Biology Quiz on Carbohydrates and Proteins

Which structural difference between α-glucose and β-glucose determines whether a polysaccharide forms starch or cellulose?

• Position of the –OH group on carbon 1

• Number of carbons in the ring

• Position of glycosidic bonds on carbon 6

• The linear or ring structure of glucose

Answer explanation

The structural difference that determines whether a polysaccharide forms starch or cellulose is the position of the –OH group on carbon 1. In α-glucose, the –OH is below the plane, while in β-glucose, it is above, affecting glycosidic bond formation.

A solution of sucrose tested directly with Benedict’s reagent remains blue. Which explains this result?

• Sucrose contains no carbonyl group

• Benedict’s reagent only reacts with polysaccharides

• Sucrose is a non-reducing sugar

• Sucrose is insoluble in water

Answer explanation

Sucrose is a non-reducing sugar because it does not have a free carbonyl group to react with Benedict's reagent, which is why the solution remains blue.

A student carries out Benedict’s test after hydrolysing starch with dilute acid. The colour change occurs because:

• Starch monomers contain β-glucose

• Hydrolysis produces reducing sugars such as glucose

• Benedict’s reagent reacts with peptide bonds

• Hydrolysis produces sucrose

Answer explanation

The correct choice is that hydrolysis produces reducing sugars such as glucose. Benedict's test detects these reducing sugars, which change the color of the reagent, indicating the presence of glucose after starch hydrolysis.

Which polysaccharide structure allows rapid release of glucose in animals?

• Amylose – unbranched helix

• Amylopectin – unbranched straight chains

• Cellulose – β-glucose microfibrils

• Glycogen – highly branched chains

Answer explanation

Glycogen, with its highly branched chains, allows for rapid release of glucose in animals. The branching provides multiple endpoints for enzymatic action, facilitating quick mobilization of glucose when energy is needed.

The strength of cellulose fibres is mainly due to:

• Glycosidic bonds between β-glucose

• Ionic bonds between adjacent chains

• Hydrogen bonding between parallel chains

• Ester bonds linking glucose monomers

Answer explanation

The strength of cellulose fibres is primarily due to hydrogen bonding between parallel chains, which provides structural stability and strength, unlike the other bond types listed.

Which sugar would give a positive Benedict’s test without prior hydrolysis?

• Sucrose

• Glucose

• Starch

• Cellulose

Answer explanation

Glucose is a reducing sugar that can directly react with Benedict's reagent, resulting in a positive test. In contrast, sucrose, starch, and cellulose do not give a positive result without hydrolysis.

When preparing a serial dilution, a student dilutes 40% glucose solution by a factor of 2 at each step. What will be the concentration after the 4th dilution?

• 5%

• 2.5%

• 10%

• 20%

Answer explanation

Starting with a 40% solution, each dilution by a factor of 2 halves the concentration. After 4 dilutions: 40% -> 20% -> 10% -> 5% -> 2.5%. Thus, the concentration after the 4th dilution is 2.5%.

Lipids are not polymers, unlike proteins and carbohydrates, because:

• They contain only carbon and hydrogen atoms

• They are not soluble in water

• They are not composed of repeating identical monomers

• They do not undergo condensation reactions

Answer explanation

Lipids are not polymers because they do not consist of repeating identical monomers like proteins and carbohydrates do. Instead, they are made up of various types of molecules, which distinguishes them from true polymers.

Which property of triglycerides makes them suitable for energy storage?

• Polar nature allowing hydrogen bonding with water

• Insolubility in water, preventing osmotic effects

• Branched structure allowing rapid hydrolysis

• Presence of double bonds increasing fluidity

Answer explanation

Triglycerides are insoluble in water, which prevents osmotic effects that could disrupt cellular function. This property makes them ideal for energy storage, as they can be stored without affecting the cell's water balance.

Which molecule is amphipathic and essential for membrane formation?

• Triglyceride

• Phospholipid

• Cholesterol

• Amylose

Answer explanation

Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. This unique structure is essential for forming cell membranes, allowing them to create a bilayer.