All Free Biology MCQs with Answers

411. Anaerobic respiration releases much less energy per glucose than aerobic respiration because

• A. glucose is only partially oxidised, leaving energy locked in products such as lactate
• B. glycolysis does not occur without oxygen
• C. oxygen directly destroys ATP molecules
• D. the enzymes of respiration denature without oxygen

Explanation: Without oxygen as the final acceptor, the Krebs cycle and electron transport chain stop, so products such as lactate or ethanol still hold most of glucose's energy. Glycolysis itself runs normally, which is why a small yield of 2 ATP is still obtained.

412. For each molecule of glucose completely respired, the total number of carbon dioxide molecules released is

• A. 2
• B. 4
• C. 6
• D. 8

Explanation: Two CO2 come from the oxidative decarboxylation of the two pyruvate molecules and four from the two turns of the Krebs cycle, giving six in all. Glycolysis releases none, which is why the link reaction and Krebs account for every CO2 breathed out.

413. The energy released as electrons pass along the electron transport chain is used directly to

• A. split water molecules
• B. pump hydrogen ions across the inner mitochondrial membrane
• C. reduce carbon dioxide
• D. phosphorylate glucose

Explanation: Three carrier complexes use the energy of falling electrons to pump H+ from matrix to intermembrane space, building the gradient that ATP synthase later harnesses. The electrons themselves finally reduce oxygen to water.

414. The net production of ATP in glycolysis is two molecules per glucose because

• A. 2 ATP are invested early and 4 ATP are produced later
• B. 4 ATP are invested and 2 ATP are produced
• C. only two of glucose's six carbons can be oxidised
• D. glycolysis occurs twice per glucose molecule

Explanation: Two ATP prime the pathway by phosphorylating glucose and its split products, and four ATP are later formed by substrate level phosphorylation, leaving a net gain of two. This investment phase is why glycolysis needs a small supply of ATP to begin with.

415. One turn of the Krebs cycle, starting from acetyl CoA, directly yields

• A. 36 ATP
• B. 2 ATP and 2 CO2
• C. 6 NADH
• D. 1 ATP, 3 NADH, 1 FADH2 and 2 CO2

Explanation: Per acetyl group the cycle turns out one ATP by substrate level phosphorylation, three reduced NAD, one reduced FAD and two CO2. Since each glucose supplies two acetyl groups, these figures double per glucose, a step examiners often expect you to make.

416. The process by which small molecules join together to build a macromolecule, with water released at each link, is called

• A. condensation
• B. hydrolysis
• C. oxidation
• D. photolysis

Explanation: In condensation, an OH from one molecule and an H from the other leave as water while a covalent bond forms, which is how disaccharides, polypeptides and triglycerides are built. Hydrolysis is the reverse: it uses water to break these bonds during digestion.

417. Which element is always present in proteins but never in simple carbohydrates or triglycerides?

• A. Phosphorus
• B. Nitrogen
• C. Magnesium
• D. Calcium

Explanation: Every amino acid carries an amino group, so nitrogen is a constant feature of proteins, and sulphur also appears in some. Simple carbohydrates and triglycerides contain only carbon, hydrogen and oxygen, and phosphorus is confined to compounds such as phospholipids and nucleic acids.

418. Which class of biological molecule is generally NOT built as a polymer of repeating monomers?

• A. Proteins
• B. Nucleic acids
• C. Lipids
• D. Carbohydrates

Explanation: Lipids are small molecules, a triglyceride being glycerol plus three fatty acids, and they do not form long repeating chains. Proteins, nucleic acids and polysaccharides are all true polymers assembled from amino acids, nucleotides and monosaccharides respectively.

419. The bond that links two monosaccharides in a disaccharide is a

• A. peptide bond
• B. ester bond
• C. phosphodiester bond
• D. glycosidic bond

Explanation: A glycosidic bond forms by condensation between the hydroxyl groups of two sugars. A peptide bond joins amino acids, an ester bond joins a fatty acid to glycerol, and a phosphodiester bond links nucleotides in nucleic acids, so each bond name points to its own family of molecules.

420. The high specific heat capacity of water benefits living organisms mainly because it

• A. makes water an excellent solvent for ions
• B. allows cell temperature to fluctuate rapidly with the surroundings
• C. keeps the temperature of cells and of water bodies relatively stable
• D. speeds up every enzyme controlled reaction

Explanation: A large amount of heat is needed to raise the temperature of water because much of the energy first breaks hydrogen bonds between molecules. This buffers cells and coastal climates against sudden change. It is a different property from solvent power, which comes from polarity.