301. Which statement best defines an essential amino acid?
ⓐ. An amino acid required from the diet
ⓑ. An amino acid that contains no \(NH_2\) group in its structure
ⓒ. An amino acid that is always absent from proteins
ⓓ. An amino acid that is made only by plants and never used by animals
Correct Answer: An amino acid required from the diet
Explanation: Essential amino acids are those that the human body cannot synthesise in sufficient quantity. They must therefore be supplied through food. Their importance is based on dietary requirement, not on absence from proteins. Non-essential amino acids can be made by the body in adequate amounts under normal conditions.
302. Which statement best describes a non-essential amino acid?
ⓐ. It is absent from every protein molecule.
ⓑ. It lacks the carboxyl group in all conditions.
ⓒ. The body can synthesise it in adequate amounts.
ⓓ. It is supplied only by carbohydrate digestion.
Correct Answer: The body can synthesise it in adequate amounts.
Explanation: A non-essential amino acid is not “unimportant”; the term only describes dietary requirement. Such amino acids can be produced by the body in sufficient quantities under normal conditions. They may still be important components of proteins and metabolism. Essential amino acids differ because they must be supplied through diet.
303. Which basis is used to classify amino acids as essential or non-essential?
ⓐ. Number of carbon atoms only
ⓑ. Requirement from diet
ⓒ. Colour of the crystalline solid
ⓓ. Presence of a glycosidic bond
Correct Answer: Requirement from diet
Explanation: Essential and non-essential classification depends on whether the body can synthesise the amino acid in sufficient quantity. If adequate synthesis is not possible, the amino acid must be obtained from the diet and is called essential. If the body can make enough of it, it is called non-essential. This classification is not based on colour, sweetness, or glycosidic linkage.
304. Which statement is the best correction of the claim “Non-essential amino acids are not needed by the body”?
ⓐ. It is actually a carbohydrate, so proteins cannot use it.
ⓑ. It cannot appear in proteins once the body makes it.
ⓒ. It contains no nitrogen, so it is outside protein chemistry.
ⓓ. It is needed, but the body can usually synthesise it.
Correct Answer: It is needed, but the body can usually synthesise it.
Explanation: The word “non-essential” can be misleading if read casually. It does not mean that the amino acid has no biological role. It means that the body can usually synthesise it in sufficient quantity. Such amino acids may still be required for protein synthesis and other biological processes.
305. Which amino acid type is expected when the side chain contains an extra carboxyl group?
ⓐ. Basic amino acid
ⓑ. Acidic amino acid
ⓒ. Neutral amino acid
ⓓ. Non-protein amino acid only
Correct Answer: Acidic amino acid
Explanation: Amino acids are classified by the nature of their side chains. If the side chain contains an extra carboxyl group, the amino acid has acidic character. This is because carboxyl groups can donate protons under suitable conditions. Basic amino acids usually contain extra amino groups or other basic nitrogen-containing groups.
306. Which amino acid type is expected when the side chain contains an extra basic amino group?
ⓐ. Acidic amino acid
ⓑ. Neutral amino acid
ⓒ. Reducing amino acid
ⓓ. Basic amino acid
Correct Answer: Basic amino acid
Explanation: A side chain containing an extra amino group gives the amino acid basic character. The amino group can accept a proton under suitable conditions. This separates basic amino acids from acidic amino acids, which have extra carboxyl groups. Neutral amino acids do not have strongly acidic or basic extra side-chain groups.
307. Which statement best describes neutral amino acids?
ⓐ. Their side chains lack extra acidic or basic groups.
ⓑ. Their molecules contain no \(NH_2\) or \(COOH\) group.
ⓒ. They are always polysaccharides made of glucose units.
ⓓ. They cannot form proteins under any condition.
Correct Answer: Their side chains lack extra acidic or basic groups.
Explanation: Neutral amino acids still contain the usual amino and carboxyl groups of amino acids. The word “neutral” mainly refers to the nature of the side chain. Their side chains do not add an extra strongly acidic or basic group. They can still participate in peptide bond formation and become part of proteins.
308. Which pair correctly matches side-chain feature with amino-acid class?
ⓐ. Extra \(COOH\) group — basic amino acid
ⓑ. Extra \(NH_2\) group — acidic amino acid
ⓒ. Extra \(COOH\) group — acidic amino acid
ⓓ. No side chain — nucleotide
Correct Answer: Extra \(COOH\) group — acidic amino acid
Explanation: An extra carboxyl group in the side chain increases acidic character. Therefore such amino acids are classified as acidic amino acids. Extra amino groups generally produce basic character instead. A nucleotide is a nucleic-acid unit and is not a side-chain class of amino acids.
309. Which statement correctly compares acidic, basic, and neutral amino acids?
ⓐ. Acidic amino acids contain only sugars, basic amino acids contain only bases, and neutral amino acids contain phosphate.
ⓑ. Acidic amino acids are always disaccharides, while basic amino acids are always polysaccharides.
ⓒ. Neutral amino acids cannot contain carbon, hydrogen, oxygen, or nitrogen.
ⓓ. Their classification depends mainly on the nature of the side chain.
Correct Answer: Their classification depends mainly on the nature of the side chain.
Explanation: Amino acids share a common backbone containing amino and carboxyl groups. The side chain \(R\) varies and largely determines whether an amino acid is treated as acidic, basic, or neutral. Extra carboxyl groups support acidic character, while extra amino groups support basic character. This classification is not based on carbohydrate or nucleotide identity.
310. Which structure best represents the zwitterionic form of a simple \(\alpha\)-amino acid?
ⓐ. \(^{+}H_3N-CH(R)-COO^{-}\)
ⓑ. \(H_3N^{+}-CH(R)-COOCH_3\)
ⓒ. \(H_2N-CH(R)-COO^{-}\)
ⓓ. \(HO-CH(R)-COO^{-}\)
Correct Answer: \(^{+}H_3N-CH(R)-COO^{-}\)
Explanation: In the zwitterionic form, an internal acid-base reaction occurs within the amino acid molecule. The carboxyl group donates a proton and becomes \(COO^{-}\). The amino group accepts that proton and becomes \(NH_3^{+}\). The molecule has both positive and negative charges but may have no net charge overall.
311. What internal change produces the zwitterion of an amino acid?
ⓐ. Transfer of phosphate from DNA to protein
ⓑ. Hydrolysis of a glycosidic linkage
ⓒ. Transfer of a proton from \(COOH\) to \(NH_2\)
ⓓ. Oxidation of glucose to gluconic acid
Correct Answer: Transfer of a proton from \(COOH\) to \(NH_2\)
Explanation: Amino acids contain both acidic and basic functional groups. The carboxyl group can donate a proton, while the amino group can accept a proton. This internal proton transfer forms \(NH_3^{+}\) and \(COO^{-}\) in the same molecule. The resulting dipolar ion is called a zwitterion.
312. Which property of amino acids is strongly explained by their zwitterionic nature?
ⓐ. Sweet taste like many monosaccharides
ⓑ. High melting point and water solubility
ⓒ. Non-reducing behaviour like sucrose
ⓓ. Formation of cellulose-like fibres in plants
Correct Answer: High melting point and water solubility
Explanation: Zwitterions contain both positive and negative charges within the same molecule. These charged centres lead to strong electrostatic interactions in the solid state, giving relatively high melting points. They also interact well with water, increasing water solubility for many amino acids. This behaviour differs from non-polar covalent molecules of similar size.
313. Which statement best describes amphoteric behaviour of amino acids?
ⓐ. They can react with both acids and bases.
ⓑ. They can only act as oxidising agents.
ⓒ. They always behave as non-reducing sugars.
ⓓ. They cannot form salts under any condition.
Correct Answer: They can react with both acids and bases.
Explanation: Amino acids are amphoteric because they contain both acidic and basic functional groups. The carboxyl group can behave as an acid, while the amino group can behave as a base. In acidic medium, the amino acid can accept a proton; in basic medium, it can lose a proton. This dual behaviour is a direct result of their functional-group structure.
314. In acidic solution, which change is most likely for the amino group of an amino acid?
ⓐ. It changes into a glycosidic bond.
ⓑ. It loses carbon dioxide immediately.
ⓒ. It becomes a ketone group.
ⓓ. It forms \(NH_3^{+}\).
Correct Answer: It forms \(NH_3^{+}\).
Explanation: In acidic solution, excess \(H^{+}\) ions are available. The amino group of an amino acid is basic and can accept a proton. This produces a positively charged \(NH_3^{+}\) group. The behaviour shows why amino acids can form salts with acids.
315. In basic solution, which change is most likely for the carboxyl group of an amino acid?
ⓐ. It forms a peptide bond without another amino acid.
ⓑ. It becomes a glycosidic linkage.
ⓒ. It loses a proton to form \(COO^{-}\).
ⓓ. It changes into a nitrogenous base.
Correct Answer: It loses a proton to form \(COO^{-}\).
Explanation: In basic medium, \(OH^{-}\) or another base can remove a proton from the carboxyl group. The carboxyl group then becomes the carboxylate ion, \(COO^{-}\). This acidic behaviour of the \(COOH\) group is one reason amino acids are amphoteric. The process is different from peptide bond formation, which requires reaction between two amino acids.
316. Which statement best describes the isoelectric point of an amino acid?
ⓐ. The temperature at which the amino acid melts
ⓑ. The \(pH\) at which it has no net charge
ⓒ. The mass of one mole of amino acid molecules
ⓓ. The number of peptide bonds in a protein
Correct Answer: The \(pH\) at which it has no net charge
Explanation: The isoelectric point is the \(pH\) at which an amino acid has no net electric charge. At this \(pH\), the positive and negative charges balance overall. The molecule may still exist largely as a zwitterion. This concept is important for understanding movement of amino acids in an electric field.
317. What happens to an amino acid at its isoelectric point in an electric field?
ⓐ. It always moves rapidly toward the anode.
ⓑ. It always moves rapidly toward the cathode.
ⓒ. It breaks completely into glucose and ammonia.
ⓓ. It shows minimum net migration.
Correct Answer: It shows minimum net migration.
Explanation: At the isoelectric point, the amino acid has no net charge overall. Since movement in an electric field depends on net charge, migration becomes minimal. The molecule may still contain internal positive and negative charges as a zwitterion. Minimum migration is therefore a key recognition feature of the isoelectric point.
318. Which assertion-reason pair is correctly evaluated?
Assertion: Amino acids can show amphoteric behaviour.
Reason: Amino acids contain both \(NH_2\) and \(COOH\) groups.
ⓐ. Both are true, and Reason explains Assertion.
ⓑ. Assertion is true, but Reason is false.
ⓒ. Assertion is false, but Reason is true.
ⓓ. Both are true, but Reason is unrelated to Assertion.
Correct Answer: Both are true, and Reason explains Assertion.
Explanation: The amino group can accept a proton and behave as a base. The carboxyl group can donate a proton and behave as an acid. Since both groups are present in amino acids, they can react with both acids and bases. The reason therefore explains the assertion.
319. Which statement best connects zwitterion formation and amphoteric behaviour?
ⓐ. Both require a glycosidic bond between two sugars.
ⓑ. Both come from acidic and basic groups in one molecule.
ⓒ. Both prove that amino acids are disaccharides.
ⓓ. Both occur only in nucleotides and not in amino acids.
Correct Answer: Both come from acidic and basic groups in one molecule.
Explanation: Amino acids contain an acidic \(COOH\) group and a basic \(NH_2\) group. These groups allow internal proton transfer to form a zwitterion. They also allow reaction with both acids and bases, producing amphoteric behaviour. Thus the same structural feature explains both properties.
320. Which statement is false about amino acid zwitterions?
ⓐ. They contain both positive and negative charges.
ⓑ. They help explain relatively high melting points of amino acids.
ⓒ. They are formed by joining two amino acids through \(-CO-NH-\).
ⓓ. They arise by internal proton transfer within an amino acid molecule.
Correct Answer: They are formed by joining two amino acids through \(-CO-NH-\).
Explanation: A zwitterion forms within a single amino acid molecule by internal proton transfer. The carboxyl group gives a proton to the amino group, forming \(COO^{-}\) and \(NH_3^{+}\). Joining two amino acids through \(-CO-NH-\) forms a peptide bond, not a zwitterion. Therefore zwitterion formation and peptide bond formation are different processes.
