BCH101: Comprehensive Page-by-Page Notes on Carbohydrates

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• Title slide content: BCH 101 Lecture, Macromolecules: Carbohydrates, Basic Biochemistry
• No additional substantive content provided on this page beyond the title and course context.

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• Carbohydrates are the most abundant biomolecules on earth.
• Photosynthesis converts more than 100 billion metric tons of CO₂ and H₂O into cellulose and other plant products.
• Carbohydrates in plant products are a dietary staple in most parts of the world.
• Oxidation of carbohydrates is the central energy-yielding pathway in most non-photosynthetic cells.
• Carbohydrate polymers (glycans) serve structural and protective roles in cell walls.
• Glycoconjugates: carbohydrate polymers covalently attached to proteins or lipids act as signals that determine intracellular destination or metabolic fate of these hybrid molecules.
• Roles of carbohydrates include:
  • Lubricating skeletal joints
  • Cell‑cell recognition and adhesion
  • Energy storage
  • Providing carbon in most heterotrophs

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• Chemical nature:
  • Carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis (polyhydroxyaldehydes/polyhydroxyketones).
  • Empirical formula often written as
    $(CH<em>2O)</em>n$
  • Some carbohydrates contain nitrogen, phosphorus, or sulfur.
• Carbon skeleton sizes: 3, 4, 5, 6, 7 carbon atoms corresponding to:
  • 3: Triose
  • 4: Tetrose
  • 5: Pentose
  • 6: Hexose
  • 7: Heptose
• Three most common monosaccharides: glucose, galactose, fructose.
  • Glucose and galactose are aldohexoses; fructose is a ketohexose.
  • Notation: glucose and galactose are aldohexoses; fructose is a ketohexose.

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• Three major size classes of carbohydrates: 1) Monosaccharides: simple sugars, single polyhydroxy aldehyde or ketone unit; most abundant in nature is D-glucose (a hexose). 2) Oligosaccharides: short chains of monosaccharide units (2–9 units) joined by glycosidic bonds; most abundant oligosaccharides are disaccharides (two monosaccharide units).
  • Example: Sucrose = D-glucose + D-fructose (a 6-carbon and a 6-carbon sugar).
  • Names end in "-ose".
  • Oligosaccharides with 3–4 units do not exist freely; they are typically bound to lipids or proteins when small.
    3) Polysaccharides: sugar polymers containing more than 10 monosaccharide units; can have hundreds or thousands of units.
  • Can be linear (e.g., cellulose) or branched (e.g., glycogen).
  • Although both cellulose and glycogen are polymers of D-glucose, the type of glycosidic linkage leads to distinct properties and biological roles.

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• Polysaccharides continued: linear vs branched structures.
  • Cellulose: linear chains of D-glucose.
  • Glycogen: highly branched polymer of glucose.
• Properties depend on glycosidic linkages; both cellulose and glycogen repeat D-glucose units, but differ in linkage type and thus function.

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• Chiral carbons and stereoisomers:
  • Asymmetric carbons exist in carbohydrates.
  • Enantiomers vs diastereomers.
  • In Fischer projections: vertical line points behind the plane; horizontal line projects out.
  • Perspective formulas (wedges): wide end of solid wedge projects toward the reader; dashed wedge goes behind.
  • A molecule with n chiral centers can have $2^n$ stereoisomers.

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• Importance of stereoisomers: real-world relevance of stereochemistry in biology and medicine.
• Thalidomide tragedy (historical example of stereoisomer importance):
  • Frances Oldham Kelsey, the American pharmacologist who stopped its approval due to safety concerns in the 1960s, highlighting regulatory and ethical dimensions.

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• Naming stereoisomers:
  • When more than one chiral center exists, determine the chiral carbon farthest from the carbonyl carbon.
  • If the –OH is on the left in the Fischer projection (with the carbonyl carbon oriented “up”), it is an L isomer; if it’s on the right, it is a D isomer.
  • Monosaccharides (for each carbon chain length) can be divided into D and L groups, differing in configuration at the chiral center most distant from the carbonyl carbon.
  • Those with the same configuration as D-glyceraldehyde are designated D; those with the same as L-glyceraldehyde are L.

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• Example exercises and homework prompts provided:
  • Given the structure for D-glucose, draw L-glucose.
  • Draw Fischer projections and identify D/L isomers for alanine and lactic acid.
  • Fischer projections of D- and L-lactic acid; D- and L-alanine examples as shown.
• Visual references include:
  • D-glucose, L-glucose
  • D- and L-alanine structures
  • D/L-lactic acid structures

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• Epimers: stereoisomers that differ in configuration at one chiral center.
  • Example: D-mannose is an epimer of D-glucose at C-2.
  • D-galactose is an epimer of D-glucose at C-4.
• Figure 7-4 illustrates epimers of D-glucose and their differences at a single chiral center.

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• Monosaccharides form ring structures in solution; open-chain forms are not common because intramolecular attack of an alcohol on the carbonyl forms a cyclic hemiacetal (pyranose) or hemiketal (furanose).
• Examples:
  • a-D-Glucopyranose and B-D-Glucopyranose (pyranose rings, six-membered rings).
  • a-D-Fructofuranose (furanose ring, five-membered).
• Haworth perspective relationships: ring forms and stereochemistry relative to Fischer projections; mutarotation interconverts anomers (α and β) at the anomeric carbon (C-1 in aldoses).

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• Mechanistic illustration of hemiacetal/hemiketal formation and acetal formation:
  • Aldehyde/ketone + alcohol → hemiacetal or hemiketal (one new chiral center at carbonyl carbon).
  • Subsequent reaction with a second alcohol to form an acetal or ketal.
  • When the second alcohol is another sugar, the glycosidic bond (glycosidic linkage) is formed.
  • Key species: hemiacetal; acetal; hemiketal; ketal.
• Additional depiction of glycosidic bond formation and hydrolysis:
  $R<em>1-C(OR</em>2)(OR<em>3) \, + \, H</em>2O \rightarrow R<em>1-C(OR</em>2)(OH) + R_3-OH$
  (illustrative hydrolysis/glycosidic bond formation context)

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• Anomers:
  • Carbon 1 is a chiral center in the cyclic form; rotation is restricted by the ring.
  • Two configurations exist: α and β; they are anomers of each other.
  • The anomeric carbon is designated C-1.

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• Reducing sugar concept:
  • Sugars with an –OH group adjacent to the carbonyl carbon (the –OH on the anomeric carbon in the cyclic form) can react with Benedict’s reagent, a basic solution of Cu²⁺.
  • This reaction yields a red-orange precipitate of copper(I) oxide: Cu₂O.
  • Sugars that undergo this reaction are reducing sugars (they reduce Cu²⁺ to Cu⁺).
• Implication: open-chain form is in equilibrium with the ring form; oxidation can proceed as the open-chain form is present.

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• Maltose formation and reducing sugar concept illustrated:
  • Maltose is formed from two D-glucose units via a glycosidic bond between one glucose’s anomeric carbon and another glucose’s hydroxyl.
  • Maltose structure: α(1→4) glycosidic linkage between the two glucose units.
  • In maltose, one anomeric carbon remains in a hemiacetal form and remains reducing; mutarotation allows interconversion between α and β forms at that position.
  • The depiction sometimes uses wavy bonds to indicate that the glycosidic linkage may be in α or β form due to mutarotation.

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• Prominent monosaccharides and backbone roles:
  • β-D-ribose: forms the backbone of RNA (RNA sugar).
  • β-D-deoxyribose: forms the backbone of DNA (deoxyribonucleic acid).
  • β-D-galactose: incorporated with glucose into lactose (milk sugar).
• Additional note: glucose is a central metabolic sugar; other absorbed sugars are converted to glucose in the liver.
• Illustrative image labels: CH₂OH groups and hydroxyl orientations for these sugars (emphasizing stereochemistry).

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• More prominent monosaccharides:
  • β-D-glucose: also called dextrose; blood sugar; prevalent in honey and fruits; metabolic energy source.
  • β-D-fructose: also called levulose; fruit sugar; sweetest monosaccharide; present in honey (in a ~1:1 ratio with glucose), fruits, and corn syrup; often used as a sweetener because less fructose is needed for the same sweetness.

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• Disaccharides: two monosaccharides linked by a glycosidic bond.
• Maltose example:
  • Structure shown: α-D-glucose + α-D-glucose; formation involves a glycosidic bond between the anomeric carbon of one glucose and the OH of another glucose, releasing water (condensation).
  • Maltose: α(1→4) glycosidic linkage.
• Important concept: a disaccharide forms via a condensation reaction; hydrolysis reverses this process to yield two monosaccharides.

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• More disaccharides with structural specifics:
  • Lactose: β-D-galactopyranosyl-(1→4)-β-D-glucopyranose (galactose linked to glucose via a β(1→4) bond).
  • Sucrose: α-D-glucopyranosyl-(1→2)β-D-fructofuranoside (glucose linked to fructose via an α1→β2 linkage).
• Visual depiction includes stereochemistry and ring forms for the involved monosaccharides.

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• Polysaccharides introduce the glucose polymers that form the bulk of energy storage and structural carbohydrates:
  • Starch, glycogen, and cellulose are all glucose polymers.
  • Polysaccharides are not reducing sugars because their anomeric carbons are linked through glycosidic bonds.
  • Three main glucose polymers to know: starch (plants), glycogen (animals), cellulose (plants).

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• Important disaccharides recap:
  • Maltose: glucose + glucose; α(1→4) glycosidic linkage; malt sugar; produced in germinating grain.
  • Lactose: glucose + galactose; β(1→4) glycosidic linkage; milk sugar; digested by lactase enzyme.
  • Sucrose: glucose + fructose; α1→β2 glycosidic linkage; table sugar.

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• Polysaccharides overview:
  • Polysaccharides contain hundreds to thousands of carbohydrate units.
  • They are not reducing sugars since the anomeric carbons are involved in glycosidic linkages.
  • We focus on three glucose polymers: starch, glycogen, cellulose.

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• Starch: storage carbohydrate in plants
  • Composed of D-glucose units.
  • Generally insoluble in water due to high molecular weight.
  • Large OH content allows thick colloidal dispersions when heated with water (e.g., in gravies or sauces).
  • Two forms: amylose (unbranched) and amylopectin (branched).

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• Amylose:
  • Long, unbranched chains of glucose connected by a(1→4) glycosidic linkages.
  • Typical chain length: 1000–2000 molecules per chain.
  • Structural depiction indicates the a(1→4) linkage and the helical conformation.
  • Approximately 10–20% of starch in plants is amylose.
  • Helical packing contributes to slower digestion relative to highly branched starches.

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• Amylopectin:
  • Long chains of glucose up to about 10^5 molecules with a(1→4) linkages, and a(16) (i.e., a(1→6)) branches every 24–30 glucose units.
  • Accounts for about 80–90% of starch in plants.
  • Structure includes a(1→4) backbone with periodic a(1→6) branch points.

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• Glycogen: storage carbohydrate for animals
  • Structurally similar to amylopectin but more highly branched.
  • Branch points occur every 8–12 glucose units, making it highly soluble and readily accessible for rapid energy release.
  • Predominantly stored in liver and muscle tissues; hydrolysis releases glucose to maintain blood sugar levels and provide energy.
• Summary of plant polysaccharides:
  • Starch in plants consists of amylose (unbranched, a(1→4)) and amylopectin (branched, a(1→4) with a(1→6) branches).
  • Glycogen in animals is highly branched (more frequent branching than amylopectin).

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• Cellulose: structural carbohydrate in plants
  • Polymer of long, unbranched D-glucose chains linked by β(1→4) glycosidic bonds.
  • Typical cellulose molecules contain 300 to 3000 glucose units.
  • Structural arrangement leads to straight chains that hydrogen-bond to one another, forming a very rigid, high-strength material.
  • Cellulose is the most important structural polysaccharide and the most abundant organic compound on Earth; major component of plant cell walls (e.g., wood is ~50% cellulose).

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• Properties and biological relevance of cellulose:
  • Because of β(1→4) linkages, cellulose chains adopt a straight, extended conformation and hydrogen-bond with adjacent chains.
  • This results in rigid, strong fibers that contribute to plant structural integrity.
  • Industrial relevance: cellulose is found in wood, paper, cotton, cellophane, rayon, linen, nitrocellulose (gun cotton), photographic films (cellulose acetate), etc.

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• Digestive and ecological notes on cellulose:
  • Most animals lack enzymes to digest cellulose directly.
  • Cellulose provides dietary fiber (roughage) that stimulates intestinal contractions and aids digestion.
  • Some herbivores (ruminants like cows, sheep, horses) use gut microbiota to digest cellulose; cellulose breakdown occurs through microbial fermentation in multi-chambered stomachs, allowing glucose production.
  • Rabbits and other species can reprocess digested material to extend fermentation time for cellulose breakdown.

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• Quick comparison: starch, cellulose, glycogen
  • Source: starch (plants), cellulose (plants), glycogen (animals).
  • Subunit: all are glucose in common form; however, the anomeric configuration varies due to glycosidic linkages.
  • Bonds:
  • Starch: mostly 1→4 linkages in amylose; also 1→4 in amylopectin with 1→6 branches.
  • Cellulose: β(1→4) linkages.
  • Glycogen: 1→4 linkages with 1→6 branch points, highly branched.
  • Branching: starch (amylose unbranched; amylopectin branched), cellulose (no branches), glycogen highly branched.
  • Diagram/shape: starch tends to helical (amylose); cellulose forms extended straight chains; glycogen forms a highly branched, compact molecule.

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• Recap of key concepts:
  • Major energy storage and carbon-supplying macromolecule is carbohydrate.
  • Functional group classes: polyhydroxyaldehydes and polyhydroxyketones.
  • Size classes: monosaccharide, oligosaccharide, polysaccharide.
  • Concept of chiral carbons and stereoisomers; pyranose rings; epimers and anomers.
  • Common monosaccharides: glucose, ribose, fructose, galactose.
  • Common oligosaccharides: maltose, lactose, sucrose.
  • Common polysaccharides: starch, glycogen, cellulose.

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• Closing: thank you.

Key terms, concepts, and relationships (quick reference)

• Empirical formula: $(CH<em>2O)</em>n$
• Glycosidic linkage notations: $\alpha(1\to 4)$, $\beta(1\to 4)$, $\alpha(1\to 2)$, $\beta(1\to 4)$, etc.
• Anomeric carbon: C-1 in aldoses; C-2 in ketoses for certain ring forms; site of mutarotation between α and β.
• Hemiacetal vs acetal; hemiacetal/hemiketal formation from carbonyl + alcohol; subsequent acetal/ketal formation with a second alcohol.
• Reducing sugars: capable of reducing Cu²⁺ to Cu⁺ in Benedict’s/Fehling’s-type reactions; open-chain form allows oxidation even if the cyclic form initially blocks oxidation.
• Epimers: differ at a single stereocenter (e.g., D-mannose vs D-glucose at C-2; D-galactose vs D-glucose at C-4).
• Epimers vs anomers: epimers differ at a non-anomeric stereocenter; anomers differ at the anomeric carbon (α vs β).
• Monosaccharide examples and roles:
  • D-glucose: central metabolic sugar; energy source; major component of starch/glycogen; exists in α- and β- forms in cyclic structures.
  • D-fructose: ketohexose; fructose in honey, fruits, corn syrup; sweetest monosaccharide.
  • D-ribose: ribose backbone of RNA.
  • D-deoxyribose: backbone of DNA.
  • D-galactose: component of lactose.
• Disaccharides:
  • Maltose: α(1→4) linkage; glucose+glucose; reducing sugar.
  • Lactose: β(1→4) linkage; galactose+glucose; reducing sugar (lactase substrate).
  • Sucrose: α1→β2 linkage; glucose+fructose; non-reducing sugar.
• Polysaccharides:
  • Starch: plant storage; amylose (a(1→4) linear) and amylopectin (a(1→4) with a(1→6) branches).
  • Glycogen: animal storage; highly branched (branch points every 8–12 glucose units).
  • Cellulose: plant structural; β(1→4) linkages; linear, rigid, hydrogen-bonded fibers; most abundant organic compound on earth.
• Biological and practical implications:
  • Digestion and fermentation differences (cellulose vs starch/glycogen).
  • Industrial relevance of cellulose and starch in materials and food.
  • Ethical and regulatory implications of stereoisomerism in biomedicine (e.g., thalidomide case).