BM - Carbohydrates Flashcards
Describe the structure of starch and explain how its structure is related to its function.
[6 marks]
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.
Compare and contrast the structure and function of glycogen and cellulose.
[6 marks]
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
Describe the biochemical test for reducing and non-reducing sugars. Explain the principles behind the test and the expected results.
[8 marks]
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.
Explain how the structure of cellulose relates to its function in plants.
[6 marks]
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.
Describe how the structure of a disaccharide and a polysaccharide differ, using maltose and glycogen as examples.
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.
Explain why starch and glycogen are suitable for energy storage but cellulose is not.
[6 marks]
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.
Describe the differences in the structure of amylose and amylopectin and explain how these differences affect their functions.
[6 marks]
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.
Explain the process by which monosaccharides join to form a disaccharide, and describe how this can be reversed.
[6 marks]
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.
Explain why glycogen is a more suitable storage molecule for animals compared to starch.
[6 marks]
- 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.
Monosaccharides and disaccharides taste sweet.
The lactose-free milk made after hydrolysis with lactase tastes sweeter than the
cow’s milk containing lactose.
Suggest why
3 marks
- (Lactose hydrolysed to) galactose and glucose;
- (So) more sugar molecules;
Give two ways in which the structure of starch is similar to cellulose
(Both)
1. Are polymers made of monomers
2. Contain glucose / carbon, hydrogen and oxygen;
3. Contain glycosidic bonds;
Give two ways in which the structure of starch is different from cellulose.
(Starch)
1. Contains α / alpha glucose;
2. Helical / coiled / compact / branched / not straight;
Using the diagram, suggest and explain one other way in which sieve cells are
adapted for mass transport.
- few organelles making it hollow
- (So) easier flow with lower resistance to flow
Using the diagram, suggest and explain one other way in which companion
cells are adapted for the transport of sugars between cells.
- Mitochondria release energy / ATP
- For active transport / uptake against concentration gradient.
Explain how cellulose molecules are adapted for their function in plant cells.
- Long and straight chains;
- Become linked together by many hydrogen bonds to form fibrils;
- Provide strength (to cell wall).
In humans, the enzyme maltase breaks down maltose to glucose.
This takes place at normal body temperature.
Explain why maltase:
* only breaks down maltose
* allows this reaction to take place at normal body temperature.
5 marks
- Tertiary structure / 3D shape of enzyme - active site
- Active site complementary to maltose / substrate / maltose fits into active
site - Description of induced fit;
- Enzyme is a catalyst / lowers activation energy
- By forming enzyme-substrate complex
Describe competitive and non-competitive inhibition of an enzyme.
5 marks
- Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
(Competitive inhibition),
2. Inhibitor similar shape to substrate;
3. (Binds) in to active site (of enzyme);
4. (Inhibition) can be overcome by more substrate;
(Non-competitive inhibition),
5. Inhibitor binds to allocentric site on enzyme
6. Prevents formation of active site / changes (shape of) active site;
7. Cannot be overcome by adding more substrate;
