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101. Which amino acid lacks a chiral $\alpha$-carbon in its structure?
ⓐ. Glycine
ⓑ. Alanine
ⓒ. Valine
ⓓ. Serine
Correct Answer: Glycine
Explanation: Glycine is the only common amino acid in which the $\alpha$-carbon is attached to two identical substituents (two hydrogen atoms), making it achiral. In other amino acids, the $\alpha$-carbon typically has four different groups: $-NH_2$, $-COOH$, $-H$, and an R group, which creates chirality. Because glycine’s R group is simply $H$, the $\alpha$-carbon does not become an asymmetric center. This structural point is frequently tested because it explains why glycine does not show optical isomerism. It also influences flexibility in proteins, since glycine’s small side chain reduces steric hindrance. Hence, glycine uniquely lacks a chiral $\alpha$-carbon.
102. Which amino acid contains a cyclic structure that makes its amino group secondary?
ⓐ. Lysine
ⓑ. Aspartic acid
ⓒ. Proline
ⓓ. Glycine
Correct Answer: Proline
Explanation: Proline is structurally distinct because its side chain links back to the amino nitrogen, forming a ring. This ring makes the amino group secondary (often described as an imino-type feature in basic biology texts), unlike the primary amino group seen in most other amino acids. The cyclic structure restricts rotation around the backbone, influencing polypeptide flexibility and commonly introducing bends in protein chains. Because the nitrogen is part of the ring, proline’s backbone geometry differs from typical amino acids. This unique feature affects secondary structure patterns and is often emphasized in conceptual questions. Therefore, proline is the amino acid with a cyclic structure and secondary amino group.
103. At about neutral pH, the $\alpha$-amino group of a typical amino acid is most commonly present as:
ⓐ. $-NH_2$
ⓑ. $-NH_3^+$
ⓒ. $-NH^-$
ⓓ. $-NH_4^+$
Correct Answer: $-NH_3^+$
Explanation: In aqueous solutions near neutral pH, amino acids behave as amphoteric molecules and the amino group tends to accept a proton. This converts the $\alpha$-amino group into the protonated form $-NH_3^+$, which is the dominant state under physiological conditions. The protonation is stabilized by water and by the simultaneous deprotonation of the carboxyl group to $-COO^-$. This paired ionization produces the common zwitterionic form and supports solubility in biological fluids. The exact balance depends on pH, but around neutral pH the amino group is predominantly protonated. Hence, $-NH_3^+$ is the correct form.
104. The zwitterionic form of an amino acid is best described as:
ⓐ. Only $-NH_2$ and only $-COOH$
ⓑ. Only $-NH_3^+$ and only $-COOH$
ⓒ. Only $-NH_2$ and only $-COO^-$
ⓓ. Both $-NH_3^+$ and $-COO^-$ in the same molecule
Correct Answer: Both $-NH_3^+$ and $-COO^-$ in the same molecule
Explanation: A zwitterion is a dipolar ion that carries both a positive and a negative charge within the same molecule. For amino acids in water, this commonly means the amino group exists as $-NH_3^+$ while the carboxyl group exists as $-COO^-$. Even though the molecule has charged groups, the net charge can be zero when the charges balance. This form explains why amino acids can act as buffers and show characteristic behavior in electric fields depending on pH. It also helps explain their solubility patterns and crystallization as ionic solids. Therefore, the zwitterionic form is the state with both $-NH_3^+$ and $-COO^-$ present.
105. The defining structural feature of an $\alpha$-amino acid is that it has:
ⓐ. $-NH_2$ and $-COOH$ attached to the same $\alpha$-carbon
ⓑ. $-NH_2$ attached to a phosphate group
ⓒ. $-COOH$ attached to a nitrogen base
ⓓ. Two $-COOH$ groups on the same carbon always
Correct Answer: $-NH_2$ and $-COOH$ attached to the same $\alpha$-carbon
Explanation: $\alpha$-Amino acids are defined by the placement of both the amino group and the carboxyl group on the same central carbon atom, called the $\alpha$-carbon. This carbon also carries a hydrogen and a variable R group, giving a consistent backbone that proteins are built from. The shared $\alpha$-carbon arrangement allows amino acids to join in a regular head-to-tail pattern, enabling polypeptide formation with a repeating backbone. It also explains common acid–base behavior because both functional groups interact in the same molecular framework. Variation in the R group then creates different amino acid properties while preserving the same core structure. Hence, attachment of $-NH_2$ and $-COOH$ to the same $\alpha$-carbon is the defining feature.
106. In the Fischer projection convention, the naturally common form of amino acids in proteins is:
ⓐ. D-form with $-NH_2$ on right
ⓑ. D-form with $-NH_2$ on left
ⓒ. L-form with $-NH_2$ on left
ⓓ. L-form with $-NH_2$ on right
Correct Answer: L-form with $-NH_2$ on left
Explanation: Most amino acids incorporated into proteins are in the L-configuration, defined using Fischer projection rules. In this convention, when the carboxyl group is placed at the top and the R group at the bottom, the amino group appears on the left for L-amino acids. This stereochemical consistency is crucial because enzymes and ribosomal machinery are stereospecific and typically recognize only one configuration efficiently. The L-form dominance is a standard biomolecules concept and helps explain why D-amino acids are uncommon in typical protein structures. Chirality affects how amino acids fit into active sites and how proteins fold in three dimensions. Therefore, the naturally common protein form is the L-form with $-NH_2$ on the left.
107. Amino acids show amphoteric behavior mainly because they contain:
ⓐ. Only a basic group and no acidic group
ⓑ. Only an acidic group and no basic group
ⓒ. Only nonpolar R groups in all cases
ⓓ. Both acidic and basic functional groups in one molecule
Correct Answer: Both acidic and basic functional groups in one molecule
Explanation: Amphoteric behavior means a substance can act as both an acid and a base, depending on the pH of the environment. Amino acids have a carboxyl group that can donate a proton (acidic behavior) and an amino group that can accept a proton (basic behavior). Because both groups exist within the same molecule, amino acids can neutralize added acids or bases to some extent and therefore show buffering capacity. This dual nature also explains their existence as zwitterions in water and their characteristic migration in electric fields. The balance between protonated and deprotonated forms changes predictably with pH. Hence, amino acids are amphoteric because they contain both acidic and basic functional groups.
108. Which amino acid is classified as basic mainly due to an additional amino group in its side chain?
ⓐ. Serine
ⓑ. Lysine
ⓒ. Valine
ⓓ. Glycine
Correct Answer: Lysine
Explanation: Lysine is classified as a basic amino acid because its side chain contains an extra amino group that can accept a proton. This additional basic site increases the likelihood that lysine carries a positive charge in many physiological conditions. The presence of a positively charged side chain affects protein folding, binding to negatively charged molecules, and interactions such as salt bridges. Because side chain chemistry largely controls amino acid classification, lysine’s extra amino group is the key reason it is considered basic. This property is frequently tested as part of structural-functional relationships in proteins. Therefore, lysine is the amino acid that is basic due to an additional amino group in its side chain.
109. Which amino acid is acidic because its side chain contains an extra carboxyl group?
ⓐ. Leucine
ⓑ. Lysine
ⓒ. Aspartic acid
ⓓ. Serine
Correct Answer: Aspartic acid
Explanation: Aspartic acid is classified as an acidic amino acid because its side chain includes an additional carboxyl group beyond the main backbone $-COOH$. This extra carboxyl group can lose a proton, commonly giving the side chain a negative charge in many biological conditions. The negative charge influences protein structure, active site chemistry, and ionic interactions with positively charged groups. Acidic side chains are important in catalysis and in stabilizing proteins through electrostatic attractions. Since classification depends on side chain functional groups, the extra carboxyl group is the defining structural reason. Hence, aspartic acid is the acidic amino acid due to an extra carboxyl group.
110. At the isoelectric point ($pI$) of an amino acid, the most accurate statement is:
ⓐ. Net charge is $0$
ⓑ. Net charge is $+1$
ⓒ. Net charge is $-1$
ⓓ. No ions are present in solution
Correct Answer: Net charge is $0$
Explanation: The isoelectric point, $pI$, is the pH at which an amino acid has an overall net charge of zero. This does not mean the molecule is uncharged in every part; it typically exists as a zwitterion with both $-NH_3^+$ and $-COO^-$ present, but the charges balance. At $pI$, amino acids often show minimal mobility in an electric field because there is no net charge to drive migration. Solubility can also be relatively low near $pI$ because ionic repulsion between molecules is reduced. This concept is central in techniques like electrophoresis and in understanding amino acid behavior across pH ranges. Therefore, at $pI$, the net charge is $0$.
111. Primary structure of a protein refers to:
ⓐ. Coiling pattern of backbone
ⓑ. Linear amino acid sequence
ⓒ. Packing of subunits together
ⓓ. Overall 3-D folding shape
Correct Answer: Linear amino acid sequence
Explanation: Primary structure means the exact order of amino acids in a polypeptide chain. It is a covalent “blueprint” written by the sequence of residues joined one after another. Even a single change in this order can alter how the chain behaves chemically and how it can fold later. This level does not describe helices, sheets, or overall shape; it only specifies which amino acid comes next in the chain. Because all higher levels of structure depend on the starting sequence, primary structure is the fundamental identity of a protein. Therefore, the linear amino acid sequence defines primary structure.
112. The bond that directly holds amino acids together in the primary structure is:
ⓐ. Hydrogen bond
ⓑ. Ionic bond
ⓒ. Disulfide bond
ⓓ. Peptide bond
Correct Answer: Peptide bond
Explanation: Primary structure is built by covalent peptide bonds linking amino acids into a continuous chain. This bond forms when the carboxyl group of one amino acid joins the amino group of the next, creating the backbone linkage $-CO-NH-$. Because it is covalent, it provides strong connectivity and preserves the sequence order of residues. Hydrogen and ionic bonds mainly stabilize higher-level folding, and disulfide bonds connect side chains rather than creating the main chain. The defining connection for the sequence itself is the peptide bond. Hence, peptide bonds hold amino acids together in primary structure.
113. The standard direction for writing a polypeptide’s primary structure is from:
ⓐ. N-terminus to C-terminus
ⓑ. C-terminus to N-terminus
ⓒ. Middle toward both ends
ⓓ. Random order of residues
Correct Answer: N-terminus to C-terminus
Explanation: Protein sequences are conventionally written starting at the N-terminus and ending at the C-terminus. The N-terminus is the end with the free amino group, while the C-terminus has the free carboxyl group. This direction matches how ribosomes synthesize polypeptides, adding new amino acids to the growing chain so the sequence extends toward the C-terminus. Using one consistent direction avoids confusion when comparing proteins and locating specific residues. It also supports clear discussion of mutations and functional regions along the chain. Therefore, the standard writing direction is N-terminus to C-terminus.
114. If one amino acid in a polypeptide chain is replaced by another, which level is directly changed first?
ⓐ. Quaternary structure
ⓑ. Secondary structure
ⓒ. Primary structure
ⓓ. Tertiary structure
Correct Answer: Primary structure
Explanation: Replacing one amino acid changes the actual residue order in the chain, which is the definition of primary structure. Higher levels such as secondary, tertiary, or quaternary may or may not change, depending on how critical that residue is for folding or interactions. But the first and guaranteed change is at the sequence level because the chain now contains a different amino acid at a specific position. This alteration can influence charge, polarity, or size at that site, potentially affecting function. The key point is that sequence changes are primary-structure changes by definition. Hence, the primary structure is directly changed first.
115. A polypeptide with $n$ amino acids has how many peptide bonds?
ⓐ. $n+1$
ⓑ. $n-1$
ⓒ. $n$
ⓓ. $2n$
Correct Answer: $n-1$
Explanation: Peptide bonds link amino acids in a chain, and each bond connects two adjacent residues. In a chain of $n$ amino acids, the first amino acid provides an N-terminus and the last provides a C-terminus, so bonds occur only between successive pairs. That means there is one fewer bond than the number of amino acids. For example, 3 amino acids form 2 peptide bonds, and 10 amino acids form 9 peptide bonds. This relationship is a direct consequence of linear connectivity in the backbone. Therefore, a polypeptide with $n$ amino acids contains $n-1$ peptide bonds.
116. The most accurate statement about primary structure and protein function is:
ⓐ. Primary structure is unrelated to folding
ⓑ. Primary structure depends on pH only
ⓒ. Primary structure changes during denaturation
ⓓ. Primary structure guides folding and function
Correct Answer: Primary structure guides folding and function
Explanation: The amino acid sequence determines the chemical properties along the chain, such as where polar, nonpolar, acidic, or basic residues occur. These patterns drive interactions that promote specific folding pathways and stabilize a functional shape. Active sites, binding regions, and structural motifs depend on having the right residues in the right positions. Even small sequence differences can change stability, binding, or catalytic efficiency. Because higher structure emerges from sequence-based interactions, primary structure is the starting control layer for function. Hence, primary structure guides folding and function.
117. Ordinary denaturation (without bond cleavage) typically affects which statement most accurately?
ⓐ. Primary sequence remains intact
ⓑ. Peptide bonds are fully hydrolyzed
ⓒ. Amino acids detach into monomers
ⓓ. Residue order is reversed
Correct Answer: Primary sequence remains intact
Explanation: Denaturation usually disrupts noncovalent interactions that maintain secondary and tertiary structure, such as hydrogen bonds, hydrophobic interactions, and ionic attractions. In most conditions, the covalent peptide bonds of the backbone are not broken, so the amino acid sequence remains the same. This is why a denatured protein can sometimes refold if normal conditions are restored. Breaking the primary structure requires hydrolysis of peptide bonds, which is a different process from typical denaturation. The key distinction is unfolding versus chain cleavage. Therefore, during ordinary denaturation, the primary sequence remains intact.
118. In a polypeptide, the end that carries a free $-NH_2$ or $-NH_3^+$ group is the:
ⓐ. C-terminus
ⓑ. R-terminus
ⓒ. N-terminus
ⓓ. O-terminus
Correct Answer: N-terminus
Explanation: The N-terminus is defined as the end of a polypeptide chain where the terminal amino group is not involved in a peptide bond. Depending on pH, this group may appear as $-NH_2$ or as the protonated form $-NH_3^+$. The opposite end, the C-terminus, carries the free carboxyl group as $-COOH$ or $-COO^-$. This directionality is essential for writing sequences, describing processing, and understanding how proteins are synthesized. It also helps identify where modifications or signal sequences occur. Hence, the end with the free amino group is the N-terminus.
119. A peptide bond is formed specifically between the:
ⓐ. Two R groups of adjacent amino acids
ⓑ. $-COOH$ of one and $-NH_2$ of the next
ⓒ. Two $-NH_2$ groups of amino acids
ⓓ. Two $-COOH$ groups of amino acids
Correct Answer: $-COOH$ of one and $-NH_2$ of the next
Explanation: The peptide bond is the covalent linkage that joins amino acids into a polypeptide backbone. It forms when the carboxyl group of one amino acid reacts with the amino group of another, creating the amide linkage $-CO-NH-$. This connection produces a continuous chain with a defined sequence and direction. The R groups remain as side chains and do not form the backbone bond that defines the residue order. Because the chain grows by repeating this same head-to-tail connection, the specific groups involved are consistent across proteins. Therefore, peptide bonds form between $-COOH$ and $-NH_2$ groups of adjacent amino acids.
120. Which representation is the most appropriate way to show a peptide’s primary structure?
ⓐ. A diagram of helices and sheets
ⓑ. A list of only the R group types
ⓒ. A 3-D surface model of the protein
ⓓ. A residue order like Ala-Gly-Ser
Correct Answer: A residue order like Ala-Gly-Ser
Explanation: Primary structure is communicated by specifying the exact sequence of amino acids in the chain. Writing a residue order such as Ala-Gly-Ser directly shows which amino acid comes first, second, and third, reflecting the covalent sequence along the backbone. Helices, sheets, and 3-D models describe higher structural levels and do not uniquely define the underlying residue order. Listing only R group types is incomplete because it loses the precise identity and position of each amino acid. Sequence notation is the standard way to record and compare proteins across organisms and conditions. Hence, a residue order like Ala-Gly-Ser best represents primary structure.
121. Secondary structure of a protein mainly refers to:
ⓐ. Exact amino acid sequence in chain
ⓑ. Local folding like helix and sheet
ⓒ. Final 3-D folding of whole chain
ⓓ. Assembly of multiple subunits
Correct Answer: Local folding like helix and sheet
Explanation: Secondary structure describes the regular, repeated folding patterns formed by the polypeptide backbone in limited regions of a protein. The two classic examples are the alpha-helix and the beta-pleated sheet, which arise because the backbone can form stable hydrogen-bonding arrangements. This level focuses on local geometry rather than the entire 3-D shape of the protein. It does not describe the amino acid order (that is primary structure) or the full spatial folding (that is tertiary). It also does not involve how separate polypeptide chains assemble (that is quaternary). Therefore, local folding patterns define secondary structure.
122. The alpha-helix is stabilized primarily by hydrogen bonds between:
ⓐ. Side chains (R groups) of adjacent amino acids
ⓑ. Phosphate groups attached to amino acids
ⓒ. Terminal ends (N-terminus to C-terminus) only
ⓓ. Backbone $C=O$ and $N-H$ groups within the same chain
Correct Answer: Backbone $C=O$ and $N-H$ groups within the same chain
Explanation: In an alpha-helix, stability comes mainly from hydrogen bonds formed along the polypeptide backbone, not from side-chain bonding. The carbonyl oxygen ($C=O$) of one peptide bond forms a hydrogen bond with the amide hydrogen ($N-H$) of another peptide bond further along the same chain. This repeating backbone hydrogen-bond pattern locks the chain into a helical coil and provides strong internal support. Because the bonding is regular and occurs throughout the helix, it creates a highly stable local structure. The side chains project outward and influence helix preference but are not the primary stabilizing bond network. Hence, backbone $C=O$ and $N-H$ hydrogen bonding stabilizes the alpha-helix.
123. Which protein is classically associated with alpha-helical secondary structure as a major feature?
ⓐ. Keratin
ⓑ. Fibroin (silk protein)
ⓒ. Collagen
ⓓ. Casein
Correct Answer: Keratin
Explanation: Keratin, found in hair, nails, and the outer layers of skin, is a common example where alpha-helical structure is prominent. The polypeptide chains in keratin form helical segments that contribute to strength and flexibility in fibrous tissues. This is frequently tested as an example of how secondary structure relates to function in structural proteins. While other proteins have their own characteristic motifs, keratin is the standard textbook example linked to alpha-helix-rich organization. The alpha-helix arrangement helps keratin resist stretching while maintaining resilience. Therefore, keratin is correctly associated with alpha-helical secondary structure.
124. The beta-pleated sheet structure is stabilized mainly by hydrogen bonds:
ⓐ. Only between side chains in the same strand
ⓑ. Only within a single helical turn
ⓒ. Between backbone groups of adjacent polypeptide strands
ⓓ. Between fatty acid chains attached to proteins
Correct Answer: Between backbone groups of adjacent polypeptide strands
Explanation: A beta-pleated sheet forms when extended segments of polypeptide chains lie next to each other and are held together by hydrogen bonding. These hydrogen bonds occur between the backbone carbonyl ($C=O$) and amide ($N-H$) groups of neighboring strands, creating a sheet-like arrangement. The strands may belong to the same polypeptide folded back on itself or to different regions brought together during folding. This backbone-based hydrogen bonding gives the sheet stability and a characteristic “pleated” appearance. Side chains alternate above and below the sheet, affecting packing but not forming the main stabilizing network. Hence, hydrogen bonds between adjacent backbone strands stabilize beta-sheets.
125. Which amino acid is most likely to disrupt an alpha-helix when present within it?
ⓐ. Proline
ⓑ. Valine
ⓒ. Leucine
ⓓ. Alanine
Correct Answer: Proline
Explanation: Proline is often called a “helix breaker” because its side chain forms a ring that restricts backbone flexibility. This rigid structure makes it difficult for the polypeptide backbone to adopt the smooth, repeating geometry needed for a stable alpha-helix. In addition, proline lacks a typical backbone hydrogen on the amide nitrogen, reducing the ability to form the regular hydrogen-bond pattern essential for helix stability. These constraints can introduce bends or kinks, interrupting helix continuity. Therefore, proline is the amino acid most likely to disrupt an alpha-helix.
126. Which pairing correctly matches a protein with beta-sheet-rich secondary structure?
ⓐ. Keratin — beta-pleated sheet
ⓑ. Hemoglobin — only beta-pleated sheet
ⓒ. Collagen — beta-pleated sheet
ⓓ. Fibroin (silk) — beta-pleated sheet
Correct Answer: Fibroin (silk) — beta-pleated sheet
Explanation: Silk fibroin is a classic example of a protein dominated by beta-pleated sheet structure. The extended beta-strands pack closely, and the sheet arrangement allows strong intermolecular interactions, producing a tough and flexible fiber. This beta-sheet-rich organization helps silk achieve high tensile strength while remaining lightweight. The example is commonly used to contrast with keratin, which is typically associated with alpha-helical structure. Because the question asks for a correct match, the silk fibroin and beta-sheet pairing is the standard, reliable association. Hence, fibroin (silk) corresponds to beta-pleated sheet structure.
127. Secondary structure elements like alpha-helix and beta-sheet are formed mainly due to:
ⓐ. Covalent bonds between side chains
ⓑ. Peptide bond hydrolysis in the backbone
ⓒ. Regular hydrogen bonding of the polypeptide backbone
ⓓ. Random collisions of amino acids in solution
Correct Answer: Regular hydrogen bonding of the polypeptide backbone
Explanation: The defining driver of secondary structure is a repeating hydrogen-bond pattern involving the polypeptide backbone. The backbone has carbonyl ($C=O$) and amide ($N-H$) groups that can form hydrogen bonds in predictable geometries. When these bonds form in a regular, repeated way, stable motifs like alpha-helices and beta-sheets arise. This process does not require breaking peptide bonds, and it is not primarily based on covalent side-chain linkages. Side chains influence which secondary structure is favored, but the key stabilizing framework is the backbone hydrogen bonding. Therefore, regular backbone hydrogen bonding is the main cause of secondary structure formation.
128. Which statement best distinguishes secondary structure from tertiary structure?
ⓐ. Secondary structure requires multiple polypeptide chains
ⓑ. Secondary structure is local backbone folding; tertiary is overall 3-D folding of one chain
ⓒ. Secondary structure describes only amino acid composition, not shape
ⓓ. Secondary structure depends only on disulfide bonds
Correct Answer: Secondary structure is local backbone folding; tertiary is overall 3-D folding of one chain
Explanation: Secondary structure refers to local, repetitive patterns of backbone folding such as alpha-helices and beta-sheets. Tertiary structure describes the complete three-dimensional shape of a single polypeptide chain formed by packing these local elements together. The tertiary level includes long-range interactions between distant parts of the same chain and reflects the final overall fold. Secondary structure does not require multiple chains, and it is not determined only by disulfide bonds. The key distinction is scope: local backbone geometry versus whole-chain 3-D architecture. Hence, local backbone folding is secondary, while overall single-chain folding is tertiary.
129. The pleated appearance of beta-sheets is mainly due to:
ⓐ. Zig-zag arrangement of the polypeptide backbone in extended strands
ⓑ. Coiling of the backbone into a tight spiral
ⓒ. Branching introduced by $\alpha(1-6)$ bonds
ⓓ. Presence of phosphate bridges between amino acids
Correct Answer: Zig-zag arrangement of the polypeptide backbone in extended strands
Explanation: In beta-sheets, the polypeptide backbone is extended rather than coiled, and the geometry of peptide bonds produces a zig-zag alignment along each strand. When multiple extended strands align side by side and hydrogen bond, the sheet shows alternating rises and dips, giving a “pleated” look. This appearance is a direct outcome of backbone bond angles and planarity, not due to carbohydrate-like branching or phosphate bridges. The pleated structure is therefore inherent to how extended peptide backbones pack together in beta-sheets. This structural feature helps explain why beta-sheets can form strong, stable layers in proteins. Hence, the zig-zag backbone arrangement creates the pleated appearance.
130. A common conceptual feature of both alpha-helix and beta-pleated sheet is that they:
ⓐ. Are held mainly by disulfide bonds between cysteines
ⓑ. Require enzymatic cleavage of peptide bonds to form
ⓒ. Are stabilized largely by hydrogen bonds involving the backbone
ⓓ. Occur only in membrane proteins and not in soluble proteins
Correct Answer: Are stabilized largely by hydrogen bonds involving the backbone
Explanation: Both alpha-helices and beta-sheets are secondary structures that depend on hydrogen bonding patterns involving the polypeptide backbone. In an alpha-helix, hydrogen bonds form within the same chain along the helix, while in a beta-sheet, hydrogen bonds form between neighboring strands. In both cases, the interacting groups are mainly backbone carbonyl ($C=O$) and amide ($N-H$) groups. This shared stabilization principle is why secondary structure is considered a backbone-driven level of organization. Disulfide bonds and other interactions can influence higher folding, but they are not the primary stabilizers of these motifs. Therefore, backbone hydrogen bonding is the common stabilizing feature.
131. Tertiary structure of a protein mainly refers to:
ⓐ. Helix pattern in one region
ⓑ. Sequence order of residues
ⓒ. Overall 3-D fold of chain
ⓓ. Subunit packing pattern
Correct Answer: Overall 3-D fold of chain
Explanation: Tertiary structure is the final three-dimensional folding of a single polypeptide chain after local helices and sheets form. It describes how the entire chain bends, loops, and packs into a specific shape that is functional. This level is driven mainly by interactions among R groups, so different residues bring different folding forces. The fold creates features like active sites, binding pockets, and surface charge patterns. Because the same sequence can fold into a reproducible shape under proper conditions, tertiary structure is a key identity of a protein. Therefore, the overall 3-D fold of one chain defines tertiary structure.
132. The most important force that drives formation of a compact globular tertiary structure in water is:
ⓐ. Hydrophobic side-chain packing
ⓑ. Phosphate backbone attraction
ⓒ. Peptide bond hydrolysis force
ⓓ. Base-pair type bonding
Correct Answer: Hydrophobic side-chain packing
Explanation: In an aqueous environment, nonpolar R groups tend to avoid water and cluster together inside the protein. This “hydrophobic collapse” is a major driving force that makes many proteins compact and globular. Once hydrophobic groups pack in the core, other interactions can fine-tune stability and shape. This core formation reduces the exposure of nonpolar surfaces to water, which is energetically favorable. The result is a stable interior and a more polar exterior that interacts well with water. Hence, hydrophobic side-chain packing is a primary driver of globular tertiary folding.
133. A disulfide bond in tertiary structure is most directly formed between side chains of:
ⓐ. Glycine residues
ⓑ. Alanine residues
ⓒ. Lysine residues
ⓓ. Cysteine residues
Correct Answer: Cysteine residues
Explanation: Disulfide bonds form when two cysteine side chains are oxidized to create a covalent link $S-S$. This linkage connects two parts of the same polypeptide chain or can connect different chains, increasing structural stability. Because it is covalent, it is stronger than most noncovalent forces used in folding. Disulfide bonds help proteins maintain shape under stress conditions and can lock specific loops or domains in place. The presence and placement of cysteine residues therefore strongly influences stable tertiary folding. Therefore, cysteine residues form disulfide bonds.
134. During typical denaturation by heat, which statement is most accurate?
ⓐ. Peptide bonds are hydrolyzed first
ⓑ. Tertiary interactions are disrupted
ⓒ. Amino acids detach as monomers
ⓓ. Residue order is rearranged
Correct Answer: Tertiary interactions are disrupted
Explanation: Heat denaturation mainly breaks the weak forces that stabilize folding, such as hydrogen bonds, ionic attractions, and hydrophobic interactions. When these interactions weaken, the protein loses its specific 3-D shape, which is largely the tertiary structure. In many cases, the primary sequence remains intact because peptide bonds are not usually broken by mild heating. Loss of tertiary structure often destroys biological activity because active sites depend on correct folding. The unfolded chain may also aggregate because hydrophobic regions become exposed. Hence, denaturation typically disrupts tertiary interactions.
135. A common structural feature of many globular proteins is:
ⓐ. Hydrophobic core, polar surface
ⓑ. Polar core, lipid surface layer
ⓒ. Sugar core, water-free surface
ⓓ. Phosphate core, neutral surface
Correct Answer: Hydrophobic core, polar surface
Explanation: Globular proteins in water often fold so that nonpolar R groups are buried inside, forming a hydrophobic core. Polar and charged side chains are more commonly exposed on the surface, where they can interact with water. This arrangement stabilizes the protein because it reduces unfavorable contact between water and nonpolar groups. It also helps maintain solubility, since the surface is more compatible with the aqueous environment. Many enzymes and carrier proteins show this organization because they function in fluid environments. Therefore, a hydrophobic core with a polar surface is a typical globular protein feature.
136. Molecular chaperones mainly help proteins by:
ⓐ. Cutting peptide bonds rapidly
ⓑ. Changing amino acid sequence
ⓒ. Preventing misfolding aggregates
ⓓ. Making $S-S$ bonds always
Correct Answer: Preventing misfolding aggregates
Explanation: During folding, polypeptide chains can expose hydrophobic regions that may stick to other chains and form aggregates. Chaperones assist by providing a protected environment or by binding transiently to unstable folding intermediates. This reduces incorrect interactions and gives the protein a better chance to reach its correct tertiary structure. Chaperones do not change the primary sequence; they support proper folding pathways. Their action is especially important under stress or when proteins are synthesized rapidly. Hence, preventing misfolding aggregates is the key role of chaperones.
137. Which protein is a standard example of a single polypeptide showing tertiary structure without quaternary assembly?
ⓐ. Hemoglobin (multi-chain protein)
ⓑ. Collagen (triple chain fiber)
ⓒ. Keratin (fibrous bundles)
ⓓ. Myoglobin (single-chain globular)
Correct Answer: Myoglobin (single-chain globular)
Explanation: Myoglobin is a compact globular protein made of a single polypeptide chain. Its functional shape depends on how that one chain folds into a stable 3-D structure, which is tertiary structure. Because it is not built from multiple different subunits, it does not require quaternary structure to exist as a functional unit. The folded form creates a specific environment for binding and holding a heme group, illustrating how tertiary folding supports function. This makes myoglobin a classic example for tertiary structure at the single-chain level. Therefore, myoglobin is the correct example.
138. In protein structure, a “domain” is best described as:
ⓐ. Compact unit folding independently
ⓑ. Random coil with no order
ⓒ. Two-chain subunit combination
ⓓ. Entire protein always one unit
Correct Answer: Compact unit folding independently
Explanation: A domain is a region of a polypeptide that can fold into a stable, compact structure on its own. Many proteins contain multiple domains, each contributing a specific function such as binding, catalysis, or regulation. Domains often behave like modular units, so similar domains can appear in different proteins. This idea helps explain how large proteins can be organized and how evolution can reuse functional building blocks. Domain organization is therefore closely linked to tertiary structure because it describes stable 3-D units within one chain. Hence, a compact independently folding unit is a domain.
139. A “salt bridge” stabilizing tertiary structure is best described as:
ⓐ. Only van der Waals attraction
ⓑ. Ionic attraction of charged R groups
ⓒ. Covalent $S-S$ linkage bond
ⓓ. Peptide bond in backbone
Correct Answer: Ionic attraction of charged R groups
Explanation: Salt bridges are ionic interactions between oppositely charged side chains within a folded protein. For example, a positively charged group like $-NH_3^+$ can interact with a negatively charged group like $-COO^-$. These attractions help stabilize the tertiary fold by holding specific regions of the chain close together. Their strength depends on the local environment and can be weakened by changes in pH or high ionic strength. Because they involve R groups, salt bridges are part of tertiary, not primary, structure stabilization. Therefore, ionic attraction of charged R groups describes a salt bridge.
140. Extreme pH can disrupt tertiary structure mainly by:
ⓐ. Creating new peptide bonds fast
ⓑ. Converting amino acids to sugars
ⓒ. Changing R-group charge states
ⓓ. Removing all backbone carbonyls
Correct Answer: Changing R-group charge states
Explanation: Many tertiary interactions depend on the charged or uncharged state of side chains, including salt bridges and hydrogen-bond networks. When pH changes greatly, acidic and basic R groups gain or lose protons, altering their charges. This can break ionic attractions and disturb the balance of forces that maintain the folded 3-D shape. As the stabilizing interactions weaken, the protein may unfold and lose activity even though the peptide backbone remains intact. The effect is therefore largely due to altered side-chain chemistry rather than new bond formation. Hence, changing R-group charge states is the main reason extreme pH disrupts tertiary structure.
141. Quaternary structure of a protein mainly refers to:
ⓐ. Association of multiple polypeptides
ⓑ. Linear order of amino acids
ⓒ. Local helix and sheet patterns
ⓓ. Overall fold of one chain
Correct Answer: Association of multiple polypeptides
Explanation: Quaternary structure describes how two or more polypeptide chains (subunits) assemble into a single functional protein complex. The key idea is subunit-to-subunit arrangement, including the number of subunits and how they fit together at interfaces. This assembly can create new functional properties that a single chain alone may not show, such as cooperative binding or improved stability. The interactions are mostly noncovalent (hydrophobic, ionic, and hydrogen bonding), though covalent links may also occur in some proteins. Because it focuses on multi-chain organization, it is distinct from tertiary folding of one chain. Hence, association of multiple polypeptides defines quaternary structure.
142. Hemoglobin is a classic example of quaternary structure because it:
ⓐ. Is only one folded polypeptide
ⓑ. Has only beta-pleated sheets
ⓒ. Has no prosthetic group at all
ⓓ. Has four interacting subunits
Correct Answer: Has four interacting subunits
Explanation: Hemoglobin functions as a multi-subunit protein in which four polypeptide chains assemble into one oxygen-transport unit. The presence of multiple interacting subunits is the defining requirement for quaternary structure. Subunit interfaces allow communication between chains, which is central to hemoglobin’s regulated oxygen binding behavior. The overall function depends not only on each subunit’s fold but also on how the subunits are arranged and influence one another. This is why hemoglobin is frequently used to explain quaternary organization and its physiological relevance. Therefore, having four interacting subunits identifies hemoglobin as a quaternary-structure example.
143. A protein made of identical subunits is best termed a:
ⓐ. Hetero-oligomeric protein complex
ⓑ. Multi-domain single-chain protein
ⓒ. Conjugated protein with cofactor
ⓓ. Homo-oligomeric protein complex
Correct Answer: Homo-oligomeric protein complex
Explanation: When all subunits in a protein complex are the same polypeptide, the assembly is described as homo-oligomeric. The term “homo” indicates identical composition of subunits, while “oligomeric” indicates multiple subunits forming one functional unit. This classification is useful because the symmetry and repeated interfaces often influence stability and regulation. Many enzymes form homo-dimers or homo-tetramers to gain functional advantages such as coordinated active sites or structural robustness. The key deciding factor is subunit identity, not the presence of cofactors. Hence, a complex of identical subunits is a homo-oligomeric protein complex.
144. Cooperative binding of $O_2$ is most directly explained by:
ⓐ. Primary sequence repeating regularly
ⓑ. Subunit interactions within quaternary structure
ⓒ. Only peptide bond planarity
ⓓ. Only alpha-helix formation
Correct Answer: Subunit interactions within quaternary structure
Explanation: Cooperative binding occurs when binding of a ligand to one subunit changes the affinity of other subunits within the same protein complex. This requires communication across subunit interfaces, which is a quaternary-structure feature. In hemoglobin, for example, binding of $O_2$ to one subunit promotes conformational changes that increase the likelihood of $O_2$ binding to the remaining subunits. The effect depends on multi-subunit architecture and the ability of subunits to shift between different functional states together. Without subunit-level coupling, such cooperative behavior is not expected. Therefore, subunit interactions in quaternary structure best explain cooperative binding.
145. If a protein’s quaternary structure is disrupted but each subunit remains folded, the primary consequence is loss of:
ⓐ. Subunit assembly-dependent function
ⓑ. Amino acid sequence identity
ⓒ. Peptide bond continuity
ⓓ. Backbone $C=O$ groups
Correct Answer: Subunit assembly-dependent function
Explanation: Quaternary structure provides function that depends on correct subunit association, such as forming complete active sites at interfaces or enabling allosteric regulation across subunits. If the complex dissociates while each subunit keeps its tertiary fold, the sequence and peptide bonds remain unchanged, but the multi-subunit architecture is lost. This can eliminate cooperative effects, reduce stability, or prevent proper substrate channeling between subunits. Many proteins are designed so that full activity emerges only when subunits assemble in the correct arrangement. Therefore, disrupting quaternary structure primarily removes subunit assembly-dependent function.
146. The interactions that most commonly stabilize quaternary structure are mainly:
ⓐ. Nucleotide base pairing forces
ⓑ. Noncovalent interface interactions
ⓒ. Phosphodiester covalent bonds
ⓓ. Sugar-phosphate backbone links
Correct Answer: Noncovalent interface interactions
Explanation: Quaternary structure is typically stabilized by the same broad classes of weak interactions that stabilize tertiary folding, but acting between different polypeptide chains. These include hydrophobic packing at subunit interfaces, hydrogen bonds, ionic attractions (salt bridges), and van der Waals forces. Because these interactions are numerous and collectively strong, they can hold subunits together while still allowing functional flexibility. In many proteins, this “reversible stability” is essential for regulation and dynamic assembly. Covalent links can occur in some cases, but the most common stabilizers are noncovalent interface interactions. Hence, noncovalent interface interactions are the typical stabilizing forces.
147. A protein complex composed of different kinds of subunits is best termed a:
ⓐ. Homo-oligomeric protein complex
ⓑ. Hetero-oligomeric protein complex
ⓒ. Random coil polypeptide chain
ⓓ. Single-chain globular protein
Correct Answer: Hetero-oligomeric protein complex
Explanation: “Hetero-oligomeric” means the protein complex contains more than one type of polypeptide subunit. The term “hetero” indicates different subunits, while “oligomeric” indicates multiple subunits forming one functional entity. This is common in many regulatory and transport proteins where different subunits contribute distinct properties, such as binding specificity, catalytic roles, or control features. Correct classification depends on subunit composition, not simply on whether the protein is globular or fibrous. Because the complex includes different chains, it is hetero-oligomeric by definition. Therefore, a complex with different kinds of subunits is a hetero-oligomeric protein complex.
148. Quaternary structure is absent in a protein that:
ⓐ. Has one chain but many domains
ⓑ. Shows many alpha-helices inside
ⓒ. Functions as a single polypeptide
ⓓ. Contains a heme-like group
Correct Answer: Functions as a single polypeptide
Explanation: Quaternary structure requires more than one polypeptide chain assembling into a functional unit. If a protein functions as a single polypeptide chain, it can still have primary, secondary, and tertiary structures, and it may even contain cofactors, but it lacks subunit association by definition. Multiple domains within one chain do not create quaternary structure because domains are parts of the same polypeptide, not separate subunits. The decisive criterion is the number of polypeptide chains in the functional protein unit. Therefore, a protein functioning as a single polypeptide does not have quaternary structure.
149. The term “subunit” in quaternary structure most accurately refers to:
ⓐ. One polypeptide chain in a complex
ⓑ. One amino acid in a protein
ⓒ. One peptide bond in backbone
ⓓ. One helix segment only
Correct Answer: One polypeptide chain in a complex
Explanation: In quaternary structure, a subunit is one complete polypeptide chain that associates with other polypeptide chains to form a functional protein complex. Each subunit typically has its own tertiary structure, and the quaternary structure describes how these folded units fit together. Subunits may be identical or different, and their interfaces often determine key functional properties like regulation, stability, or cooperative binding. The term does not refer to a single amino acid, a single peptide bond, or one helix, because those are not independent functional building blocks at the multi-chain level. Hence, a subunit is one polypeptide chain within a complex.
150. In a multi-subunit protein, an allosteric effect most directly involves:
ⓐ. Breaking peptide bonds in backbone
ⓑ. Removing all side chains at once
ⓒ. Converting protein into lipid-like form
ⓓ. Structural change transmitted between subunits
Correct Answer: Structural change transmitted between subunits
Explanation: Allosteric effects occur when binding at one site causes a conformational change that alters activity or binding at another site. In multi-subunit proteins, this often means a structural shift in one subunit influences neighboring subunits through their interfaces. This transmission depends on quaternary organization because subunits must be physically connected in a coordinated assembly. The outcome can be increased or decreased affinity for a ligand, altered catalytic activity, or switching between functional states. The mechanism is conformational communication, not peptide bond cleavage or side-chain removal. Therefore, an allosteric effect most directly involves structural change transmitted between subunits.
151. Protein denaturation is best described as:
ⓐ. Breaking peptide bonds into amino acids
ⓑ. Loss of native shape and activity
ⓒ. Conversion of protein into carbohydrate
ⓓ. Removal of nitrogen from amino acids
Correct Answer: Loss of native shape and activity
Explanation: Denaturation means a protein loses its native three-dimensional conformation that is required for biological function. Heat, extreme $pH$, detergents, or chemicals can disrupt hydrogen bonds, ionic attractions, and hydrophobic packing that stabilize folding. When these interactions are disturbed, the active site or binding surface no longer matches the target, so activity decreases or stops. In most ordinary denaturation, the amino acid sequence stays the same because peptide bonds are not the first bonds to break. Denatured proteins may also aggregate because hydrophobic regions become exposed. Therefore, denaturation is correctly defined as loss of native shape and activity.
152. During typical heat denaturation, which level is most directly disrupted first?
ⓐ. Primary structure
ⓑ. Peptide bond connectivity
ⓒ. Amino acid sequence order
ⓓ. Secondary/tertiary interactions
Correct Answer: Secondary/tertiary interactions
Explanation: Heat mainly weakens the noncovalent forces that stabilize protein folding, such as hydrogen bonds, salt bridges, and hydrophobic interactions. These forces are responsible for maintaining secondary patterns (helix/sheet) and the overall tertiary fold of a single chain. Because peptide bonds are strong covalent links, they usually remain intact under mild heating, so the primary sequence does not immediately change. As the fold loosens, active sites deform and proteins can lose function rapidly. Exposed hydrophobic regions can cause clumping, making denaturation effectively irreversible in many cases. Hence, secondary/tertiary interactions are disrupted first.
153. The most accurate statement about primary structure during ordinary denaturation is:
ⓐ. It usually remains unchanged
ⓑ. It is reversed from C- to N-terminus
ⓒ. It becomes a lipid-like polymer
ⓓ. It is replaced by random sugars
Correct Answer: It usually remains unchanged
Explanation: Primary structure refers to the linear amino acid sequence connected by peptide bonds. In most common denaturing conditions (heat, moderate chemicals, or $pH$ shifts), the protein unfolds because weak stabilizing forces are disrupted, not because peptide bonds are cleaved. Therefore, the residue order along the chain typically stays the same even when the protein loses shape and function. This is why some proteins can refold if denaturing conditions are removed, provided aggregation has not occurred. Breaking primary structure requires hydrolysis or strong chemical attack on peptide bonds, which is not the usual first event. Thus, the primary structure usually remains unchanged.
154. A major reason detergents can denature many proteins is that they:
ⓐ. Convert peptide bonds into ester bonds
ⓑ. Remove all amino acids from the chain
ⓒ. Disrupt hydrophobic interactions in the core
ⓓ. Create new disulfide bonds everywhere
Correct Answer: Disrupt hydrophobic interactions in the core
Explanation: Many proteins maintain a compact fold because nonpolar side chains cluster away from water, forming a hydrophobic core. Detergents have both hydrophobic and hydrophilic parts, so they can surround and solubilize nonpolar regions that are normally buried. This interferes with the hydrophobic packing that drives and stabilizes the tertiary structure. When the core interactions weaken, the protein can unfold and lose its functional geometry, especially at binding sites. This effect is not about changing the amino acid sequence, but about weakening the forces that keep the fold intact. Therefore, detergents denature proteins mainly by disrupting hydrophobic interactions.
155. A protein is most likely to lose function at extreme $pH$ because:
ⓐ. The polypeptide chain becomes shorter automatically
ⓑ. The sequence order of residues is rearranged
ⓒ. Peptide bonds are instantly hydrolyzed in water
ⓓ. R-group charge states change and break interactions
Correct Answer: R-group charge states change and break interactions
Explanation: Many stabilizing forces in proteins depend on side-chain ionization, especially salt bridges and hydrogen-bond networks. At extreme $pH$, acidic and basic R groups gain or lose protons, so charges shift and ionic attractions may disappear or invert. This can destabilize the tertiary fold and distort active sites even if the backbone remains intact. Changes in charge also alter surface solubility, increasing the risk of aggregation or precipitation. Because these effects directly disrupt the interaction map that holds the fold together, function is often lost quickly. Hence, extreme $pH$ causes loss of function mainly by changing R-group charge states and breaking key interactions.
156. Which example best represents irreversible denaturation in daily life?
ⓐ. Cooling a warm enzyme solution to restore activity
ⓑ. Coagulation of egg albumin on heating
ⓒ. Temporary binding of oxygen to a protein
ⓓ. Reversible folding of a small peptide in water
Correct Answer: Coagulation of egg albumin on heating
Explanation: When egg white is heated, its proteins unfold and then aggregate into a new network that appears as a solid coagulum. The unfolding exposes hydrophobic regions and reactive groups, promoting extensive protein–protein interactions that lock the structure in place. Once this aggregated network forms, simply cooling does not restore the original soluble, functional conformation. This is a classic illustration of denaturation followed by aggregation, making the change effectively irreversible under normal conditions. The primary sequence remains present, but the functional native fold cannot be regained because of the new cross-interactions. Therefore, coagulation of egg albumin on heating is a standard example of irreversible denaturation.
157. Which option correctly matches a major protein function with a common example?
ⓐ. Transport — hemoglobin
ⓑ. Storage — insulin
ⓒ. Hormone — collagen
ⓓ. Defense — keratin
Correct Answer: Transport — hemoglobin
Explanation: Proteins show functional diversity because different sequences fold into structures specialized for specific tasks. Hemoglobin is a well-known transport protein that binds and carries gases in blood using a specialized binding environment, enabling efficient delivery to tissues. This transport role depends on its structural features and regulated binding behavior, which are core examples in biomolecules. In contrast, insulin is a hormone, collagen is structural, and keratin is structural/protective rather than defense. The question targets correct pairing of function and example, and hemoglobin is the classic transport match. Hence, “Transport — hemoglobin” is correct.
158. Enzymes are functionally diverse mainly because different proteins:
ⓐ. Have identical active sites but different sizes
ⓑ. Are always fibrous and insoluble in water
ⓒ. Fold to create different active-site shapes and chemistries
ⓓ. Contain no amino acids with charged side chains
Correct Answer: Fold to create different active-site shapes and chemistries
Explanation: Enzymes work because their folding creates an active site with a precise 3-D shape and a specific arrangement of functional groups. Different amino acid sequences position side chains differently, producing unique microenvironments that bind particular substrates and stabilize particular transition states. This explains why one enzyme may catalyze oxidation, another may hydrolyze bonds, and another may transfer groups, even though all are proteins. The active site depends on tertiary structure and the chemical nature of residues present at key positions. Small sequence changes can alter specificity or efficiency by reshaping the active site. Therefore, enzyme diversity arises because proteins fold to create different active-site shapes and chemistries.
159. A fibrous protein is most likely to be specialized for:
ⓐ. Rapid catalytic turnover in solution
ⓑ. Structural support and strength
ⓒ. Genetic information storage
ⓓ. High-speed ion pumping in membranes
Correct Answer: Structural support and strength
Explanation: Fibrous proteins typically form elongated, cable-like structures that are optimized for mechanical roles rather than rapid catalysis. Their structure often emphasizes repeated secondary motifs and strong intermolecular packing, which contributes to tensile strength and durability in tissues. This makes them well suited for support, protection, and maintaining shape at the tissue level. In contrast, many enzymes and transport proteins are globular and optimized for soluble activity and binding. The functional diversity of proteins includes these structural roles as a major category. Therefore, fibrous proteins are most commonly specialized for structural support and strength.
160. The most direct link between protein structure and function is that:
ⓐ. Function depends only on total amino acid count
ⓑ. Function is unchanged even if folding is lost
ⓒ. Function depends only on having peptide bonds
ⓓ. Function depends on correct 3-D conformation
Correct Answer: Function depends on correct 3-D conformation
Explanation: A protein’s function is determined by how its amino acid sequence folds into a specific three-dimensional shape. This shape creates binding pockets, active sites, interaction surfaces, and flexible regions required for biological action. If the conformation changes significantly (as in denaturation), these functional features may no longer align with their targets, causing loss of activity. The same sequence can be functional or nonfunctional depending on whether it achieves and maintains the correct fold. This is why conditions that disturb tertiary or quaternary structure often inactivate proteins without breaking the chain. Hence, protein function depends on correct 3-D conformation.
161. Denaturation of a protein most directly means disruption of:
ⓐ. Peptide bonds in the backbone
ⓑ. Amino acid sequence order
ⓒ. Noncovalent forces maintaining folding
ⓓ. Formation of new amino acids
Correct Answer: Noncovalent forces maintaining folding
Explanation: Denaturation primarily disrupts the weak interactions that stabilize the folded structure, such as hydrogen bonds, ionic attractions, and hydrophobic packing. When these forces are disturbed, the protein loses its native 3-D shape and typically loses biological activity. This process usually does not require breaking peptide bonds, so the primary sequence often remains unchanged. The exposed hydrophobic regions can also lead to aggregation, making the loss of function effectively irreversible in many cases. The core concept is “unfolding without chain cleavage.” Therefore, denaturation is best linked to disruption of noncovalent forces that maintain folding.
162. Which condition is most likely to denature a protein by changing side-chain ionization states?
ⓐ. Extreme $pH$
ⓑ. Low light intensity
ⓒ. Mild cooling
ⓓ. Low oxygen level
Correct Answer: Extreme $pH$
Explanation: Extreme $pH$ alters the protonation state of acidic and basic side chains, changing their charges. This can break salt bridges and disturb hydrogen-bonding networks that stabilize tertiary and quaternary structure. Once these interactions weaken, the protein can unfold and lose function even though the peptide backbone remains intact. Charge changes can also reduce solubility and promote aggregation or precipitation. The main driver here is altered side-chain chemistry rather than mechanical stress or oxygen availability. Hence, extreme $pH$ is a common cause of denaturation through ionization changes.
163. Which statement about heat denaturation is most accurate?
ⓐ. It first breaks peptide bonds into amino acids
ⓑ. It mainly disrupts hydrogen bonds and hydrophobic interactions
ⓒ. It converts proteins into carbohydrates
ⓓ. It removes all R groups from amino acids
Correct Answer: It mainly disrupts hydrogen bonds and hydrophobic interactions
Explanation: Heat increases molecular motion and weakens the weak interactions that stabilize protein folding. Hydrogen bonds that support secondary structure and hydrophobic packing that supports tertiary structure become less stable, causing the protein to unfold. Because peptide bonds are strong covalent bonds, they usually do not break under ordinary heating conditions used in daily life or typical lab denaturation. The result is loss of native conformation and often loss of activity. Exposed hydrophobic regions can trigger aggregation, reinforcing irreversible loss of function. Therefore, heat denaturation mainly disrupts hydrogen bonds and hydrophobic interactions.
164. A protein often loses activity on denaturation mainly because:
ⓐ. Its amino acids are removed from the chain
ⓑ. Its active site geometry is altered
ⓒ. Its nitrogen content becomes zero
ⓓ. Its peptide bonds are replaced by glycosidic bonds
Correct Answer: Its active site geometry is altered
Explanation: Protein activity depends on the precise 3-D arrangement of residues that form binding sites and catalytic centers. Denaturation distorts or destroys this arrangement, so substrates or partners can no longer bind correctly and catalysis cannot proceed efficiently. The amino acid sequence may still be present, but the functional shape is lost. This is why enzymes can become inactive even when their chains remain intact. The key point is that function comes from structure, especially the shape and chemical environment of the active site. Hence, denaturation often inactivates proteins by altering active site geometry.
165. Which is the best example of denaturation followed by aggregation?
ⓐ. Starch dissolving in hot water
ⓑ. Egg white turning solid on heating
ⓒ. Glucose dissolving in water
ⓓ. Fat melting into oil on warming
Correct Answer: Egg white turning solid on heating
Explanation: Egg white contains proteins that unfold when heated, exposing hydrophobic regions and reactive groups. These exposed parts interact with each other, causing proteins to stick together and form a network-like aggregate. This aggregation changes the texture from transparent and fluid to opaque and solid, a hallmark of coagulation. The process is typically irreversible under normal conditions because the aggregated state prevents refolding. This example cleanly illustrates denaturation (loss of native structure) followed by aggregation (protein–protein association). Therefore, egg white turning solid on heating is the best example.
166. Ordinary denaturation generally does NOT directly break:
ⓐ. Hydrogen bonds
ⓑ. Ionic interactions
ⓒ. Hydrophobic interactions
ⓓ. Peptide bonds
Correct Answer: Peptide bonds
Explanation: Denaturation usually disrupts weak interactions that stabilize folding, such as hydrogen bonding, salt bridges, and hydrophobic packing. Peptide bonds are covalent links forming the primary backbone, and they are much stronger than the forces that hold higher structure. Under typical denaturing conditions like heating, pH shifts, or detergents, peptide bonds remain intact while the protein unfolds. Breaking peptide bonds requires hydrolysis or strong chemical conditions that specifically attack the backbone. This distinction is crucial for understanding why proteins can sometimes refold after denaturation. Hence, peptide bonds are generally not directly broken in ordinary denaturation.
167. Which agent denatures proteins mainly by competing for hydrogen bonding and altering solvent environment?
ⓐ. Urea
ⓑ. Oxygen
ⓒ. Sodium chloride (normal level)
ⓓ. Glucose
Correct Answer: Urea
Explanation: Urea is a classic chemical denaturant that disrupts the stabilizing interactions within proteins by altering the solvent environment. It can interfere with hydrogen bonding and weaken hydrophobic interactions, making the unfolded state more favorable. As a result, the protein’s tertiary structure can collapse even though the primary sequence remains intact. This loss of fold leads to loss of function because active sites and binding surfaces are no longer properly formed. Urea is therefore widely used in laboratories to unfold proteins for analysis. Hence, urea is the agent that denatures proteins by disrupting stabilizing interactions through solvent effects.
168. Denaturation is often irreversible in cells mainly when:
ⓐ. The unfolded protein aggregates
ⓑ. The amino acids become chiral
ⓒ. The protein gains extra peptide bonds
ⓓ. The sequence becomes longer automatically
Correct Answer: The unfolded protein aggregates
Explanation: When proteins unfold, hydrophobic regions that were buried inside become exposed to the aqueous environment. These nonpolar patches tend to stick together across different protein molecules, leading to aggregation. Aggregates prevent the polypeptide from finding its correct folding pathway again, making refolding difficult or impossible. Aggregation also reduces solubility and can cause precipitation, further locking proteins into a nonfunctional state. This explains why some denaturation events become irreversible even though peptide bonds are not broken. Therefore, denaturation often becomes irreversible when the unfolded protein aggregates.
169. Which property is most likely to decrease when a soluble globular protein denatures?
ⓐ. Covalent backbone length
ⓑ. Peptide bond count
ⓒ. Number of amino acids
ⓓ. Solubility in water
Correct Answer: Solubility in water
Explanation: Many globular proteins are soluble because their surfaces are enriched in polar and charged residues, while hydrophobic residues are buried. Denaturation exposes hydrophobic parts to water, which promotes protein–protein sticking and aggregation. This reduces solubility and may cause precipitation. The backbone length and amino acid count do not change during ordinary denaturation because peptide bonds typically remain intact. The major change is in surface properties due to unfolding. Hence, solubility in water is the property that most often decreases upon denaturation.
170. Which statement best summarizes the relationship between denaturation and function?
ⓐ. Denaturation increases enzyme specificity
ⓑ. Denaturation creates new peptide bonds automatically
ⓒ. Denaturation always changes amino acid order
ⓓ. Denaturation can destroy function without changing sequence
Correct Answer: Denaturation can destroy function without changing sequence
Explanation: Protein function depends strongly on the correct folding into a native 3-D conformation. Denaturation disrupts folding by breaking weak stabilizing interactions, so active sites and binding surfaces lose their precise geometry. Even when the primary sequence remains unchanged, the functional arrangement of residues is lost, causing inactivation. This explains why enzymes can become inactive after heating or exposure to extremes of $pH$ without any change in amino acid order. The key concept is structure–function dependence beyond the sequence itself. Therefore, denaturation can destroy function without changing the sequence.
171. Lipids are often called “not true polymers” mainly because they:
ⓐ. Are not made of repeating monomer units
ⓑ. Always have peptide bonds in their structure
ⓒ. Always contain long chains of amino acids
ⓓ. Are always built from repeating nucleotide units
Correct Answer: Are not made of repeating monomer units
Explanation: True polymers typically consist of long chains formed by repetition of a basic monomeric unit, as seen in proteins (amino acids) and nucleic acids (nucleotides). Lipids, however, are a diverse group that do not share a single repeating monomer pattern across the group. Many common lipids like fats are assembled by esterifying glycerol with fatty acids rather than forming long monomer-repeat chains. Because the “polymer” idea depends on repeated building blocks in a chain, lipids do not fit that definition well. This is why, in biomolecules classification, lipids are treated as not true polymers. Hence, the correct reason is the lack of repeating monomer units.
172. A neutral fat (triacylglycerol) is formed by esterification of glycerol with how many fatty acid molecules?
ⓐ. $5$
ⓑ. $4$
ⓒ. $3$
ⓓ. $2$
Correct Answer: $3$
Explanation: A triacylglycerol is a storage fat where glycerol provides three hydroxyl groups that can each form an ester bond. Each hydroxyl group reacts with the carboxyl group of a fatty acid, so three fatty acids become attached to one glycerol molecule. This “three-acyl” arrangement is why it is called triacylglycerol and why it is a compact energy storage form. The structure also explains why fats are largely hydrophobic, since the attached fatty acid chains are nonpolar. Therefore, triacylglycerol contains $3$ fatty acid molecules per glycerol.
173. Which statement correctly describes a saturated fatty acid?
ⓐ. It contains a phosphate group as the head
ⓑ. It contains many carbon–carbon double bonds
ⓒ. It contains no carbon–carbon double bonds
ⓓ. It contains a peptide bond in the chain
Correct Answer: It contains no carbon–carbon double bonds
Explanation: Saturated fatty acids have hydrocarbon chains in which all carbon–carbon bonds are single bonds, meaning the chain is “fully saturated” with hydrogen. This structural feature allows the chains to pack closely together, often making such fats more solid at room temperature. The absence of double bonds also means the chain remains relatively straight compared with many unsaturated chains. Because the key defining criterion is bond type within the hydrocarbon chain, “no carbon–carbon double bonds” is the correct description. This concept is frequently tested to connect structure with physical properties. Hence, saturated fatty acids contain no carbon–carbon double bonds.
174. Fats are insoluble in water mainly because they:
ⓐ. Have long nonpolar hydrocarbon chains
ⓑ. Have many charged amino groups on surfaces
ⓒ. Contain repeating sugar units with many $-OH$
ⓓ. Contain phosphate groups only in the backbone
Correct Answer: Have long nonpolar hydrocarbon chains
Explanation: Water is a polar solvent, so it dissolves substances that can form favorable polar interactions or hydrogen bonds with it. Fats are dominated by long hydrocarbon chains that are nonpolar, so they do not interact well with water molecules. As a result, water excludes these nonpolar chains, causing fats to separate out rather than dissolve. This hydrophobic behavior is a direct consequence of the chemical nature of fatty acid chains attached in triacylglycerols. The property is structural, not due to any shortage of “size” or “weight,” but due to polarity mismatch. Therefore, fats are insoluble mainly because of long nonpolar hydrocarbon chains.
175. Waxes are best described as esters formed between:
ⓐ. Glucose and fatty acids in repeating units
ⓑ. Glycerol and three fatty acids only
ⓒ. Amino acids and glycerol units only
ⓓ. Long-chain fatty acids and long-chain alcohols
Correct Answer: Long-chain fatty acids and long-chain alcohols
Explanation: Waxes are simple lipids formed by esterification of a long-chain fatty acid with a long-chain alcohol. This gives them a strongly hydrophobic character and makes them useful for waterproofing and protective coatings in organisms. Their structure differs from fats because fats are typically triesters of glycerol, whereas waxes are esters of fatty acids with alcohols that are not glycerol. This difference explains why waxes tend to be more rigid and water-repellent. The key identification point is the long-chain alcohol component in addition to the fatty acid. Hence, waxes are esters of long-chain fatty acids and long-chain alcohols.
176. The “amphipathic” nature of many phospholipids is due to the presence of:
ⓐ. Only nonpolar tails with no polar region
ⓑ. A polar head and nonpolar tails
ⓒ. Only polar regions with no hydrocarbon chains
ⓓ. A repeating monomer chain like proteins
Correct Answer: A polar head and nonpolar tails
Explanation: Phospholipids contain a hydrophilic (polar) head region and hydrophobic (nonpolar) fatty acid tails in the same molecule. The polar head interacts with water, while the nonpolar tails avoid water, producing amphipathic behavior. This dual nature is essential for membrane formation because phospholipids spontaneously arrange into bilayers with tails inward and heads outward. The structural logic directly explains membrane stability and selective permeability features. Because amphipathic means “both water-loving and water-fearing parts,” the polar head and nonpolar tails are the defining reason. Therefore, phospholipids are amphipathic due to a polar head and nonpolar tails.
177. A higher iodine value of an oil indicates that it has:
ⓐ. A higher degree of unsaturation
ⓑ. A higher degree of saturation only
ⓒ. A higher proportion of peptide bonds
ⓓ. A higher proportion of glucose units
Correct Answer: A higher degree of unsaturation
Explanation: Iodine reacts by adding across carbon–carbon double bonds in unsaturated fatty acids. Oils with more double bonds can take up more iodine, resulting in a higher iodine value. This measurement therefore reflects the degree of unsaturation in the fatty acid chains present. The concept is used to compare oils and fats and to infer how “unsaturated” the lipid mixture is. More unsaturation typically affects physical properties like fluidity and melting behavior. Hence, a higher iodine value indicates a higher degree of unsaturation.
178. Compared with carbohydrates, fats yield more energy per gram mainly because fats:
ⓐ. Are primarily made of phosphate and nitrogen
ⓑ. Contain more oxygen atoms per carbon always
ⓒ. Are polymers of repeating glucose monomers
ⓓ. Are more reduced and rich in $C-H$ bonds
Correct Answer: Are more reduced and rich in $C-H$ bonds
Explanation: Energy yield on oxidation depends largely on how reduced a molecule is, meaning how many electrons it can donate during metabolic oxidation. Fatty acids contain many $C-H$ bonds and relatively less oxygen compared with carbohydrates, making them more reduced. When oxidized, this provides a larger release of energy per gram compared with carbohydrate molecules that are already more oxygen-rich. This chemical basis explains why fats serve as compact long-term energy stores in organisms. The point is not that fats are polymers, but that their composition supports higher oxidation energy. Therefore, fats yield more energy because they are more reduced and rich in $C-H$ bonds.
179. Hydrogenation of vegetable oils generally results in:
ⓐ. Immediate formation of cellulose-like fibers
ⓑ. Complete conversion into proteins and amino acids
ⓒ. Increased saturation and possible trans fat formation
ⓓ. Removal of all ester bonds from the fat
Correct Answer: Increased saturation and possible trans fat formation
Explanation: Hydrogenation adds hydrogen across carbon–carbon double bonds in unsaturated fatty acids, increasing the degree of saturation. This raises the melting point and can convert oils into more solid or semi-solid forms. During partial hydrogenation, some double bonds may shift configuration and form trans fatty acid isomers, which is why trans fats can appear in such products. The key idea is chemical modification of double bonds within fatty acid chains, not breakdown of the glycerol ester framework. This concept links bond chemistry to physical properties and health-related discussions in exams. Hence, hydrogenation increases saturation and can produce trans fats.
180. Saponification of a fat (triacylglycerol) with alkali produces:
ⓐ. Glycerol and fatty acid salts (soap)
ⓑ. Amino acids and short peptides only
ⓒ. Glucose and glycogen granules only
ⓓ. Nucleotides and phosphate sugars only
Correct Answer: Glycerol and fatty acid salts (soap)
Explanation: Saponification is the alkaline hydrolysis of ester bonds present in triacylglycerols. The ester linkages between glycerol and fatty acids are broken, releasing glycerol and converting fatty acids into their salt forms. These fatty acid salts are what we commonly call soaps, and their amphipathic nature helps emulsify oils in water. The reaction demonstrates a key property of fats as esters and is often used to test understanding of lipid chemistry. Since the products directly reflect ester hydrolysis under basic conditions, glycerol and fatty acid salts are expected. Therefore, saponification yields glycerol and fatty acid salts (soap).
181. Lipids are termed “not true polymers” primarily because most lipids:
ⓐ. Have repeating amino acid units linked by peptide bonds
ⓑ. Have repeating nucleotide units linked by phosphodiester bonds
ⓒ. Lack a long chain of repeating monomer units as a single theme
ⓓ. Always contain only one carbon atom as a backbone
Correct Answer: Lack a long chain of repeating monomer units as a single theme
Explanation: A true polymer typically has a repeated monomeric unit forming a long chain, like amino acids in proteins or nucleotides in nucleic acids. Lipids do not share one universal repeating unit pattern across the group; instead, they include fats, oils, waxes, and phospholipids with different architectures. For example, fats are formed by esterifying glycerol with fatty acids rather than building a monomer-repeat chain. This diversity of structures means “polymer” does not describe lipids consistently. Hence, in biomolecules classification, lipids are described as not true polymers because they lack a single repeating monomer-chain theme.
182. Oils are generally liquid at room temperature mainly because they contain:
ⓐ. More unsaturated fatty acids with $C=C$ bonds
ⓑ. Only saturated fatty acids with straight chains
ⓒ. Only glucose units joined by glycosidic bonds
ⓓ. Many peptide bonds that increase flexibility
Correct Answer: More unsaturated fatty acids with $C=C$ bonds
Explanation: Oils typically have a higher proportion of unsaturated fatty acid chains, which include one or more $C=C$ double bonds. These double bonds introduce bends in the hydrocarbon chain, preventing tight packing of molecules. Poor packing reduces intermolecular attractions, lowering the melting point and making the lipid remain liquid at room temperature. This is a structural explanation that links bond type to physical state. The effect is especially noticeable when unsaturation is higher, as packing becomes even less efficient. Therefore, oils are commonly liquid because they contain more unsaturated fatty acids with $C=C$ bonds.
183. A higher iodine value of an oil indicates:
ⓐ. Lower number of double bonds in fatty acid chains
ⓑ. Higher degree of unsaturation in fatty acid chains
ⓒ. Higher proportion of glycerol units in the oil
ⓓ. Higher amount of peptide bonds in the mixture
Correct Answer: Higher degree of unsaturation in fatty acid chains
Explanation: Iodine adds across $C=C$ double bonds present in unsaturated fatty acids. If an oil contains more double bonds, it can react with and “take up” more iodine, giving a higher iodine value. This makes iodine value a practical indicator of how unsaturated the fatty acid composition is. Greater unsaturation also relates to higher fluidity and generally lower melting point. The measure is therefore tied directly to double-bond content rather than glycerol quantity. Hence, a higher iodine value means a higher degree of unsaturation.
184. Waxes are commonly used by organisms for waterproofing mainly because waxes are:
ⓐ. Highly soluble in water due to many $-OH$ groups
ⓑ. Polymers of glucose that form water-absorbing gels
ⓒ. Proteins with strong ionic bonds and charged surfaces
ⓓ. Highly hydrophobic esters with long hydrocarbon chains
Correct Answer: Highly hydrophobic esters with long hydrocarbon chains
Explanation: Waxes are composed of long-chain fatty acids esterified to long-chain alcohols, producing molecules dominated by nonpolar hydrocarbon regions. Because water is polar, these nonpolar chains do not mix with water, making waxes strongly water-repellent. This property allows wax coatings to reduce water loss and protect surfaces, such as on leaves, fruits, and feathers. The stability and low reactivity of wax layers further support their protective role. The key concept is hydrophobicity arising from long hydrocarbon chains. Therefore, waxes are effective waterproofing materials because they are highly hydrophobic esters.
185. Compared with saturated fats, oils usually have lower melting points because unsaturated chains:
ⓐ. Form stronger hydrogen bonds with each other
ⓑ. Form more peptide bonds within the molecule
ⓒ. Pack less tightly due to bends near $C=C$
ⓓ. Always contain shorter chains with fewer carbons
Correct Answer: Pack less tightly due to bends near $C=C$
Explanation: Unsaturated fatty acids contain $C=C$ double bonds that introduce kinks in the chain, disrupting straight alignment. When chains cannot align closely, intermolecular attractions become weaker, so less energy is needed to separate molecules during melting. This explains why many oils remain liquid at room temperature while saturated fats, with straighter chains, pack tightly and melt at higher temperatures. The relationship is primarily about packing efficiency rather than chain length as a universal rule. The bond geometry created by $C=C$ is the crucial factor. Hence, oils melt more easily because unsaturated chains pack less tightly due to bends near $C=C$.
186. Partial hydrogenation of oils is mainly aimed at:
ⓐ. Increasing saturation and raising the melting point
ⓑ. Breaking ester bonds to release free glycerol
ⓒ. Converting fatty acids into amino acids
ⓓ. Converting oils into polysaccharides for storage
Correct Answer: Increasing saturation and raising the melting point
Explanation: Hydrogenation adds hydrogen across $C=C$ double bonds, reducing unsaturation and increasing saturation in fatty acid chains. As saturation increases, chains become straighter and can pack more closely, which raises the melting point and makes the product more solid or semi-solid. This is why hydrogenation has been used to modify texture and shelf stability of lipid products. The chemistry targets double bonds rather than dismantling the glycerol–fatty acid ester framework. The central outcome is a shift toward more saturated character with higher melting behavior. Therefore, partial hydrogenation is aimed at increasing saturation and raising melting point.
187. Chemically, a typical wax molecule is best described as:
ⓐ. Glycerol linked to three fatty acids
ⓑ. Long-chain fatty acid esterified to a long-chain alcohol
ⓒ. Two monosaccharides linked by a glycosidic bond
ⓓ. Amino acids linked by a peptide bond
Correct Answer: Long-chain fatty acid esterified to a long-chain alcohol
Explanation: Waxes are simple lipids formed when the carboxyl group of a long-chain fatty acid forms an ester bond with the hydroxyl group of a long-chain alcohol. This structure creates a highly nonpolar molecule with strong water-repellent properties. Unlike fats and oils (triacylglycerols), waxes are not based on glycerol carrying three fatty acids. Their long hydrocarbon segments explain why waxes are solid, protective, and resistant to water penetration. This definition is frequently tested to distinguish waxes from other lipid classes. Hence, waxes are long-chain fatty acid esters of long-chain alcohols.
188. Oils are more prone to oxidative rancidity mainly because:
ⓐ. They contain many peptide bonds that break easily
ⓑ. They are polymers that depolymerize in air
ⓒ. They contain only saturated fatty acids with no reactive sites
ⓓ. Their unsaturated chains have reactive regions near $C=C$
Correct Answer: Their unsaturated chains have reactive regions near $C=C$
Explanation: Oxidative rancidity commonly involves reactions at or near $C=C$ double bonds in unsaturated fatty acid chains. These regions are more chemically reactive toward oxygen, leading to formation of peroxides and subsequent breakdown products that produce unpleasant odors and flavors. Oils generally have more unsaturation than solid fats, so they present more such reactive sites. This is why exposure to air, light, and heat can accelerate rancidity in many oils. The explanation depends on bond chemistry, not on peptide bonds or polymer breakdown. Therefore, oils are more prone to oxidative rancidity because unsaturated chains have reactive regions near $C=C$.
189. Soap helps wash oily stains mainly because soap molecules:
ⓐ. Convert oils into amino acids during washing
ⓑ. Break peptide bonds present in oils
ⓒ. Have a hydrophobic tail and hydrophilic head that form micelles
ⓓ. Are sugars that dissolve oils by glycosidic bonding
Correct Answer: Have a hydrophobic tail and hydrophilic head that form micelles
Explanation: Soap molecules are amphipathic, meaning they possess a nonpolar (hydrophobic) tail and a polar/ionic (hydrophilic) head. The hydrophobic tails embed into oil droplets while the hydrophilic heads remain in water, allowing the formation of micelles that suspend oil in water. This emulsification lets oily dirt be carried away during rinsing. The mechanism is based on molecular structure and polarity, not on breaking peptide bonds or converting oils chemically into amino acids. Because oils are nonpolar, the hydrophobic tail interaction is essential for trapping them. Hence, soap cleans oils by forming micelles using hydrophobic tails and hydrophilic heads.
190. Which statement correctly differentiates oils from waxes?
ⓐ. Oils are triacylglycerols; waxes are fatty acid–alcohol esters
ⓑ. Oils are amino acid chains; waxes are nucleotide chains
ⓒ. Oils are polysaccharides; waxes are proteins
ⓓ. Oils are phosphates only; waxes are sugars only
Correct Answer: Oils are triacylglycerols; waxes are fatty acid–alcohol esters
Explanation: Oils are mainly triacylglycerols, meaning glycerol is esterified with three fatty acids to form a neutral fat that is typically liquid due to higher unsaturation. Waxes, in contrast, are formed by esterification of long-chain fatty acids with long-chain alcohols, producing strongly hydrophobic protective substances. This structural distinction explains why oils function largely in energy storage, while waxes commonly serve as waterproofing and protective coatings. The key is the backbone and ester-partner: glycerol with three fatty acids for oils versus fatty acid plus long-chain alcohol for waxes. This is a standard biomolecules classification point. Therefore, oils are triacylglycerols, while waxes are fatty acid–alcohol esters.
191. Phospholipids are considered lipids but “not true polymers” mainly because they:
ⓐ. Are always made of repeating monomers in long chains
ⓑ. Lack a universal repeating monomer-chain pattern
ⓒ. Are built from nucleotides by phosphodiester bonds
ⓓ. Are formed only by peptide bond repetition
Correct Answer: Lack a universal repeating monomer-chain pattern
Explanation: True polymers are typically long chains made by repeating a common monomer unit, such as amino acids in proteins or nucleotides in nucleic acids. Phospholipids, like other lipids, do not form such monomer-repeat chains; they are assembled from a few components into a single molecule. A typical phospholipid has a glycerol backbone linked to fatty acids and a phosphate-containing head, not a long repeating sequence. Because the group “lipids” includes diverse structures rather than one repeating building-block architecture, they are described as not true polymers. This conceptual point is about structure classification, not about whether phospholipids are important biomolecules. Hence, phospholipids lack a universal repeating monomer-chain pattern.
192. A typical phospholipid molecule is best described as having:
ⓐ. Only polar groups with no hydrocarbon chains
ⓑ. Two polar heads and one nonpolar tail
ⓒ. Only nonpolar tails with no head region
ⓓ. One polar head and two nonpolar tails
Correct Answer: One polar head and two nonpolar tails
Explanation: Most common membrane phospholipids have a glycerol backbone esterified to two fatty acids, creating two hydrophobic tails. The third glycerol position is linked to a phosphate group that carries or connects to a polar head group, making that region hydrophilic. This gives the molecule amphipathic character: part interacts with water, part avoids it. The two-tail design is important for forming stable bilayers, because tails pack inward while heads face the aqueous environment. This structural arrangement is a core concept explaining membrane formation and behavior. Therefore, a typical phospholipid has one polar head and two nonpolar tails.
193. The amphipathic nature of phospholipids is mainly due to:
ⓐ. Presence of peptide bonds and glycosidic bonds together
ⓑ. A charged/polar head and hydrophobic fatty acid tails
ⓒ. Repeating glucose units with many $-OH$ groups
ⓓ. A polymeric chain of amino acids with R groups
Correct Answer: A charged/polar head and hydrophobic fatty acid tails
Explanation: Phospholipids contain two chemically distinct regions: a polar head that often carries charge due to phosphate and associated groups, and nonpolar hydrocarbon tails from fatty acids. Because water interacts well with polar/charged groups but poorly with nonpolar chains, the molecule behaves as amphipathic. This dual nature drives self-assembly into bilayers and vesicles, which is essential for membrane structure. The head group stabilizes interaction with aqueous surroundings, while the tails cluster away from water to minimize unfavorable interactions. This is a direct structure-to-property relationship tested frequently in biomolecules. Hence, amphipathic behavior arises from a polar head and hydrophobic tails.
194. In aqueous solution, phospholipids most spontaneously form:
ⓐ. A bilayer with tails inward and heads outward
ⓑ. A single straight chain like cellulose
ⓒ. A crystalline lattice of amino acids
ⓓ. A sugar polymer with branched points
Correct Answer: A bilayer with tails inward and heads outward
Explanation: Because phospholipids are amphipathic, they arrange themselves to satisfy both water-loving and water-avoiding parts. The hydrophobic tails cluster away from water, while polar heads remain exposed to water, leading to formation of a bilayer. In this structure, tails face each other inside the membrane, and heads face the aqueous environment on both sides. This self-assembly lowers free energy by reducing tail–water contact and maximizing head–water interactions. The bilayer is the fundamental structural basis of biological membranes. Therefore, phospholipids spontaneously form bilayers with tails inward and heads outward.
195. The major role of phospholipids in cells is to:
ⓐ. Store genetic information in nuclei
ⓑ. Catalyze most metabolic reactions as enzymes
ⓒ. Form the basic framework of membranes
ⓓ. Build cellulose microfibrils in cell walls
Correct Answer: Form the basic framework of membranes
Explanation: Phospholipids are the principal structural components of biological membranes because they can form stable bilayers in water. This bilayer provides a selective barrier separating internal cellular compartments from the external environment. The amphipathic design gives membranes both stability and fluidity, allowing proteins to embed and function. Membrane formation is not the role of cellulose (structural polysaccharide) or nucleic acids (information storage). The key biological significance of phospholipids is therefore architectural and compartment-forming. Hence, phospholipids form the basic framework of membranes.
196. Compared with triacylglycerols, phospholipids differ mainly because phospholipids:
ⓐ. Contain a phosphate group and polar head region
ⓑ. Have three fatty acids and no other groups
ⓒ. Have no fatty acid chains at all
ⓓ. Are polymers of repeating monomers
Correct Answer: Contain a phosphate group and polar head region
Explanation: Triacylglycerols (fats/oils) are glycerol esterified with three fatty acids and are largely nonpolar storage molecules. Phospholipids typically have two fatty acids plus a phosphate-containing head group, creating a polar region. This head group makes phospholipids amphipathic and suitable for membrane formation, unlike triacylglycerols which form fat droplets for energy storage. The structural substitution of a phosphate-based head in place of a third fatty acid is the key distinguishing feature. This difference drives very different biological roles and physical behaviors. Therefore, phospholipids differ mainly by having a phosphate group and polar head.
197. The “head” of a phospholipid is hydrophilic mainly because it:
ⓐ. Is rich in hydrocarbon $C-H$ bonds only
ⓑ. Is built only from repeating sugars
ⓒ. Is made of long nonpolar alcohols only
ⓓ. Contains phosphate that can carry charge
Correct Answer: Contains phosphate that can carry charge
Explanation: The phosphate group in phospholipids is strongly polar and often ionized, so it interacts favorably with water. In addition, many phospholipids have further polar head groups attached to phosphate, increasing hydrophilicity. This polarity allows the head region to remain in contact with aqueous surroundings, stabilizing membrane surfaces. The contrast with the hydrophobic tails creates amphipathic behavior essential for bilayer formation. The hydrophilic nature is therefore linked to charge and polarity, not to nonpolar hydrocarbon content. Hence, the head is hydrophilic because it contains phosphate that can carry charge.
198. Membrane fluidity is strongly influenced by phospholipid tails because:
ⓐ. Tails are made of nucleotides that pair
ⓑ. Tails contain peptide bonds that break easily
ⓒ. Tail saturation and length affect packing
ⓓ. Tails are sugars that form hydrogen-bond networks
Correct Answer: Tail saturation and length affect packing
Explanation: Phospholipid tails are fatty acid chains, and their ability to pack together determines how rigid or fluid a membrane is. Unsaturated tails with $C=C$ bonds introduce bends that reduce packing efficiency, increasing fluidity. Longer and more saturated tails pack more tightly, lowering fluidity and raising the melting tendency. These structural effects alter membrane permeability and the mobility of embedded proteins. Thus, tail chemistry is a central factor controlling membrane physical properties. Therefore, membrane fluidity is strongly influenced because tail saturation and length affect packing.
199. A key exam-focused statement about phospholipids is that they:
ⓐ. Are the main energy reserve in adipose tissue
ⓑ. Are primarily structural molecules in membranes
ⓒ. Are polymers built by repeating monomers
ⓓ. Are stored as crystalline fibers in cell walls
Correct Answer: Are primarily structural molecules in membranes
Explanation: Phospholipids are designed for membrane architecture because they self-assemble into bilayers due to amphipathic structure. This makes them the primary structural foundation of cell and organelle membranes, providing compartment boundaries and a platform for membrane proteins. While they can be metabolized, their hallmark role is structural rather than long-term energy storage, which is mainly triacylglycerol function. The concept is tested to distinguish “membrane lipids” from “storage fats.” Their structure fits bilayer formation more than dense energy storage. Hence, phospholipids are primarily structural molecules in membranes.
200. The best reason phospholipids can form a stable bilayer is that they:
ⓐ. Have a polar head that avoids water and tails that attract water
ⓑ. Have only hydrophobic parts and no polar region
ⓒ. Are amphipathic with heads interacting with water and tails avoiding water
ⓓ. Are polymers that crystallize into sheets like cellulose
Correct Answer: Are amphipathic with heads interacting with water and tails avoiding water
Explanation: A stable bilayer forms when phospholipids arrange so that their hydrophilic heads face water while their hydrophobic tails are shielded from water. This arrangement minimizes unfavorable tail–water contact and maximizes favorable head–water interactions, lowering free energy. Because each molecule contains both polar and nonpolar regions, large numbers of molecules can self-organize into extended bilayers. The same principle explains formation of vesicles and membrane dynamics in cells. This stability is not due to polymer-like repetition but due to amphipathic geometry and polarity. Therefore, phospholipids form stable bilayers because they are amphipathic with heads interacting with water and tails avoiding water.