Carbohydrates: Structure and Functions

Comprehensive review flashcards covering carbohydrate classification, monosaccharide nomenclature, chirality, Fischer and Haworth projections, disaccharides, reducing versus non-reducing sugars, and structural and storage polysaccharides.

What is the general chemical formula representing carbohydrates?

The general formula is $C_n(H_2O)_n$, reflecting the historical term "hydrate of carbon."

What is the chemical definition of a carbohydrate?

A polyhydroxyaldehyde or polyhydroxyketone, or a substance that yields these compounds upon hydrolysis.

What are the four major classifications of carbohydrates based on molecular size?

Monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

What is a monosaccharide, and what is its general formula?

A carbohydrate that cannot be hydrolyzed into a simpler compound, having the general formula $C_n H_{2n} O_n$.

How do aldoses and ketoses differ in terms of their functional groups?

Aldoses are monosaccharides that contain an aldehyde functional group ($-\text{CHO}$), whereas ketoses contain a ketone functional group ($-\text{C=O}$).

What are the physical solubility properties of monosaccharides?

Monosaccharides are colorless, crystalline solids that are water soluble, slightly soluble in ethanol, and insoluble in nonpolar solvents such as diethyl ether, dichloromethane, and benzene.

How is a six-carbon monosaccharide containing a ketone group systematically classified?

A ketohexose (combining the prefix "keto-", the carbon-count syllable "hex-", and the suffix "-ose").

What is the definition of a chiral object or molecule?

An object or molecule that is nonsuperimposable on its mirror image.

Under what symmetry conditions is a molecule guaranteed to be achiral?

A molecule is achiral if it has a mirror plane of symmetry or an inversion center.

Why is the molecule shown below classified as chiral?

Bromochlorofluoromethane is chiral because its central carbon is an asymmetric center bonded to four different groups ($\text{H}$, $\text{Cl}$, $\text{F}$, $\text{Br}$), making it nonsuperimposable on its mirror image.

How many chiral carbon centers are present in aldotrioses, aldotetroses, aldopentoses, and aldohexoses?

Aldotrioses have $1$, aldotetroses have $2$, aldopentoses have $3$, and aldohexoses have $4$ chiral carbons.

How many chiral carbon centers are present in ketotrioses, ketotetroses, ketopentoses, and ketohexoses?

Ketotrioses have $0$ (none), ketotetroses have $1$, ketopentoses have $2$, and ketohexoses have $3$ chiral carbons.

In a Fischer projection, what do horizontal and vertical lines represent?

Horizontal lines represent bonds projecting forward from the stereocenter toward the viewer, while vertical lines represent bonds projecting to the rear away from the viewer.

How are D- and L-monosaccharides distinguished in a Fischer projection?

In a D-monosaccharide, the $-OH$ group on the penultimate carbon is on the right; in an L-monosaccharide, the $-OH$ group on the penultimate carbon is on the left.

What is an epimer?

Stereoisomers that differ in configuration at only one chiral center (for example, D-glucose and D-galactose are epimers at carbon-$4$).

Identify the two ring systems shown below that serve as models for cyclic monosaccharides.

The six-membered cyclic ether on the left is Pyran (giving rise to pyranoses), and the five-membered cyclic ether on the right is Furan (giving rise to furanoses).

What is the anomeric carbon in a cyclic carbohydrate?

The newly created carbon stereocenter formed during hemiacetal ring closure; it is carbon-$1$ in aldoses and carbon-$2$ in ketoses.

What is the difference between an (\alpha) and (\beta) anomer in a D-sugar Haworth projection?

In the $\alpha$-anomer, the anomeric $-OH$ group points downward (trans to the terminal $-\text{CH}_2\text{OH}$ group), while in the $\beta$-anomer, it points upward (cis to the terminal $-\text{CH}_2\text{OH}$ group).

What is the most stable, strain-free conformation adopted by pyranose rings?

The chair conformation.

What is an amino sugar?

A carbohydrate derivative that contains an $-\text{NH}_2$ group in place of an $-OH$ group.

What are the common names and the normal blood fasting concentration range for D-glucose?

Also called grape sugar, blood sugar, or dextrose; normal fasting blood glucose concentration is $70\text{--}100\,\text{mg/dL}$.

Why is D-galactose referred to as "brain sugar," and where is it synthesized in the human body?

It is called brain sugar because it is found in brain and nerve tissue; it is synthesized from glucose in mammary glands for use in lactose.

What are two biological roles of D-mannose?

It is an aldohexose crucial for the glycosylation of proteins in the body and participates in immune defense.

What are the key characteristics of D-fructose?

It is a ketohexose known as levulose or fruit sugar, the sweetest tasting of all sugars, and predominantly forms a five-membered furanose cyclic ring.

What structural feature defines a reducing sugar?

The presence of a hemiacetal group (an $-OH$ and an $-OR$ attached to the same carbon), allowing equilibrium with an open-chain aldehyde that can reduce reagents like Benedict's and Tollens'.

What is the monosaccharide composition and glycosidic linkage of maltose?

It consists of two D-glucose units (one of which must be $\alpha$-D-glucose) connected by an $\alpha(1\rightarrow 4)$ glycosidic linkage; it is a reducing sugar.

What disaccharide is shown in this diagram, and what specific linkage connects its units?

Lactose, which is composed of a $\beta$-D-galactopyranose unit linked to a D-glucopyranose unit via a $\beta(1\rightarrow 4)$ glycosidic bond.

What causes lactose intolerance?

A deficiency of the lactase enzyme in the small intestine, leading to undigested lactose being fermented by gut bacteria to produce gas and organic acids.

What disaccharide is depicted in this structural formula, and why is it non-reducing?

Sucrose; it is non-reducing because both anomeric carbons are tied up in an $\alpha, \beta(1\rightarrow 2)$ glycosidic bond between $\alpha$-D-glucose and $\beta$-D-fructose, leaving no free hemiacetal.

How does cellobiose differ from maltose?

Cellobiose is composed of two D-glucose units joined by a $\beta(1\rightarrow 4)$ glycosidic linkage and cannot be digested by humans, whereas maltose is linked by an $\alpha(1\rightarrow 4)$ linkage and is digestible.

What are oligosaccharides, and how many monosaccharide units do they contain?

Carbohydrates containing $3\text{--}10$ monosaccharide units joined by glycosidic bonds, commonly found in onions, cabbage, broccoli, and whole wheat.

What oligosaccharide-derived toxin is found in potato plants, and what indicates its presence?

Solanine; excessive levels make potatoes bitter, and greening of the potato indicates high solanine levels.

Based on the diagram below, which terminal monosaccharides determine Type A and Type B blood antigens?

Type A blood contains an additional terminal (N)-acetylgalactosamine, whereas Type B blood contains an additional terminal galactose.

What are the two structural polysaccharides that constitute starch, and how do their linkages and proportions differ?

Amylose constitutes $15\%\text{--}20\%$ (unbranched chain with $\alpha(1\rightarrow 4)$ linkages), while Amylopectin constitutes $80\%\text{--}85\%$ (branched chain with $\alpha(1\rightarrow 4)$ main chain and $\alpha(1\rightarrow 6)$ branch linkages).

What is glycogen, and where is it primarily stored in the human body?

A glucose storage homopolysaccharide featuring $\alpha(1\rightarrow 4)$ and $\alpha(1\rightarrow 6)$ glycosidic bonds that is more highly branched than amylopectin; stored primarily in liver cells and muscle cells.

Why are humans unable to digest cellulose?

Cellulose is a linear homopolysaccharide with $\beta(1\rightarrow 4)$ glycosidic bonds, and humans lack the digestive enzymes capable of hydrolyzing $\beta(1\rightarrow 4)$ linkages.

What is chitin, and what repeating monomer unit comprises it?

A linear structural polysaccharide with $\beta(1\rightarrow 4)$ glycosidic linkages composed of (N)-acetyl-D-glucosamine (an (N)-acetyl amino derivative of glucose).

What is hyaluronic acid, and what are its physiological roles?

A glycosaminoglycan composed of alternating residues of (N)-acetyl-$\beta$-D-glucosamine and D-glucuronate; it functions as a joint lubricant, a cushion in cartilage, and a lubricant in the vitreous humor of the eye.

Where is heparin found, and what is its primary physiological function?

Found in mast cells (especially within the liver, lungs, and skin); it functions as an acidic polysaccharide blood anticoagulant ("blood thinner").