BM - Carbohydrates

Question 1

Describe the structure of starch and explain how its structure is related to its function.
[6 marks]

Answer 1

Structure:
* Made of alpha-glucose monomers joined by glycosidic bonds.
* Amylose: unbranched, helical structure.
* Amylopectin: branched, allowing rapid hydrolysis.

Function:
* Energy storage in plants.
* Compact: stores large amounts of glucose in a small space.
* Insoluble: doesn’t affect water potential.
* Branched structure allows enzymes to access ends for quick energy release.

Question 2

Compare and contrast the structure and function of glycogen and cellulose.
[6 marks]

Answer 2

Glycogen:
* Made of alpha-glucose, highly branched.
* Energy storage in animals.
* Compact and soluble, allowing rapid glucose release for respiration.

Cellulose:
* Made of beta-glucose, straight unbranched chains.
* Chains linked by hydrogen bonds to form strong microfibrils.
* Provides structural support in plant cell walls.

Comparison: Both are polysaccharides

Question 3

Describe the biochemical test for reducing and non-reducing sugars. Explain the principles behind the test and the expected results.
[8 marks]

Answer 3

Reducing sugars:
* Add Benedict’s reagent and heat.
* Positive result: blue to brick-red precipitate.

Non-reducing sugars:
* First hydrolyze by heating with dilute HCl, then neutralize with sodium hydrogen carbonate.
* Repeat Benedict’s test.
* Positive result: blue to brick-red precipitate.

Principles:
* Reducing sugars donate electrons to reduce Cu²⁺ to Cu⁺.
* Non-reducing sugars must first be broken into monosaccharides to reveal reducing groups.

Question 4

Explain how the structure of cellulose relates to its function in plants.
[6 marks]

Answer 4

Structure:
* Made of beta-glucose joined by 1,4-glycosidic bonds.
* Straight, unbranched chains.
* Chains linked by hydrogen bonds to form microfibrils.

Function:
* Provides tensile strength and rigidity.
* Helps maintain shape and resist turgor pressure in plant cells.

Question 5

Describe how the structure of a disaccharide and a polysaccharide differ, using maltose and glycogen as examples.

Answer 5

Disaccharide (Maltose):
* Composed of two alpha-glucose molecules joined by a single 1,4-glycosidic bond.
* Small, soluble, used in transport and metabolism.

Polysaccharide (Glycogen):
* Composed of many alpha-glucose molecules.
* Highly branched structure with 1,4- and 1,6-glycosidic bonds.
* Used for energy storage in animals, compact and rapidly hydrolyzed.

Differences:
* Disaccharides are small and consist of two monomers; polysaccharides are large and consist of many.

Question 6

Explain why starch and glycogen are suitable for energy storage but cellulose is not.
[6 marks]

Answer 6

Starch and Glycogen:
* Composed of alpha-glucose, which is easily hydrolyzed into glucose for respiration.
* Compact and branched, allowing efficient storage and quick energy release.
* Insoluble, so they do not affect water potential.

Cellulose:
* Made of beta-glucose, forming straight, unbranched chains.
* Hydrogen bonding makes cellulose rigid and insoluble, suited for structural support rather than energy storage.

Question 7

Describe the differences in the structure of amylose and amylopectin and explain how these differences affect their functions.
[6 marks]

Answer 7

Amylose:
* Unbranched, helical structure.
* Compact, ideal for long-term energy storage.

Amylopectin:
* Branched structure with 1,6-glycosidic bonds.
* Provides more accessible ends for enzymes, allowing rapid glucose release.

Function:
* Combination of both forms allows plants to store energy efficiently while providing flexibility in energy release.

Question 8

Explain the process by which monosaccharides join to form a disaccharide, and describe how this can be reversed.
[6 marks]

Answer 8

Formation:
* Two monosaccharides (e.g., glucose) join via a condensation reaction.
* Glycosidic bond is formed, and water is released.

Reversal:
* Hydrolysis reaction breaks the glycosidic bond.
* Water is added, producing two monosaccharides.

Example:
Formation of maltose and its breakdown during digestion.

Question 9

Explain why glycogen is a more suitable storage molecule for animals compared to starch.
[6 marks]

Answer 9

• Glycogen is more highly branched than starch, allowing faster hydrolysis.
• Rapid glucose release meets the higher metabolic demands of animals.
• Glycogen is compact, storing large amounts of energy in a small space.
• Both are insoluble, so they do not affect water potential, but glycogen’s structure is better suited to animals’ mobile lifestyles.