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Questions and Answers

Which form of D-glucose predominates in solution due to reduced steric hindrances?

Furanose form

Boat form

Chair form

Pyranose form (correct)

What is the ratio of β to α anomer conformations for D-glucose in an equilibrium solution?

1:1

2:1 (correct)

1:2

3:2

Which of the following best describes why D-glucose is an important fuel for most organisms?

It is exclusively used by aerobic organisms for energy.

It forms from nucleic acids and is readily available.

It is the only fuel used by brain and red blood cells in non-starvation conditions. (correct)

It is highly reactive and interacts with all cells.

Which statement about reducing sugars is correct?

All monosaccharides can adopt linear structures and are thus reducing sugars. Signup and view all the answers

What happens during mutarotation of cyclic sugars?

The sugar changes its specific rotation while reaching an equilibrium. Signup and view all the answers

Which form of glucose does not react with oxidizing agents?

Non-reducing sugars Signup and view all the answers

What is true about the chair and boat forms of β-D-glucose?

The chair form is more stable due to its less steric hindrance. Signup and view all the answers

Which of the following sugars is a known reducing sugar?

Fructose Signup and view all the answers

What is the empirical formula for carbohydrates?

(CH2O)n Signup and view all the answers

Which of the following describes the structural classification of monosaccharides?

They can be classified based on the number of carbon atoms and the type of most oxidized group. Signup and view all the answers

What is a constitutional isomer?

Molecules with identical molecular formulas that differ in how the atoms are ordered. Signup and view all the answers

How many carbon atoms can monosaccharides contain?

3 to 7 Signup and view all the answers

Which type of monosaccharide is classified as a pentose?

A monosaccharide with five carbon atoms. Signup and view all the answers

What term is used to describe sugars that differ in configuration only at a single asymmetric center?

Epimers Signup and view all the answers

What happens to an aldehyde when it reacts with an alcohol?

It forms a hemiacetal. Signup and view all the answers

What is the role of the anomeric carbon in glucose?

It becomes an asymmetric center upon cyclic formation. Signup and view all the answers

Which of the following terms refers to diastereoisomers that are not mirror images of each other?

Diastereoisomers Signup and view all the answers

What type of cyclic structure is formed by sugars that resemble furan?

Furanose Signup and view all the answers

Study Notes

Carbohydrates and Glycoproteins

Carbohydrates are carbon-based molecules with a high proportion of hydroxyl groups.
Their empirical formula is (CH2O)n, but they can have additional groups or modifications.
They are better described as polyhydroxy aldehydes and ketones.
Monosaccharides are the simplest carbohydrates and consist of three to seven carbons.
They are also called simple sugars.

Monosaccharide Nomenclature

Nomenclature is based on carbon chain length:
- Three carbons: trioses
- Four carbons: tetroses
- Five carbons: pentoses
- Six carbons: hexoses
- Seven carbons: heptoses
Nomenclature is also based on the identity of the most oxidized group:
- Keto group: ketose
- Aldehyde group: aldose

Isomers

Constitutional Isomers: Molecules with identical molecular formulas but differ in the order of their atoms.
Stereoisomers: Molecules that differ in spatial arrangement but not bonding order.
- They have either D or L configuration.
- Can be enantiomers (mirror images of each other) or diastereoisomers (not mirror images of each other).
- The number of possible stereoisomers is 2n, where n is the number of asymmetric carbon atoms.

Common Monosaccharides

Epimers: Sugars that are diastereoisomers differing in configuration only at a single asymmetric center.

Cyclic Forms of Monosaccharides

Most monosaccharides exist primarily in their ring forms.
An aldehyde can react with an alcohol to form a hemiacetal.
A ketone can react with an alcohol to form a hemiketal.

Pyranose Formation

Occurs when a six-membered ring is formed.
Named after the compound pyran.

Furanose Formation

Occurs when a five-membered ring is formed.
Named after the compound furan.

Anomers of Glucose

Anomer: A diastereoisomeric form of sugars that forms when a cyclic hemiacetal forms and an additional asymmetric center is created.
In glucose, C-1 (the anomeric carbon atom) becomes an asymmetric center, forming two ring structures:
- α-D-glucopyranose: Hydroxyl group attached to C-1 is on the opposite side of the ring as C-6.
- β-D-glucopyranose: Hydroxyl group attached to C-1 is on the same side of the ring as C-6.

D-Fructose Ring Structures

C-2 is the anomeric carbon atom.
The pyranose form predominates in solution due to reduced steric hindrances.
The furanose form predominates in fructose derivatives.

Chair and Boat Forms of β-D-Glucose

The chair form predominates because all axial positions are occupied by hydrogens.
The boat form is disfavored because it is sterically hindered.

D-Glucose as a Fuel

Blood Sugar: D-glucose circulating in the blood.
Used as the sole fuel by the brain (in non-starvation conditions) and red blood cells.
Potential reasons why D-glucose is an important fuel:
- Glucose is formed from formaldehyde under prebiotic conditions and may have been available as a fuel source for primitive biochemical systems.
- It is relatively inert.
- The most stable ring structure is β-D-glucopyranose.

Solutions of Glucose

Two anomeric forms (α and β) are in equilibrium that passes through the open-chain form.
The β anomer is more stable in solution.

Reducing Sugars

Reducing Sugars: Sugars that react with oxidizing agents.
- All monosaccharides can adopt linear structures in solution.
Non-Reducing Sugars: Sugars that do not react with oxidizing agents.
Fehling's solution: Solutions of Cu2+ used to test for the presence of reducing sugars.