All Free Biology MCQs with Answers

• A. Isocitric acid → Oxalosuccinic acid → α-Ketoglutaric acid
• B. Oxalosuccinic acid → Isocitric acid → α-Ketoglutaric acid
• C. α-Ketoglutaric acid → Isocitric acid → Oxalosuccinic acid
• D. Isocitric acid → α-Ketoglutaric acid → Succinic acid

Explanation: The correct sequence in the Krebs cycle is: Isocitric acid → α-Ketoglutaric acid → Succinic acid. During the cycle, isocitric acid undergoes oxidation and decarboxylation to form oxalosuccinic acid, a transient intermediate that quickly converts to α-ketoglutaric acid. α-Ketoglutaric acid then experiences oxidative decarboxylation, leading to the formation of succinyl-CoA, which is subsequently converted to succinic acid. The other options are incorrect as they either misorder the intermediates or inaccurately describe the chemical transformations that occur between these compounds.

• A. Light absorption
• B. Consumption of carbon dioxide
• C. Light emission
• D. Both A and B

Explanation: An action spectrum typically illustrates the relative effectiveness of different wavelengths of light in driving a specific biological process, such as photosynthesis. It shows peaks at wavelengths where absorption is most effective. However, it does not directly indicate the consumption of carbon dioxide; rather, it shows how various wavelengths of light contribute to processes that can lead to carbon dioxide consumption. Therefore, the correct answer is 'Consumption of carbon dioxide.' Option A ('Light absorption') is incorrect because the action spectrum inherently represents light absorption. Option C ('Light emission') is also incorrect as the spectrum does not measure light emission. Option D is incorrect because it incorrectly groups 'Light absorption' as not being represented, which it is.

• A. Presence of enzyme in high concentration
• B. Absence of Rubisco
• C. PEP carboxylase being strictly carboxylase
• D. High yield of enzyme

Explanation: PEP carboxylase is an enzyme that plays a crucial role in the C4 photosynthetic pathway. It fixes CO2 into a 4-carbon compound, oxaloacetate, in the mesophyll cells of C4 plants. This enzyme is highly selective for CO2 and does not react with O2, which prevents oxygenation reactions and photorespiration. By avoiding photorespiration, C4 plants are able to conserve energy and maintain high photosynthetic efficiency, even in hot and dry environments.

• A. derived from vitamins.
• B. composed of amino acids.
• C. loosely attached to enzymes.
• D. covalently attached to enzymes.

Explanation: Prosthetic groups are non-protein molecules that are covalently attached to an enzyme, making them a permanent part of the enzyme's structure. This is in contrast to coenzymes, which are usually loosely bound and can dissociate from the enzyme. Option A is incorrect because prosthetic groups are not typically derived from vitamins, though some coenzymes are. Option B is incorrect because prosthetic groups are not composed of amino acids; they are usually small organic molecules or metal ions. Option C is incorrect because it describes coenzymes, which are often loosely attached and can easily bind and release from the enzyme.

• A. Lyases
• B. Ligases
• C. Isomerases
• D. Transferases

Explanation: Lyases are a class of enzymes that catalyze the non-hydrolytic removal or addition of groups to substrates, often resulting in the formation or breaking of double bonds. They are distinct from other enzyme classes because they do not require water to break bonds, unlike hydrolases. Instead, they facilitate the conversion of molecules by removing functional groups, such as carbon dioxide or ammonia, without the need for hydrolysis. e.g: Histidine decarboxylase breaks the covalent bond between carbon atoms in histidine forming carbon dioxide and histamine.Ligases, on the other hand, are involved in forming new bonds between molecules, typically requiring energy input from ATP. Isomerases catalyze the rearrangement of atoms within a molecule, altering its geometric or structural form. Transferases are responsible for transferring functional groups between molecules, such as methyl or phosphate groups.Therefore, the correct answer is lyases, as they specifically match the description of enzymes that catalyze the non-hydrolytic removal of a chemical group from a substrate.

• A. Lyases
• B. Ligases
• C. Hydrolases
• D. Isomerases

Explanation: The correct answer is ligases. RNA polymerase catalyzes the formation of phosphodiester bonds between RNA nucleotides, which is characteristic of ligase activity. Ligases are involved in joining two molecules together, making them the appropriate class for RNA polymerase, which links nucleotides together during transcription.Lyases break bonds without hydrolysis or oxidation, which is not the function of RNA polymerase. Hydrolases use water to break bonds, which does not occur in the synthesis of RNA strands. Isomerases rearrange molecules to form isomers without adding or removing atoms, a process unrelated to the action of RNA polymerase during transcription.

• A. oxidoreductases
• B. Hydrolases
• C. Transferases
• D. lyases

Explanation: These enzymes catalyze the transfer of specific functional group other than hydrogen from one substrate to another. The chemical group transferred in the process is not in a free state.

• A. Substrate
• B. Coenzyme
• C. Apoenzyme
• D. Holoenzyme

Explanation: An activated enzyme made of a polypeptide chain and a cofactor is referred to as a "holoenzyme." The holoenzyme is the complete, catalytically active form of an enzyme. The holoenzyme structure ensures that the enzyme is in its active conformation and is capable of facilitating the desired biochemical reactions.

• A. Isoenzyme
• B. Coenzyme
• C. Holoenzyme
• D. Apoenzyme

Explanation: The correct term for an organic substance bound to an enzyme and essential for its activity is a "coenzyme." A coenzyme is a non-protein organic molecule that binds to an enzyme and participates in the catalytic reaction, assisting the enzyme in carrying out its function. Coenzymes are often derived from vitamins or other small organic molecules. They work by transferring chemical groups or electrons between enzymes and their substrates, allowing the enzyme to perform its specific role in a biochemical reaction. Examples of coenzymes include NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), both derived from vitamin B3, and coenzyme A, derived from pantothenic acid (vitamin B5). These coenzymes play crucial roles in various metabolic pathways by accepting or donating electrons or chemical groups during enzymatic reactions.

• A. Coenzyme
• B. Prosthetic group
• C. Holoenzyme
• D. Activator

Explanation: The detachable cofactor is known as an activator if it is an inorganic ion. If the non-protein part is covalently bonded, it is known as a prosthetic group. If it is loosely attached to the protein part, it is known as a coenzyme. The enzyme with its cofactor is called holoenzyme.