All Free Biology MCQs with Answers

311. Essential amino acids are those that

• A. are found in every protein
• B. cannot be synthesised by the body and must be supplied in the diet
• C. are needed only during childhood
• D. make up the active site of every enzyme

Explanation: About nine of the twenty amino acids cannot be made by the human body at a useful rate, so they have to come from food, and a protein supplying all of them in good proportion is called a complete protein. Animal proteins are generally complete while most single plant proteins are not, which is why combinations such as rice with pulses matter in a vegetarian diet. All twenty are essential to the cell in the ordinary sense; the term refers only to dietary need.

312. The tertiary structure of a protein is held in shape by all of the following EXCEPT

• A. hydrogen bonds between R groups
• B. ionic bonds between charged R groups
• C. disulphide bridges between cysteine residues
• D. peptide bonds between R groups

Explanation: Peptide bonds join the amino group of one residue to the carboxyl group of the next along the backbone, so they build the primary structure and never form between R groups. The fold itself is maintained by the weaker hydrogen, ionic and hydrophobic interactions between side chains, reinforced by the covalent disulphide bridge where two cysteines meet. That is why gentle heating or a pH change can unfold a protein without breaking it apart.

313. An unsaturated fatty acid differs from a saturated one in that it

• A. contains one or more carbon to carbon double bonds in its hydrocarbon chain
• B. contains more hydrogen atoms per carbon
• C. has no carboxyl group
• D. cannot be used in respiration

Explanation: Each double bond puts a kink in the chain, so the molecules pack together less closely and the fat has a lower melting point, which is why plant oils are liquid while animal fats are solid at room temperature. Saturated means the chain carries the maximum possible hydrogen, so the unsaturated form has fewer hydrogens, not more. Diets high in saturated fat are linked with atherosclerosis.

314. Cholesterol is important in the human body because it

• A. is the main energy store of adipose tissue
• B. carries oxygen in the blood
• C. is the precursor of the steroid sex hormones, bile salts and vitamin D
• D. forms the walls of plant cells

Explanation: Cholesterol is a steroid and the starting material for testosterone, oestrogen, bile salts and vitamin D, and it also sits between the phospholipids of animal membranes and regulates their fluidity. Its bad reputation comes from the deposits it forms in artery walls when the blood level is high, not from any lack of function. Triglycerides, not cholesterol, are the energy store of adipose tissue.

315. 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.

316. 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.

317. 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.

318. 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.

319. 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.

320. In a chloroplast, the light dependent reactions take place in the

• A. stroma
• B. thylakoid membranes of the grana
• C. outer membrane
• D. intermembrane space

Explanation: Chlorophyll and the electron carriers are embedded in the thylakoid membranes, which are stacked into grana to give the largest possible area for absorbing light. The stroma is the surrounding fluid, and it is where the light independent reactions fix carbon dioxide using the ATP and reduced NADP made in the membranes. Dividing the organelle this way keeps the two stages physically separate.