Question 1

Buffering behavior of polyprotic acids: At which pH values can a polyprotic acid like H3PO4 act as an effective buffer?

Accepted Answer

Introduction / Context: Buffers resist changes in pH when small amounts of acid or base are added. Polyprotic acids such as phosphoric acid (H3PO4) have multiple dissociation steps and therefore multiple pKa values, enabling useful buffering over several pH regions. Given Data / Assumptions: Polyprotic acid with distinct pKa1, pKa2, pKa3. Conjugate acid/base pairs coexist near each pKa. Standard aqueous conditions. Concept / Approach: Buffers are most effective when pH ≈ pKa because both the acid and its conjugate base are present in comparable amounts. A polyprotic system provides several such pH plateaus. Phosphate buffering in biology illustrates this with a key pair around pKa2 ≈ 7.2 (H2PO4−/HPO4^2−). Step-by-Step Solution: Identify that each dissociation step has its own pKa.Recall buffer effectiveness is maximized near pH = pKa.Conclude buffering is possible near any of the acid’s pKa values. Verification / Alternative check: Henderson–Hasselbalch: pH = pKa + log([base]/[acid]). When the ratio is about 1, buffering is strongest. Why Other Options Are Wrong: A/B: Buffering does not require combination with polyprotic bases nor low concentration; it requires appropriate acid/base ratio. C: Not limited to neutrality; any pKa region qualifies. E: Buffers work in water; nonaqueous media are not required. Common Pitfalls: Assuming buffers only work at pH 7; in reality, they work around their specific pKa values. Final Answer: At pH values near any of its pKa values.

Question 2

Organic reactivity principle: The reactions of molecules are most directly the reactions of which structural features?

Accepted Answer

Introduction / Context: In organic and biochemical reactions, functional groups dictate how molecules behave. Recognizing reactive sites allows prediction of mechanisms and products without memorizing every individual molecule’s behavior. Given Data / Assumptions: Standard organic functional groups: hydroxyl, carbonyl, carboxyl, amino, phosphate, thiol, etc. Solvent: typically water in biochemistry. Normal temperature/pressure for biological systems. Concept / Approach: Functional groups carry characteristic electron distributions and polarities. They undergo well-defined reaction types (for example, nucleophilic acyl substitution at carboxyls, addition to carbonyls, phosphorylation at phosphates). Therefore, when two molecules react, it is their functional groups that engage first and determine the pathway. Step-by-Step Solution: Identify the functional groups on each reactant.Map each group to typical reactivity patterns (acid/base, nucleophile/electrophile).Predict outcomes based on those group-specific mechanisms. Verification / Alternative check: Peptide bond formation occurs between specific functional groups (carboxyl and amino) via condensation, illustrating the rule. Why Other Options Are Wrong: Independent of functional groups: Contradicts core principles of organic chemistry. Only enzyme-catalyzed sites: Enzymes accelerate reactions but do not create entirely new chemistry unrelated to the functional groups. Crystal lattice defects: Irrelevant to molecules in solution. Common Pitfalls: Overlooking solvent and pH effects that modulate functional group reactivity. Final Answer: Functional groups.

Question 3

Properties of water in biochemistry: Which listed statement does NOT correctly describe liquid water?

Accepted Answer

Introduction / Context: Water’s unique properties drive protein folding, membrane assembly, and acid–base chemistry in life. Distinguishing hydrophilic from hydrophobic effects is essential for understanding biomolecular structure. Given Data / Assumptions: Liquid water near room temperature. Hydrogen bonding network among molecules. Hydrophobic interactions occur among nonpolar solutes in water. Concept / Approach: Water is polar and forms extensive hydrogen bonds. Hydrophobic interactions are not bonds formed by water; rather, they are entropy-driven associations of nonpolar molecules that minimize disruption of water’s hydrogen-bonded network. Thus, saying “water readily forms hydrophobic interactions” is incorrect; water drives hydrophobes together but does not “form” those interactions itself. Step-by-Step Solution: Recall geometry: bent, asymmetric; strong dipole.Recognize hydrogen-bond donors/acceptors: H (donor) and O lone pairs (acceptor).Identify the false statement: water forming hydrophobic interactions. Verification / Alternative check: Consider oil droplets coalescing in water; the driving force is water’s preference to hydrogen bond with itself, excluding nonpolar surfaces. Why Other Options Are Wrong: Asymmetry/polar bonds/dipole: All true. Hydrogen-bonding by all atoms: Oxygen accepts; hydrogens donate; net effect involves all three atoms. Common Pitfalls: Equating “hydrophobic interaction” with a physical bond type; it is an emergent thermodynamic phenomenon. Final Answer: Water readily forms hydrophobic interactions.

Question 4

Bronsted–Lowry acid–base identity: What does a Bronsted acid become after it donates a proton (H+)?

Accepted Answer

Introduction / Context: The Bronsted–Lowry definition classifies acids as proton donors and bases as proton acceptors. Tracking the species before and after proton transfer clarifies buffering, titration, and enzymatic acid–base catalysis. Given Data / Assumptions: Acid HA donates H+ to a base B. Reaction medium is water unless otherwise noted. Conjugate pairs differ by exactly one proton. Concept / Approach: When HA donates H+, it becomes A−, which is called the conjugate base of HA. Simultaneously, the acceptor (for example, H2O) becomes its conjugate acid (H3O+). This reciprocal relationship underlies buffer action: both members of a conjugate pair coexist near pKa. Step-by-Step Solution: Write a generic reaction: HA + H2O → A− + H3O+.Identify donor: HA (acid). After donation, HA → A−.Name A− as the conjugate base of HA. Verification / Alternative check: Acetic acid/acetic base pair: CH3COOH ⇌ CH3COO− + H+. The acetate ion is the conjugate base. Why Other Options Are Wrong: Conjugate acid / hydronium: These describe the proton acceptor’s product, not the donor’s. Free radical / elemental hydrogen: Not produced in standard acid–base transfer. Common Pitfalls: Confusing the identities of conjugate pairs; always compare species that differ by one proton. Final Answer: Its conjugate base.

Question 5

Buffer capacity and effectiveness: Which factors most strongly determine how effective a buffer is?

Accepted Answer

Introduction / Context: Buffers are indispensable in biological experiments. Their “capacity” is the amount of strong acid or base that can be added before pH changes appreciably. Choosing the right buffer conditions ensures reliable enzymatic activity and stability of macromolecules. Given Data / Assumptions: Weak acid/conjugate base pair in water. Henderson–Hasselbalch relationship applies. Constant temperature for routine use. Concept / Approach: Buffer capacity increases with the total concentration of the conjugate pair and is maximal when pH ≈ pKa (that is, when the acid/base ratio is ~1). Very dilute buffers or those far from pKa have poor capacity. Temperature affects pKa slightly, but core determinants are concentration and ratio. Step-by-Step Solution: Use pH = pKa + log([base]/[acid]).Maximize capacity by choosing pH near pKa and sufficient total concentration.Select the option combining both determinants. Verification / Alternative check: Titration curves are flattest (best buffering) near pKa and become steeper (poor buffering) far from pKa or at very low concentration. Why Other Options Are Wrong: Temperature only: Can shift pKa modestly but is not the primary determinant of capacity. Glassware: Not a chemical factor for capacity. Common Pitfalls: Using extremely dilute buffers or targeting pH more than 1 unit away from pKa, which yields unstable pH. Final Answer: Both (a) and (b).

Question 6

Hydrophobic proteins and water: If a protein were built exclusively from nonpolar amino acids, how would it interact with water?

Accepted Answer

Introduction / Context: Protein–water interactions drive folding and function. Nonpolar amino acids (for example, leucine, isoleucine, valine, phenylalanine) avoid contact with water, a phenomenon underlying the hydrophobic effect. Given Data / Assumptions: Hypothetical protein composed exclusively of nonpolar side chains. Environment: aqueous solution. No detergents or chaperones present. Concept / Approach: Water forms a hydrogen-bonded network; nonpolar surfaces disrupt it without compensating interactions. To minimize this disruption, nonpolar residues cluster together, reducing exposed hydrophobic surface area. Thus, an all-nonpolar protein would be extremely insoluble and “repelled” by water, aggregating or partitioning into nonpolar phases. Step-by-Step Solution: Identify the side chain chemistry: nonpolar = hydrophobic.Predict solubility: poor in water, tendency to aggregate.Choose the option indicating repulsion from water. Verification / Alternative check: Membrane proteins bury nonpolar regions within lipid bilayers and expose polar regions to water; all-nonpolar sequences are typically membrane-embedded or insoluble aggregates. Why Other Options Are Wrong: Attracted to water/buffer behavior: Requires polar/ionizable groups. Spontaneous lipid membrane formation: Lipids, not proteins, form bilayers. Common Pitfalls: Confusing hydrophobic aggregation of proteins with membrane self-assembly by amphipathic lipids. Final Answer: It would be repelled by water (strongly hydrophobic).

Question 7

Physiological buffers in vivo: Which acid/base pairs serve as natural buffers in living systems?

Accepted Answer

Introduction / Context: Maintaining pH is vital for enzyme activity and metabolic control. Several buffering systems operate in blood, intracellular fluid, and within proteins themselves. Given Data / Assumptions: Bicarbonate buffer predominates in blood plasma. Phosphate buffer is important intracellularly. Histidine side chains buffer locally in proteins (pKa near physiological pH). Concept / Approach: The bicarbonate system (CO2/H2CO3/HCO3−) couples to respiration and renal function to maintain blood pH. The phosphate pair (H2PO4−/HPO4^2−) buffers cytosolic and urinary pH. Histidine’s imidazole toggles between protonated and deprotonated states around neutral pH, stabilizing local microenvironments and participating in enzyme catalysis. Step-by-Step Solution: Identify each pair’s physiological niche.Relate pKa values to effectiveness near physiological pH.Select the inclusive option acknowledging all contribute. Verification / Alternative check: Clinical acid–base physiology emphasizes bicarbonate; cell biology texts highlight phosphate and amino acid side chain buffering. Why Other Options Are Wrong: Any single pair alone: Incomplete; multiple buffers operate in parallel. None: Contradicts established physiology. Common Pitfalls: Ignoring protein side-chain buffering, which fine-tunes local pH near active sites. Final Answer: All of these.

Question 8

Identifying a biopolymer by its monomers: A polymer composed of alanine, tyrosine, and lysine would be classified as what type of molecule?

Accepted Answer

Introduction / Context: Biopolymers are recognized by their repeating monomer units. Correctly identifying the polymer class from its monomers is a core skill in biochemistry and cell biology. Given Data / Assumptions: Monomers listed: alanine, tyrosine, lysine. These are standard alpha-amino acids. No other chemical classes are implied. Concept / Approach: Proteins are polymers of amino acids linked by peptide bonds. Nucleic acids (DNA/RNA) are polymers of nucleotides; phospholipids are built from glycerol, fatty acids, and phosphate-containing headgroups; polysaccharides are polymers of monosaccharides. Therefore, a chain of amino acids defines a protein or peptide. Step-by-Step Solution: Recognize alanine, tyrosine, lysine as amino acids.Recall that amino acids form polypeptide chains via peptide bonds.Classify the polymer as a protein. Verification / Alternative check: Tyrosine brings an aromatic phenolic side chain; lysine contributes a basic side chain; alanine is nonpolar—typical amino acid diversity seen in proteins. Why Other Options Are Wrong: DNA: Built from nucleotides (A, T, G, C), not amino acids. Phospholipid: Not a polymer of amino acids. Strong base: A property, not a macromolecule class. Polysaccharide: Built from sugars. Common Pitfalls: Confusing amino acid names with nucleotides or fatty acids; learn characteristic functional groups. Final Answer: Protein.

Question 9

Core elements in biomolecules: Which statements correctly describe oxygen, nitrogen, and carbon as used in living systems?

Accepted Answer

Introduction / Context: Carbon, nitrogen, and oxygen are central to biochemistry. Their bonding capabilities and prevalence underpin the structures of amino acids, nucleotides, carbohydrates, and lipids. Given Data / Assumptions: Covalent bonding dominates organic chemistry. Stable isotopes of C, N, O possess protons and neutrons. These elements are abundant in biological macromolecules. Concept / Approach: Carbon forms four covalent bonds, enabling diverse backbones and rings. Nitrogen commonly forms three covalent bonds and bears lone pairs, contributing to basicity and hydrogen bonding. Oxygen typically forms two covalent bonds and is highly electronegative, central to hydrogen bonding and polarity. All three are ubiquitous in biomolecules and occur as nuclei containing protons and neutrons (with minor isotopic variations). Step-by-Step Solution: Evaluate bonding capability: covalent bonding is true for all three.Assess nuclear composition: protons and neutrons present in stable isotopes.Confirm ubiquity in biomolecules: proteins, nucleic acids, sugars, lipids contain C, with O and N widely represented. Verification / Alternative check: Periodic trends and standard biomolecular formulae (for example, C, H, O, N composition) corroborate these facts. Why Other Options Are Wrong: Any single statement alone: Incomplete; all are correct. None: Contradicts basic chemical knowledge. Common Pitfalls: Overlooking hydrogen, phosphorus, and sulfur—also important but not part of this item’s focus. Final Answer: All of the above.

Question 10

Acids vs. bases in aqueous biology: what is the core difference? Choose the statement that best describes the fundamental Brønsted–Lowry difference between an acid and a base in water (physiology/biochemistry context).

Accepted Answer

Introduction / Context: Understanding acids and bases under the Brønsted–Lowry framework is foundational for biochemistry, physiology, and pharmaceutical sciences. In aqueous systems such as blood plasma or cytosol, hydrogen ion (proton) transfer controls enzyme activity, buffer behavior, and membrane transport. Given Data / Assumptions: Medium is water (aqueous solutions relevant to biology). We compare acids and bases using Brønsted–Lowry definitions. We assume normal physiological temperature and ionic strength. Concept / Approach: Brønsted–Lowry acid: proton donor. Brønsted–Lowry base: proton acceptor. In water, an acid increases the concentration of hydronium (H3O+), while a base decreases it by accepting a proton (often from water) to form species like NH4+ from NH3. This view generalizes the Arrhenius idea and fits complex biological buffers. Step-by-Step Solution: Define acid: a species that donates H+ to another species.Define base: a species that accepts H+ from another species.Map to water: HA + H2O → A− + H3O+ (acid donates), and B + H2O → BH+ + OH− (base accepts).Select the statement that matches: acids donate H+ in water; bases accept H+. Verification / Alternative check: Common biochemical buffers (for example, H2PO4−/HPO4^2−, bicarbonate/carbonic acid) behave according to this donor–acceptor model, confirming the rule across physiological pH. Why Other Options Are Wrong: Polarity alone does not define acid–base properties; some acids and bases are both polar. Saying bases donate H+ reverses the definition. Claiming both donate H+ contradicts proton balance. Common Pitfalls: Confusing Lewis and Brønsted–Lowry definitions; assuming acidity strictly correlates with molecular polarity. Final Answer: Acids donate hydrogen ions in water while bases accept hydrogen ions.

Question 11

Hydrogen bonding with water: what does it imply about the solute? A compound capable of forming hydrogen bonds with water typically has which key bonding characteristic?

Accepted Answer

Introduction / Context: Solubility and interactions of biomolecules in water depend on polarity and hydrogen bonding. Many metabolic intermediates and drug molecules rely on hydrogen bonding to achieve adequate aqueous solubility and specific binding. Given Data / Assumptions: We consider typical small molecules or functional groups in biology. Hydrogen bonding requires appropriate donors/acceptors. Medium is water (a strong hydrogen-bonding solvent). Concept / Approach: Hydrogen bonds form when a hydrogen attached to an electronegative atom (N, O, sometimes S) interacts with a lone pair on another electronegative atom. Such atoms are created by polar covalent bonds (for example, O–H, N–H, C=O). Therefore, the presence of polar covalent bonds is a prerequisite for forming hydrogen bonds with water. Step-by-Step Solution: Identify hydrogen-bond donors: X–H where X is O or N.Identify hydrogen-bond acceptors: atoms like O or N with lone pairs (for example, carbonyl oxygen).Conclude that polar covalent bonds (unequal electron sharing) enable these interactions. Verification / Alternative check: Alcohols, amides, and carboxylic acids show high water miscibility due to H-bonding, while hydrocarbons without polar bonds do not. Why Other Options Are Wrong: Noncovalent bonds do not describe intramolecular structure; buffers require conjugate acid–base pairs, not merely H-bond capacity; insolubility contradicts typical H-bonding behavior; purely ionic compounds interact differently and may dissolve without H-bonding donors/acceptors. Common Pitfalls: Assuming any electronegative atom guarantees solubility; overall solubility also depends on hydrophobic surface area. Final Answer: Contains at least some polar covalent bonds.

Question 12

Functional groups in biochemistry: typical elemental makeup and polarity Most important functional groups in biological molecules share what general feature?

Accepted Answer

Introduction / Context: Biomolecular reactivity stems from functional groups such as hydroxyls, carbonyls, carboxylates, amines, amides, and phosphates. Their polarity governs solubility, binding, and catalysis in aqueous environments. Given Data / Assumptions: We consider common moieties in proteins, nucleic acids, carbohydrates, and lipids. Focus is on elemental composition and polarity. Concept / Approach: Oxygen and nitrogen introduce significant electronegativity differences, creating polar covalent bonds and hydrogen-bonding capacity. Many groups are polar but not necessarily acidic (for example, amide). Thus, the best generalization is that key biological functional groups contain O and/or N and are polar. Step-by-Step Solution: List examples: hydroxyl (O), carbonyl (C=O), carboxyl (O and O−), amine/ammonium (N), amide (O and N), phosphate esters (O, P).Recognize their polarity and hydrogen-bonding roles.Select the statement that captures polarity without overclaiming acidity. Verification / Alternative check: Protein backbones (amide) are polar; nucleic acid bases contain multiple N and O sites enabling H-bond networks. Why Other Options Are Wrong: Not all are acidic; phosphates appear widely but not universally; “only C and H” fails for most biomolecules. Common Pitfalls: Equating polarity with charge; many polar groups are neutral at physiological pH. Final Answer: Oxygen and/or nitrogen and are polar.

Water, pH and Macromolecules Questions