Free Biological Molecules MCQs with Answers

31. A triglyceride is formed when glycerol combines with three fatty acids by

• A. hydrolysis, forming glycosidic bonds
• B. condensation, forming ester bonds and releasing three molecules of water
• C. oxidation, forming peptide bonds
• D. reduction, forming hydrogen bonds

Explanation: Each of the three hydroxyl groups of glycerol reacts with the carboxyl group of a fatty acid, so three ester bonds form and three water molecules are released. Digestion reverses this by hydrolysis, adding water back and freeing the fatty acids and glycerol for absorption. Glycosidic bonds join sugars and peptide bonds join amino acids, so neither term applies to a lipid.

32. The function of transfer RNA during protein synthesis is to

• A. carry the genetic message out of the nucleus
• B. form the structural core of the ribosome
• C. unwind the DNA double helix
• D. bring the correct amino acid to the ribosome according to the codon being read

Explanation: Each tRNA carries an anticodon at one end and its matching amino acid at the other, so it acts as the adaptor that translates the language of bases into the language of amino acids. Messenger RNA is the molecule that carries the message from the nucleus, and ribosomal RNA forms the structure of the ribosome, so all three types work together. Unwinding DNA is the job of helicase.

33. Messenger RNA differs from transfer RNA in that mRNA

• A. carries a copy of the base sequence of a gene from the nucleus to the ribosome
• B. is folded into a clover leaf shape
• C. carries an anticodon at one end
• D. is the most abundant type of RNA in the cell

Explanation: Messenger RNA is transcribed from one strand of a gene and is read in triplets by the ribosome, so its length varies with the gene it copies and it is usually short lived. The clover leaf shape and the anticodon belong to transfer RNA, and ribosomal RNA is the most abundant type by mass. Because mRNA is a working copy, the original DNA is never exposed to the risks of the cytoplasm.

34. The double helix structure of DNA was proposed in 1953 by

• A. Mendel and Morgan
• B. Griffith and Avery
• C. Watson and Crick, using the X ray diffraction data of Rosalind Franklin
• D. Sanger and Gilbert

Explanation: Watson and Crick built the model from Franklin's X ray photographs, which showed the helix and its dimensions, together with Chargaff's finding that A equals T and G equals C. The structure immediately suggested how the molecule could be copied, since each strand is a template for the other. Griffith and Avery had earlier shown that DNA is the transforming material, and Sanger later developed sequencing.

35. Haemoglobin is classified as a conjugated protein because it

• A. consists of four identical polypeptide chains
• B. contains only amino acids joined by peptide bonds
• C. is made in the bone marrow rather than the liver
• D. contains a non protein haem group attached to its polypeptide chains

Explanation: A conjugated protein is a protein permanently joined to a non protein prosthetic group, and in haemoglobin each of the four chains carries an iron containing haem that actually binds the oxygen. Without the haem the globin protein alone could carry nothing. Glycoproteins and lipoproteins are conjugated in the same sense, with a carbohydrate or lipid group instead.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.