ducts in the mammary gland, expelling stored milk (let-down). Prolactin from the anterior pituitary promotes milk production by alveolar epithelial cells. Vasopressin regulates water retention and vascular tone, not milk ejection. Step-by-Step Solution: 1) Identify function: milk ejection requires smooth muscle-like contraction of myoepithelial cells.2) Map hormone to effect: oxytocin stimulates these cells via G-protein–coupled receptors and intracellular calcium.3) Distinguish from synthesis: prolactin increases milk synthesis but does not eject milk.4) Conclude: Oxytocin is the correct choice. Verification / Alternative check: Clinical use of intranasal oxytocin can aid let-down in specific lactation difficulties, confirming its role. Suckling rapidly elevates plasma oxytocin measured in mothers during breastfeeding. Why Other Options Are Wrong: Prolactin: production, not ejection. Vasopressin: water balance; no direct role in let-down. All of these: overinclusive and incorrect. Somatostatin: inhibitory hormone; unrelated to lactation reflex. Common Pitfalls: Conflating milk synthesis with ejection; they are coordinated but hormonally distinct processes. Final Answer: Oxytocin.

Lipid Questions