Free Biological Molecules MCQs with Answers

1. Which property of water allows it to act as a temperature buffer for living cells?

• A. Its low surface tension
• B. Its high specific heat capacity
• C. Its low density in the liquid state
• D. Its inability to form hydrogen bonds

Explanation: Water has an unusually high specific heat capacity because a large amount of energy is absorbed breaking hydrogen bonds before the temperature rises. A cell can therefore take in or lose considerable heat with only a small change in internal temperature, which protects enzymes from denaturation. Surface tension in water is high, not low, and water is denser as a liquid than as ice, so the other options invert real properties.

2. Water is described as a universal solvent primarily because its molecules are

• A. non-polar
• B. polar
• C. ionised at all temperatures
• D. held together by covalent bonds between molecules

Explanation: The oxygen atom pulls the shared electrons away from the two hydrogens, leaving a partial negative charge on the oxygen and partial positive charges on the hydrogens. This polarity lets water surround and separate ions and other polar solutes, which is why almost every reaction in a cell happens in aqueous solution. The bonds between water molecules are hydrogen bonds, not covalent bonds.

3. Sucrose is formed from glucose and fructose by which type of reaction?

• A. Hydrolysis
• B. Condensation
• C. Oxidation
• D. Neutralisation

Explanation: A condensation reaction joins the two monosaccharides through a glycosidic linkage and releases one molecule of water. Hydrolysis is the reverse process and would split sucrose back into glucose and fructose, which is what sucrase does in the small intestine. No change in oxidation state and no acid-base pairing is involved.

4. Cellulose and starch are both polymers of glucose, yet humans can digest only starch. The reason is that

• A. cellulose contains beta 1,4 linkages that human enzymes cannot hydrolyse
• B. cellulose molecules are too large to enter the gut
• C. cellulose is a protein rather than a carbohydrate
• D. starch contains no glycosidic bonds at all

Explanation: Starch is built from alpha glucose and its alpha 1,4 linkages are hydrolysed by amylase, while cellulose is built from beta glucose and its beta 1,4 linkages need cellulase, an enzyme humans do not produce. The polymer size is not the obstacle, since starch molecules are also very large. Both are carbohydrates and both contain glycosidic bonds.

5. Which of the following is a reducing sugar?

• A. Sucrose
• B. Starch
• C. Maltose
• D. Cellulose

Explanation: Maltose keeps a free anomeric carbon, so its open-chain form has an aldehyde group that reduces Benedict's reagent to a brick red precipitate. In sucrose both anomeric carbons are locked into the glycosidic bond, which is why it is the classic non-reducing disaccharide. Starch and cellulose are polysaccharides with only one free end in an enormous molecule, so no detectable reduction occurs.

6. The bond that joins two amino acids in a polypeptide chain forms between

• A. two amino groups
• B. the carboxyl group of one amino acid and the amino group of the next
• C. two R groups
• D. the carboxyl group of one amino acid and the R group of the next

Explanation: A peptide bond is a condensation between the carboxyl group of one amino acid and the alpha amino group of the next, releasing water. The R groups stay free and determine how the chain folds and what the protein does. Interactions between R groups matter at the tertiary level, but they are not peptide bonds.

7. The alpha helix and beta pleated sheet of a protein are examples of its

• A. primary structure
• B. secondary structure
• C. tertiary structure
• D. quaternary structure

Explanation: Secondary structure is the regular local folding held together by hydrogen bonds between the backbone carbonyl and amide groups, giving the alpha helix and the beta pleated sheet. Primary structure is only the sequence of amino acids. Tertiary structure is the overall three-dimensional fold of one chain, and quaternary structure appears when two or more chains associate, as in haemoglobin.

8. Haemoglobin contains four polypeptide chains. This makes it an example of a protein with

• A. primary structure only
• B. secondary structure only
• C. tertiary structure only
• D. quaternary structure

Explanation: Quaternary structure exists whenever a functional protein is built from more than one polypeptide chain. Haemoglobin has two alpha and two beta chains, each holding a haem group, and their cooperative interaction is what produces the sigmoid oxygen dissociation curve. A single-chain protein such as myoglobin stops at tertiary structure.

9. Compared with carbohydrates, lipids yield more energy per gram because they

• A. contain more oxygen atoms per carbon
• B. are more highly reduced, with more carbon-hydrogen bonds per gram
• C. are soluble in water
• D. are broken down without any oxygen

Explanation: Energy is released when carbon-hydrogen bonds are oxidised, and a fatty acid chain is almost entirely carbon and hydrogen with very little oxygen already attached. Carbohydrates are partly oxidised to begin with, so less energy remains to be released, which is why fat yields about 9 kcal per gram against roughly 4 for carbohydrate. Lipids are insoluble in water and their breakdown is fully aerobic.

10. A phospholipid molecule is described as amphipathic because it has

• A. two hydrophobic ends
• B. a hydrophilic head and hydrophobic tails
• C. two hydrophilic ends
• D. no charged groups at all

Explanation: The phosphate head is charged and interacts with water, while the two fatty acid tails are non-polar and are excluded from it. Placed in water, the molecules therefore arrange themselves into a bilayer with the tails facing inward, which is the basis of every cell membrane. A molecule with only one kind of end could not form that structure.