Draw the two possible Haworth structures (both alpha and beta anomers) for the following monosaccharides and give their corresponding systematic names. Show the stepwise process D-Ribose (b) D-Galactose (c) D-Fructose 2. Draw the structure and give the systematic name of the given disaccharides. No need to show the stepwise process Lactose which is also known as milk sugar. β-D-Gal(14)ẞ-D-Glc, where Gal is galactose and Glc is glucose. 1. POLUTION.PDF (b) Sucrose which is also known as table sugar: a-D-Glc (1->2) β-D-Fru where Gic is glucose and Fru is fructose. 3. Draw the structure of cellulose and amylose and state their structural similarities and differences
The Correct Answer and Explanation is :
1. Haworth Structures for Monosaccharides and Systematic Names
(a) D-Ribose
Step 1: D-Ribose is an aldopentose (a five-carbon monosaccharide with an aldehyde group). The D-isomer has the hydroxyl group (-OH) on the right of carbon 3 in the Fischer projection.
Step 2: In solution, D-ribose cyclizes to form a furanose ring (five-membered ring) with the oxygen atom being part of the ring.
- Alpha-anomer: The hydroxyl group attached to carbon 1 (anomeric carbon) is on the opposite side of the ring (below the plane).
- Beta-anomer: The hydroxyl group attached to carbon 1 is on the same side of the ring (above the plane).
Step 3: The systematic names for the two anomers:
- Alpha-D-ribopyranose (for the alpha anomer).
- Beta-D-ribopyranose (for the beta anomer).
(b) D-Galactose
Step 1: D-Galactose is an aldohexose (six-carbon monosaccharide with an aldehyde group). The D-isomer has the hydroxyl group on the right of carbon 4 in the Fischer projection.
Step 2: Galactose forms a six-membered pyranose ring.
- Alpha-anomer: The hydroxyl group on carbon 1 (anomeric carbon) is positioned opposite the CH2OH group (below the plane).
- Beta-anomer: The hydroxyl group on carbon 1 is positioned above the plane, the same side as the CH2OH group.
Step 3: The systematic names for the two anomers:
- Alpha-D-galactopyranose (for the alpha anomer).
- Beta-D-galactopyranose (for the beta anomer).
(c) D-Fructose
Step 1: D-Fructose is a ketohexose (six-carbon monosaccharide with a ketone group). The D-isomer has the hydroxyl group on the right of carbon 3 in the Fischer projection.
Step 2: Fructose can cyclize to form a furanose ring (five-membered ring) or a pyranose ring (six-membered ring), but typically it forms a furanose ring.
- Alpha-anomer: The hydroxyl group on the anomeric carbon (carbon 2) is on the opposite side of the CH2OH group (below the plane).
- Beta-anomer: The hydroxyl group on the anomeric carbon is above the plane (same side as the CH2OH group).
Step 3: The systematic names for the two anomers:
- Alpha-D-fructofuranose (for the alpha anomer).
- Beta-D-fructofuranose (for the beta anomer).
2. Disaccharides
(a) Lactose
Lactose consists of a β-D-galactose linked to α-D-glucose by a β-1,4-glycosidic bond.
- Systematic name: β-D-galactopyranosyl-(1→4)-D-glucopyranose.
(b) Sucrose
Sucrose consists of an α-D-glucose linked to β-D-fructose by a 1→2 glycosidic bond.
- Systematic name: α-D-glucopyranosyl-(1→2)-β-D-fructofuranose.
3. Structure of Cellulose and Amylose: Similarities and Differences
Cellulose:
- Structure: Cellulose is a polymer made of D-glucose units linked by β-1,4-glycosidic bonds. Each glucose unit in cellulose is oriented in the opposite direction (β-configuration), which causes the polymer to form long straight chains that are able to hydrogen bond with adjacent chains, giving cellulose its rigidity and strength.
- Function: It provides structural support in plant cell walls.
Amylose:
- Structure: Amylose is a form of starch, composed of D-glucose units linked by α-1,4-glycosidic bonds. Amylose forms a helical structure rather than a linear one, unlike cellulose.
- Function: It serves as an energy storage polysaccharide in plants.
Similarities:
- Both cellulose and amylose are polysaccharides made of D-glucose units.
- Both are linear chains of glucose, but they differ in the type of glycosidic bonds linking the glucose units.
Differences:
- Glycosidic Bond Type: Cellulose has β-1,4-glycosidic bonds, while amylose has α-1,4-glycosidic bonds.
- Structure: Cellulose forms straight chains that are rigid and form hydrogen bonds between chains, giving it structural strength. Amylose, on the other hand, forms helical structures, which are more flexible and suitable for energy storage.
- Function: Cellulose provides structural integrity in plant cell walls, whereas amylose serves as an energy storage form in plants (part of starch).
In summary, while both are polysaccharides of glucose, the differences in their glycosidic bonds lead to vastly different structural properties and biological functions.