AS Level Biology MCQs on Molecules & Water

Which best explains why glycogen is a more effective short-term energy store than amylose?

• Glycogen has β-glucose units linked with β-1,4 bonds

• Glycogen is less branched, so more compact

• Glycogen has more α-1,6 branches, increasing sites for enzyme action

• Glycogen forms stronger hydrogen bonds between chains

Answer explanation

Glycogen's structure includes more α-1,6 branches compared to amylose, which increases the number of sites for enzyme action. This allows for quicker mobilization of glucose, making glycogen a more effective short-term energy store.

The main difference between α- and β-glucose affects:

• The type of glycosidic bond they can form

• Their ability to form condensation products

• Their solubility in water

• The number of carbon atoms in the molecule

Answer explanation

The main difference between α- and β-glucose lies in the orientation of the hydroxyl group on the first carbon, which affects the type of glycosidic bond they can form during polymerization, influencing carbohydrate structure.

Cellulose is difficult to digest for most animals because:

• β-1,4 bonds cannot be broken without specific enzymes

• It is highly branched, limiting enzyme access

• It is hydrophobic and repels digestive enzymes

• It contains α-glucose instead of β-glucose

Answer explanation

Cellulose has β-1,4 bonds that most animals cannot break down without specific enzymes, making it difficult to digest. Other options do not accurately describe cellulose's structure or its digestibility.

Amylopectin is less compact than amylose because:

• It contains β-1,4 bonds

• It has α-1,6 branches that prevent tight coiling

• It has fewer glycosidic bonds overall

• It forms hydrogen bonds with water

Answer explanation

Amylopectin has α-1,6 branches that create a more open structure, preventing tight coiling. This makes it less compact than amylose, which primarily consists of linear α-1,4 bonds.

Which statement about condensation reactions is correct?

• They occur only in carbohydrate synthesis

• They require the input of water

• They join monomers with the release of water

• They break down polymers into monomers

Answer explanation

Condensation reactions join monomers to form polymers, releasing water as a byproduct. This process is essential in synthesizing various biological macromolecules, not just carbohydrates.

Which feature of glycogen allows rapid release of glucose during intense activity?

• High number of β-1,4 bonds

• Long unbranched chains

• Highly branched structure

• Insolubility in water

Answer explanation

The highly branched structure of glycogen allows for multiple glucose molecules to be released simultaneously during intense activity, facilitating rapid energy availability compared to unbranched forms.

A student tests a sample for starch and gets a negative iodine test, but the sample still contains a plant storage carbohydrate. Which molecule is most likely present?

• Amylose

• Amylopectin

• Glycogen

• Cellulose

Answer explanation

Glycogen is a storage carbohydrate that does not react with iodine, unlike starch (amylose and amylopectin). Therefore, a negative iodine test suggests the presence of glycogen, which is commonly found in animals.

Hydrolysis of sucrose will yield:

• Glucose only

• Glucose and galactose

• Glucose and fructose

• Fructose only

Answer explanation

Hydrolysis of sucrose, a disaccharide, breaks it down into its monosaccharide components: glucose and fructose. Therefore, the correct answer is glucose and fructose.

Which property of triglycerides explains why they yield more energy per gram than carbohydrates?

• Higher proportion of hydrogen atoms to oxygen atoms

• Presence of double bonds in fatty acid chains

• Lower molecular mass

• Stronger ester bonds

Answer explanation

Triglycerides have a higher proportion of hydrogen atoms to oxygen atoms compared to carbohydrates. This means they can release more energy during oxidation, making them more energy-dense per gram.

Saturated fatty acids are generally solid at room temperature because:

• They contain double bonds that cause kinks

• They pack closely together due to straight chains

• They contain fewer hydrogen atoms per carbon

• They repel water

Answer explanation

Saturated fatty acids are solid at room temperature because they have straight chains that allow them to pack closely together, resulting in a solid structure. This is in contrast to unsaturated fatty acids, which have kinks due to double bonds.