All Free Biology MCQs with Answers

301. Which of the following is NOT a macromolecule?

• A. Starch
• B. Glycogen
• C. Glucose
• D. Cellulose

Explanation: Glucose is a monosaccharide, a single small unit and therefore a micromolecule, while the other three are polysaccharides built from thousands of glucose units joined by glycosidic bonds. Macromolecules are polymers assembled from repeating monomers by condensation, so the test is whether the molecule can be hydrolysed into smaller repeating units. Glucose cannot.

302. The four elements that together make up about 99 per cent of the mass of living matter are

• A. carbon, hydrogen, oxygen and nitrogen
• B. carbon, hydrogen, oxygen and sulphur
• C. sodium, potassium, calcium and chlorine
• D. carbon, nitrogen, phosphorus and sulphur

Explanation: Carbon, hydrogen, oxygen and nitrogen are the backbone of carbohydrates, lipids, proteins and nucleic acids, with phosphorus and sulphur adding roughly another one per cent. Sodium, potassium and calcium are essential but are needed only in small quantities as ions. Carbon is central because it forms four stable covalent bonds and so builds long chains and rings.

303. The monomer unit from which a nucleic acid is built is

• A. an amino acid
• B. a monosaccharide
• C. a nucleotide
• D. a fatty acid

Explanation: Each nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base, and successive nucleotides are linked by phosphodiester bonds between the sugar of one and the phosphate of the next. Amino acids are the monomers of proteins and monosaccharides those of polysaccharides. Fatty acids are components of lipids, which are not true polymers at all.

304. Water makes up approximately what proportion of the fresh mass of a typical living cell?

• A. about 10 per cent
• B. about 30 per cent
• C. about 50 per cent
• D. 70 to 90 per cent

Explanation: Most cells are between seventy and ninety per cent water, which is why it is treated as the medium of life rather than as one nutrient among others. Every metabolic reaction occurs in aqueous solution, water is itself a reactant in hydrolysis and a product of condensation, and it transports dissolved substances. Seeds and spores are the low water exceptions, and their metabolism is correspondingly almost stopped.

305. 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.

306. 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.

307. 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.

308. 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.

309. 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.

310. 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.