All Free Chemistry MCQs with Answers

351. The complete hydrolysis of a protein produces

• A. glucose molecules
• B. fatty acids and glycerol
• C. a mixture of amino acids
• D. urea and ammonia

Explanation: Every peptide bond is broken by boiling with concentrated acid or by protease enzymes, releasing the constituent amino acids, which can then be separated by chromatography or electrophoresis. Glucose comes from the hydrolysis of carbohydrates and fatty acids with glycerol from fats. Urea is a later metabolic product formed when surplus amino acids are deaminated in the liver.

352. Which of the following is NOT a function of proteins in the body?

• A. Acting as enzymes
• B. Transporting oxygen
• C. Providing antibodies for defence
• D. Serving as the main long term energy store

Explanation: The body stores energy as fat and, in the short term, as glycogen, and proteins are broken down for energy only when those stores are exhausted, since doing so sacrifices functional tissue. Enzymes, haemoglobin and antibodies are all proteins, as are hormones such as insulin and structural materials such as collagen. This breadth of function is why protein is an essential part of the diet.

353. The biuret test for proteins gives

• A. a violet colour with copper two ions in alkaline solution
• B. a brick red precipitate
• C. a silver mirror
• D. a blue black colour

Explanation: Copper two ions form a violet complex with the peptide linkages, so the test detects the presence of peptide bonds rather than any particular amino acid and requires at least two of them. A blue black colour with iodine is the test for starch and a brick red precipitate with Fehling's solution indicates a reducing sugar. The ninhydrin test is the alternative for free amino acids.

354. Enzymes are described as biological catalysts because they

• A. are used up in the reactions they catalyse
• B. raise the activation energy of a reaction
• C. supply energy to drive unfavourable reactions
• D. speed up reactions by lowering the activation energy and are recovered unchanged

Explanation: An enzyme provides an alternative pathway with a smaller energy barrier, so a far larger proportion of collisions are effective at body temperature, and it emerges from each cycle ready to work again. It cannot make an energetically unfavourable reaction happen, only accelerate one that is already possible. A single enzyme molecule may convert thousands of substrate molecules per second.

355. The high specificity of an enzyme for its substrate is due to

• A. the shape and chemistry of its active site
• B. its molar mass
• C. the temperature of the reaction
• D. the concentration of the substrate

Explanation: Only a substrate whose shape and charge distribution complement the active site can bind, which is why sucrase acts on sucrose but not on the closely related maltose. The induced fit model refines this picture by allowing the site to mould itself around the substrate as binding occurs. Specificity is a structural property and is unaffected by temperature or concentration.

356. Enzyme activity is lost above about 45 degrees Celsius because

• A. the substrate evaporates
• B. the enzyme is denatured as the weak bonds holding its tertiary structure break
• C. the enzyme is used up faster
• D. the activation energy increases

Explanation: Heat disrupts the hydrogen and ionic interactions that hold the chain in shape, so the active site loses its precise geometry and can no longer bind the substrate, and the loss is usually permanent. Below the optimum, raising the temperature increases the rate in the normal way, which is why the curve rises to a peak and then falls sharply. The peak for human enzymes lies near 37 degrees.

357. A competitive inhibitor reduces the rate of an enzyme catalysed reaction by

• A. denaturing the enzyme permanently
• B. binding an allosteric site
• C. occupying the active site because it resembles the substrate
• D. lowering the temperature

Explanation: Substrate and inhibitor compete for the same site, so the inhibition can be overcome by increasing the substrate concentration and the maximum rate is eventually still reached. A non competitive inhibitor binds elsewhere, changes the shape of the enzyme and cannot be outcompeted, so it genuinely lowers the maximum rate. Many drugs are designed as competitive inhibitors of a specific enzyme.

358. Which pair correctly matches a macromolecule with its monomer?

• A. Protein and glucose
• B. Starch and amino acid
• C. Nucleic acid and fatty acid
• D. Protein and amino acid

Explanation: Proteins are polymers of amino acids, starch and cellulose of glucose, and nucleic acids of nucleotides, while fats are not true polymers at all but esters of glycerol and fatty acids. Matching each macromolecule to its monomer and its linkage is the core of this topic. All these polymers are built by condensation and broken by hydrolysis.

359. The quaternary structure of a protein exists only when the molecule

• A. contains a disulphide bridge
• B. is fibrous
• C. consists of two or more polypeptide chains associated together
• D. contains more than one hundred amino acids

Explanation: Quaternary structure describes how separate polypeptide subunits fit together, as in haemoglobin with its two alpha and two beta chains, and a single chain protein such as myoglobin therefore stops at the tertiary level. The subunits are held by the same weak interactions that stabilise tertiary structure. Chain length alone has nothing to do with it.

360. Conjugated proteins differ from simple proteins in that they contain

• A. only amino acids
• B. a non protein prosthetic group in addition to the polypeptide
• C. no peptide bonds
• D. two identical chains

Explanation: A conjugated protein is bound to a non protein group, so haemoglobin carries a haem group, glycoproteins carry carbohydrate and lipoproteins carry lipid, and in each case the prosthetic group is essential to the function. Simple proteins yield only amino acids on hydrolysis. Removing the prosthetic group leaves the protein inactive.