Carbohydrates Overview and Classification

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Questions and Answers

What structural role does cellulose serve in plants?

Energy storage

Structural support (correct)

Water solubility

Nutrient transport

Why can animals not utilize cellulose as a fuel source?

Lack of enzymes to hydrolyze β1→4 glycosidic bonds (correct)

Cellulose is easily digestible by animal enzymes

Cellulose is water-soluble and thus lost in digestion

Cellulose is primarily a digestive inhibitor

What makes chitin more hydrophobic compared to cellulose?

Linear structure without branches

Presence of N-acetylglucosamine residues (correct)

Higher number of hydroxyl groups

Greater molecular weight

What is the most stable three-dimensional structure of starch and glycogen?

Helical structure Signup and view all the answers

What role do steric factors and hydrogen bonding play in homopolysaccharides?

They influence folding and stability of structures Signup and view all the answers

Which of the following disaccharides is categorized as nonreducing?

Trehalose Signup and view all the answers

What characterizes a homopolysaccharide?

It is made up of a single type of monomeric sugar. Signup and view all the answers

What is the primary storage polysaccharide in plant cells?

Starch Signup and view all the answers

What is a distinguishing feature of glycogen compared to starch?

It is more extensively branched. Signup and view all the answers

What is the purpose of storing glucose as polymers like glycogen?

To avoid elevating osmolarity in the cell. Signup and view all the answers

Which type of bond is present between glucose residues in amylose?

(α1→4) linkage Signup and view all the answers

What is a defining characteristic of heteropolysaccharides?

They contain two or more types of monomers. Signup and view all the answers

Why are hydroxyl groups significant in starch and glycogen?

They allow for hydrogen bonding with water. Signup and view all the answers

What happens to aldoses during oxidation to form aldonic acids?

Oxidation of the carbonyl carbon Signup and view all the answers

Which property of phosphorylated sugars aids in trapping them inside cells?

They bear a negative charge Signup and view all the answers

Which of the following sugars is a reducing disaccharide?

Lactose Signup and view all the answers

What type of bond is formed by the reaction of the hydroxyl group of one sugar molecule with the anomeric carbon of another?

O-glycosidic bond Signup and view all the answers

Which of the following sugars can tautomerize to form aldehydes, making them reducing sugars?

Fructose Signup and view all the answers

In naming reducing oligosaccharides, what is indicated in parentheses?

The two carbon atoms joined by the glycosidic bond Signup and view all the answers

What is the outcome of the interaction of reducing sugars with Cu2+ in alkaline conditions?

Formation of a brick-red precipitate Signup and view all the answers

Which of the following disaccharides is classified as a nonreducing sugar?

Sucrose Signup and view all the answers

What is the defining feature of carbohydrates?

They are aldehydes or ketones with at least two hydroxyl groups. Signup and view all the answers

What type of carbohydrate consists of a single sugar unit?

Monosaccharides Signup and view all the answers

Which of the following is an example of a disaccharide?

Sucrose Signup and view all the answers

What characterizes aldoses in terms of their structure?

They have a carbonyl group at an end of the carbon chain. Signup and view all the answers

What term describes two sugars that differ only in the configuration around one carbon atom?

Epimers Signup and view all the answers

Which structure is formed when a hydroxyl group reacts with a carbonyl group in monosaccharides?

Hemiacetal Signup and view all the answers

What are the cyclic forms of D-glucose known as?

Pyranoses Signup and view all the answers

In the context of Fischer projections, what do the horizontal bonds represent?

Bonds that project out of the plane of the paper. Signup and view all the answers

What process involves the interconversion of α and β anomers?

Mutarotation Signup and view all the answers

Which statement accurately describes furanoses?

They are five-membered ring compounds. Signup and view all the answers

What is the primary difference between D and L isomers?

Their configuration at the reference carbon Signup and view all the answers

What do α and β configurations refer to in cyclic sugars?

The orientation of the hydroxyl group at the anomeric carbon. Signup and view all the answers

How many stereoisomers can a molecule with n chiral centers have?

$2^n$ Signup and view all the answers

What characteristic feature do hemiketals and hemiacetals share?

They are derived from reactions involving carbonyl compounds. Signup and view all the answers

What is the main structural role of peptidoglycan in bacteria?

Provides rigidity to the cell wall Signup and view all the answers

What are glycosaminoglycans primarily composed of?

Repeating monosaccharide units Signup and view all the answers

Which component is unique to the extracellular matrix?

Proteoglycans Signup and view all the answers

How do heparin and heparan sulfate primarily differ?

Heparin is primarily produced by mast cells Signup and view all the answers

Which statement about hyaluronan is accurate?

It consists of alternating D-glucuronic acid and N-acetylglucosamine Signup and view all the answers

What structural feature distinguishes starch from cellulose?

Type of linkages between monosaccharides Signup and view all the answers

Which statement accurately describes proteoglycans?

They contain sulfated glycosaminoglycan chains linked to proteins Signup and view all the answers

What is a common feature of glycoproteins?

They have one or more oligosaccharides joined covalently to proteins Signup and view all the answers

What is the main function of glycosphingolipids in neurons?

Play a role in signal transduction Signup and view all the answers

Which of the following polysaccharides is primarily involved in energy storage in animal cells?

Glycogen Signup and view all the answers

What distinguishes chondroitin sulfate from hyaluronan?

Chondroitin sulfate is connected to specific proteins Signup and view all the answers

What is a key characteristic of the extracellular matrix?

It provides a porous pathway for nutrient diffusion Signup and view all the answers

Which polysaccharide provides structural support primarily in the exoskeletons of insects?

Chitin Signup and view all the answers

Which linkages characterize the structure of cellulose?

β1→4 linkages Signup and view all the answers

Study Notes

Carbohydrates

Carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis.
Many carbohydrates have the empirical formula (CH2O)n.
Monosaccharides, oligosaccharides, and polysaccharides are the three main classes of carbohydrates.

Classes of Carbohydrates

Monosaccharides are single polyhydroxy aldehyde or ketone units, such as D-glucose.
Oligosaccharides are short chains of monosaccharide units linked by glycosidic bonds, with disaccharides being a type with two monosaccharide units.
Polysaccharides are sugar polymers with more than 10 monosaccharide units, like cellulose and glycogen.
Polysaccharides form the most abundant carbohydrates in nature, playing important roles in storage, structure, and signaling.

Monosaccharides and Disaccharides

The two families of monosaccharides are aldoses and ketoses, classified by the position of their carbonyl group.
Aldoses have their carbonyl group at the end in an aldehyde group, while ketoses have it at any other position as a ketone group.
All monosaccharides except dihydroxyacetone have chiral carbons and occur in optically active isomeric forms, with enantiomers being mirror image optical isomers.
Fischer projection formulas are used to represent the three-dimensional structures of sugars on paper, with horizontal bonds projecting outwards and vertical bonds projecting behind the plane of the paper.
D isomers and L isomers are two groups of optical isomers determined by the configuration at the reference carbon, the chiral carbon furthest from the carbonyl carbon.
D isomers have the same configuration as D-glyceraldehyde, the hydroxyl group on the right in a projection formula, while L isomers share the configuration of L-glyceraldehyde, with the hydroxyl group on the left in a projection formula.
Epimers are sugars that differ only in the configuration around one carbon atom.
Aldotetroses and monosaccharides with 5+ backbone carbon atoms occur as cyclic structures in an aqueous solution due to the formation of a covalent bond between the carbonyl group and an oxygen of a hydroxyl group.
Hemiacetals and hemiketals are formed through the reaction of alcohols and aldehydes or ketones.
The anomeric carbon is the carbonyl carbon, which forms two stereoisomeric configurations: α and β.
Mutarotation is the interconversion between α and β anomers.
Pyranoses are six-membered ring compounds formed from the reaction of the hydroxyl group at C-5 and the keto group at C-1, while furanoses are five-membered ring compounds formed when the hydroxyl group at C-5 reacts with the keto group at C-2.
Haworth perspective formulas are more accurate representations of cyclic sugar structures than Fischer projections, with the rings tilted to make their plane almost perpendicular to the paper.
Hexose derivatives:
- Aldonic acids form following oxidation of the carbonyl carbon of aldoses.
- Uronic acids form following oxidation at C-6.
- Phosphorylated derivatives, like glucose 6-phosphate, trap sugar inside the cell because most cells do not have transporters for phosphorylated sugars.
Reducing sugars undergo a redox reaction with Cu2+ under alkaline conditions, reducing Cu2+ to Cu+ and forming a brick-red precipitate.
Ketoses can also be reducing sugars if they can tautomerize to form aldehydes.

Polysaccharides

Polysaccharides are also called glycans and consist of multiple monosaccharides.
Homopolysaccharides contain only one type of monosaccharide and serve as storage forms or structural elements.
Heteropolysaccharides contain two or more types of monomers, often providing extracellular support.
Polysaccharides do not have defined lengths or molecular weights due to the absence of a template for their synthesis.

Storage Forms of Fuel

Starch is a storage polysaccharide in plants, containing amylose and amylopectin.
- Amylose is a long, unbranched chain of D-glucose residues connected by (α1→4) linkages.
- Amylopectin is larger than amylose and has (α1→4) linkages between glucose residues with (α1→6) linkages for branching.
Glycogen is a polymer of (α1→4)-linked glucose subunits with (α1→6) branched linkages, making it more extensively branched and compact than starch.
Starch and glycogen molecules are heavily hydrated due to their exposed hydroxyl groups, which can form hydrogen bonds.

Structural Roles

Cellulose is a tough, fibrous, water-insoluble homopolysaccharide composed of D-glucose units linked by (β1→4) glycosidic bonds.
Chitin is a linear homopolysaccharide made of N-acetylglucosamine residues linked by (β1→4) linkages.
The three-dimensional structures of homopolysaccharides are stabilized by weak interactions, with hydrogen bonding being especially important due to the hydroxyl groups in polysaccharides.
The helical structure of starch and glycogen, with six residues per turn, allows for the stable three-dimensional arrangement of these molecules.
Cellulose has a linear, extended structure, with each residue turned 180° relative to its neighbor.

Peptidoglycan and Glycosaminoglycans

Peptidoglycan is a rigid component of bacterial cell walls, composed of alternating (β1→4)-linked N-acetylglucosamine and N-acetylmuramic acid residues cross-linked by short peptides.
The extracellular matrix (ECM) is a gel-like material in the extracellular space of tissues, composed of heteropolysaccharides and fibrous proteins.
Glycosaminoglycans are heteropolysaccharides found in the ECM, composed of repeating disaccharide units, one of which is always N-acetylglucosamine or N-acetylgalactosamine, and the other is typically a uronic acid. They generally contain esterified sulfate groups.
Hyaluronan has alternating residues of D-glucuronic acid and N-acetylglucosamine.
Chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparan sulfate differ from hyaluronan by being shorter, linked to proteins, and having different monomer units.
Heparan sulfate has variable, nonrandom arrangements of sulfated and nonsulfated sugars and interacts specifically with proteins.
Heparin is a highly sulfated, intracellular form of heparan sulfate produced primarily by mast cells, used therapeutically to inhibit blood coagulation.

Glycoconjugates

Glycoconjugates are biologically active molecules consisting of an informational carbohydrate joined to a protein or lipid.
Proteoglycans are macromolecules found in the cell surface and ECM, with one or more sulfated glycosaminoglycan chains joined covalently to a protein, playing a major role in all extracellular matrices.
Glycoproteins contain one or several oligosaccharides joined covalently to a protein. They are found on the plasma membrane, in the ECM, in blood, and in organelles, with their heterogenous oligosaccharide portions providing information.
Glycolipids are plasma membrane components with oligosaccharides as their hydrophilic head groups.
Glycosphingolipids are a class of glycolipids with specific backbone structures, rich in neurons and involved in signal transduction.

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Description

This quiz covers the basics of carbohydrates, including their definitions, classifications into monosaccharides, oligosaccharides, and polysaccharides, and their chemical properties. It also explores the significance of these compounds in nature and biology. Test your understanding of these essential biomolecules!