Free Biological Molecules MCQs with Answers

21. Sweating cools the body efficiently because water has a high

• A. specific heat capacity
• B. latent heat of vaporisation
• C. surface tension
• D. density

Explanation: A large amount of energy is needed to break the hydrogen bonds holding liquid water together and convert it to vapour, and that energy is drawn from the skin, so a small volume of sweat removes a great deal of heat. High specific heat capacity is a different property, the one that lets a cell resist temperature change in the first place. Confusing the two is the usual error in this question.

22. Ice floats on water, and this is biologically important because

• A. the ice layer insulates the water below, letting aquatic organisms survive the winter
• B. it allows water to evaporate faster in cold weather
• C. it increases the amount of oxygen dissolved in the lake
• D. it lowers the freezing point of the remaining water

Explanation: Water is unusual in becoming less dense as it freezes, because the hydrogen bonds lock the molecules into an open lattice, so ice forms at the surface instead of sinking. The layer then insulates the water beneath, which stays liquid and allows fish and other organisms to survive. If ice sank, a pond would freeze solid from the bottom upwards and life in it would be destroyed.

23. The rise of water in the xylem as an unbroken column depends mainly on

• A. the low viscosity of water
• B. the high density of water
• C. cohesion between water molecules caused by hydrogen bonding
• D. the fact that water is a poor solvent for gases

Explanation: Hydrogen bonds make water molecules stick to one another, so a column pulled by evaporation at the leaves does not break, and adhesion to the walls of the narrow xylem vessels supports the column as well. The same cohesion gives water its high surface tension, which lets small insects rest on a pond. Viscosity and density play no part in maintaining the transpiration stream.

24. Glucose and fructose have the same molecular formula, C6H12O6, but different structures. They are therefore

• A. isomers
• B. polymers
• C. isotopes
• D. identical sugars

Explanation: Isomers share a molecular formula but differ in the arrangement of their atoms, and here glucose carries an aldehyde group while fructose carries a ketone group, which is why only glucose forms a six membered ring. The difference in structure gives a different taste and a different metabolic route despite the identical formula. Isotopes are forms of one element differing in neutron number.

25. Glycogen is a better short term storage molecule in animals than starch is in plants partly because glycogen

• A. contains no glycosidic bonds
• B. is soluble and diffuses freely out of the cell
• C. is built from beta glucose
• D. is far more branched, so many ends can be hydrolysed at once

Explanation: Extensive branching gives a large number of terminal glucose units, so enzymes can release glucose very rapidly when an active animal needs it. Being a large polymer it is also osmotically inactive and compact enough to store in liver and muscle. Starch is less branched and is adequate for a plant, which does not need sudden bursts of energy.

26. When a protein is denatured by heat

• A. its peptide bonds are hydrolysed and amino acids are released
• B. its tertiary structure is destroyed but its primary structure remains intact
• C. its amino acid sequence is rearranged
• D. it becomes more soluble and more active

Explanation: Heat breaks the relatively weak hydrogen, ionic and hydrophobic interactions that hold the chain in its three dimensional shape, so the protein unfolds and loses function, but the strong covalent peptide bonds of the backbone survive. This is why boiling an egg changes the albumen irreversibly without breaking it into amino acids. Denatured proteins usually become less soluble, which is why the egg white turns opaque.

27. 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.

28. 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.

29. 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.

30. 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.