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Questions and Answers
What are the structural roles of insoluble carbohydrate polymers in biological systems?
What are the structural roles of insoluble carbohydrate polymers in biological systems?
Which type of monosaccharide contains a carbonyl group at the end of its carbon chain?
Which type of monosaccharide contains a carbonyl group at the end of its carbon chain?
What distinguishes enantiomers from other stereoisomers?
What distinguishes enantiomers from other stereoisomers?
What is the simplest form of a ketose monosaccharide?
What is the simplest form of a ketose monosaccharide?
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In Fischer projection formulas, which bonds project out of the plane towards the reader?
In Fischer projection formulas, which bonds project out of the plane towards the reader?
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Which of the following statements about monosaccharides is correct?
Which of the following statements about monosaccharides is correct?
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How are glycoconjugates defined?
How are glycoconjugates defined?
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What main function do carbohydrate polymers have in lubricating skeletal joints?
What main function do carbohydrate polymers have in lubricating skeletal joints?
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What differentiates reducing sugars from non-reducing sugars?
What differentiates reducing sugars from non-reducing sugars?
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Which of the following statements about sucrose is true?
Which of the following statements about sucrose is true?
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What is Fehling's reaction used to test for?
What is Fehling's reaction used to test for?
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How does glycogen primarily store energy in the body?
How does glycogen primarily store energy in the body?
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Which monosaccharide is involved in the oxidation process described?
Which monosaccharide is involved in the oxidation process described?
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What is the role of glucose oxidase in measuring blood glucose levels?
What is the role of glucose oxidase in measuring blood glucose levels?
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What is the difference between amylose and amylopectin in starch?
What is the difference between amylose and amylopectin in starch?
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What characterizes a non-reducing sugar like sucrose?
What characterizes a non-reducing sugar like sucrose?
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Study Notes
Carbohydrates and Glycobiology
- Carbohydrates are the most abundant macromolecules by mass
- Functions of carbohydrates include: energy source, energy storage, components of cell walls, recognition and signaling, components of coenzymes and nucleic acids
- Carbohydrates are produced from CO2 and H2O via photosynthesis in plants
- Carbohydrates range in size from small molecules like glyceraldehyde (Mw = 90 g/mol) to large molecules like amylopectin (Mw = 200,000,000 g/mol)
- Carbohydrates can be covalently linked with proteins to form glycoproteins and proteoglycans
- Insoluble carbohydrate polymers are structural and protective components in cell walls (bacteria and plants) and connective tissues (animals)
- Other carbohydrate polymers lubricate joints, and participate in recognition and adhesion between cells
- Complex carbohydrate polymers covalently attached to proteins or lipids act as signals that dictate intracellular location or metabolic fate of these hybrid molecules (glycoconjugates)
Monosaccharides
- Monosaccharides (simple sugars): one polyhydroxy aldehyde or ketone unit (e.g., glucose)
- Disaccharides: two monosaccharide units joined by a glycosidic linkage (e.g., sucrose)
- Oligosaccharides: a few monosaccharide units joined together; in cells, most are joined to a non-sugar molecule
- Polysaccharides: sugar polymers consisting of more than 20 monosaccharide units
Monosaccharides (cont'd)
- Monosaccharides are (CH2O)n where n ≥ 3
- Two families of monosaccharides exist: aldoses (aldehyde functional group) and ketoses (ketone functional group)
- Examples of aldoses and ketoses: glyceraldehyde (aldotriose), dihydroxyacetone (ketotriose)
Stereoisomers
- All monosaccharides (except dihydroxyacetone) are chiral
- Molecules with n chiral centers can have 2n stereoisomers
- Glyceraldehyde (21=2) has 2 enantiomers
- Aldohexoses (24=16) have 16 stereoisomers
Enantiomers
- Enantiomers are nonsuperimposable mirror images
- Designate the most distant chiral carbon from the carbonyl carbon as D (right) or L (left)
- Most hexoses in living organisms are D stereoisomers
- Simple sugars can also occur in the L form (e.g., L-arabinose)
Epimers
- Epimers are two sugars that differ only in the configuration around one carbon atom
- Example: D-glucose and D-galactose are epimers at carbon-4
Structures to Know
- Glyceraldehyde and dihydroxyacetone are standard 3-carbon sugars
- Ribose is a standard 5-carbon sugar
- Glucose is a standard 6-carbon sugar
- Galactose is an epimer of glucose
- Mannose is an epimer of glucose
- Fructose is a ketose form of glucose
D-Aldoses
- Diagrams of D-aldoses (3,4,5, and 6 carbons)
D-Ketoses
- Diagrams of D-ketoses (3,4, and 5 carbons)
Monosaccharides (cont'd)
- In solution, monosaccharides with 5 or more carbons form cyclic compounds
Hemiacetals & Hemiketals
- Aldehyde and ketone carbons are electrophilic
- Alcohol oxygen atoms are nucleophilic
- When aldehydes are attacked by alcohols, hemiacetals form
- When ketones are attacked by alcohols, hemiketals form
Cyclization of Monosaccharides
- Pentoses and hexoses readily undergo intramolecular ring formation
- Carbonyl carbon becomes a new chiral center (anomeric carbon)
- Carbonyl oxygen becomes a hydroxyl group
- Positioning of the hydroxyl group (a or β) depends on the trans/cis position relative to CH2OH
Formation of the Two Cyclic Forms of D-Glucose
- Reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage
- Mutarotation = the interconversion of α and β anomers
Pyranoses & Furanoses
- Pyranoses: six-membered rings (e.g., α- and β-D-glucopyranose)
- Formed when the hydroxyl group at C-6 reacts with the keto group at C-2
- Furanoses: five-membered rings (e.g., α- and β-D-fructofuranose)
- Formed by reaction of the hydroxyl group at C-5 reacting with the keto group at C-2
Monosaccharides Are Reducing Agents
- Monosaccharides can be oxidized by relatively mild oxidizing agents (e.g., ferric [Fe3+] or cupric [Cu2+] ions)
- Carbonyl carbon oxidized to a carboxyl group
- Glucose and other oxidizing sugars capable of reducing ferric or cupric ions are called reducing sugars
Disaccharides Contain a Glycosidic Bond
- Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon
- Glycosidic bond is less reactive than the hemiacetal
- Second monomer with hemiacetal is reducing
- Anomeric carbon involved in glycosidic linkage is nonreducing
Maltose
- Disaccharide formed upon condensation of two glucose molecules via a 1→4 bond
The Reducing End
- Formation of a glycosidic bond renders a sugar nonreducing
- Reducing end = the end of a disaccharide or polysaccharide chain with a free anomeric carbon
Non-reducing Sugars
- Two sugar molecules can be joined via a glycosidic bond between two anomeric carbons
- Product has two acetal groups and no hemiacetals
- No reducing ends; this is a nonreducing sugar
- Trehalose is a constituent of insect hemolymph (protection from drying)
Sucrose
- Disaccharide of glucose and fructose
- Formed by plants, not animals
- No free anomeric carbon; a nonreducing sugar
Difference between reducing and non-reducing sugar
- Summary table differentiating reducing and non-reducing sugars
Polysaccharides
- Natural carbohydrates are usually polymers (homopolysaccharides and heteropolysaccharides)
- Homopolysaccharides: contain a single monomeric sugar species (storage or structural elements) Examples: starch (amylose and amylopectin) and glycogen
- Heteropolysaccharides: contain 2+ kinds monomers (extracellular support)
Starch and Glycogen: Homopolysaccharides
- Starch: (amylose and amylopectin): polymers of glucose
- Amylose: unbranched chains of D-glucose connected by (α1→4) linkages
- Amylopectin: larger than amylose, with (α1→6) linkages and highly branched
- Glycogen: polymer of (α1→4)-linked glucose subunits; more extensively branched, more compact than starch
Metabolism of Glycogen and Starch
- Glycogen and starch often form granules in cells
- Granules contain enzymes that synthesize & degrade the polymers
- Glycogen and amylopectin have one reducing end and many nonreducing ends
Structure of Starch and Glycogen
- Diagrams of amylose, amylopectin, and glycogen structures
Mixture of Amylose and Amylopectin in Starch
- Amylopectin and amylose strands form double helices in starch granules and help with energy production
Structure and Roles of Some Polysaccharides
- Table summarizing structures and roles of some polysaccharides (including starch, glycogen, cellulose, chitin, dextran, peptidoglycan, hyaluronan)
Some Homopolysaccharides Serve Structural Roles
- Cellulose: tough, fibrous, water-insoluble substance
- Linear, unbranched homopolysaccharide made of 10,000-15,000 β-D-glucose units linked by β(1→4) glycosidic bonds
- Animals lack the enzyme to hydrolyze these bonds
Chitin
- Linear homopolysaccharide composed of N-acetylglucosamine residues in β(1→4) linkage
- More hydrophobic and water-resistant than cellulose due to the acetylated amino group
Peptidoglycan Reinforces the Bacterial Cell Wall
- Rigid component of bacterial cell walls
- Heteropolymer of alternating (β1→4)-linked N-acetylglucosamine and N-acetylmuramic acid residues; cross-linked by short peptides
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Description
Explore the essential role of carbohydrates in biology through this quiz. Learn about their functions, structures, and significance in energy storage, signaling, and cellular components. Delve into the intricacies of glycoproteins and their biological implications.
